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Implementing Access Lists and Prefix Lists

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Implementing Access Lists and Prefix Lists

Contents

Implementing Access Lists and Prefix Lists

An access control list (ACL) consists of one or more access control entries (ACE) that collectively define the network traffic profile. This profile can then be referenced by Cisco IOS XR softwarefeatures such as traffic filtering, route filtering, QoS classification, and access control. Each ACL includes an action element (permit or deny) and a filter element based on criteria such as source address, destination address, protocol, and protocol-specific parameters.

Prefix lists are used in route maps and route filtering operations and can be used as an alternative to access lists in many Border Gateway Protocol (BGP) route filtering commands. A prefix is a portion of an IP address, starting from the far left bit of the far left octet. By specifying exactly how many bits of an address belong to a prefix, you can then use prefixes to aggregate addresses and perform some function on them, such as redistribution (filter routing updates).

This module describes the new and revised tasks required to implement access lists and prefix lists on the Cisco ASR 9000 Series Router


Note


For a complete description of the access list and prefix list commands listed in this module, refer to the Cisco ASR 9000 Series Aggregation Services Router IP Addresses and Services Command ReferenceTo locate documentation of other commands that appear in this chapter, use the command reference master index, or search online.


Feature History for Implementing Access Lists and Prefix Lists

Release

Modification

Release 3.7.2

This feature was introduced.

Prerequisites for Implementing Access Lists and Prefix Lists

The following prerequisite applies to implementing access lists and prefix lists:

All command task IDs are listed in individual command references and in the Cisco IOS XR Task ID Reference Guide.If you need assistance with your task group assignment, contact your system administrator.

Restrictions for Implementing Access Lists

The following restriction applies to implementing access lists and prefix lists:


  • IPv6 ACL configuration to Layer2 interfaces or Ethernet Flow Points (EFP)s is not supported.

Hardware Limitations


  • Support for ABF is only for IPv4 and Ethernet line cards. IPv6 and other interfaces are not supported.

  • ABF is an ingress line card feature and the egress line card must be ABF aware.

  • SIP-700 is not ABF aware and hence drops ABF packets.

Restrictions


  • The following nexthop configurations are not supported: attaching ACL having a nexthop option in the egress direction, modifying an ACL attached in the egress direction having nexthop, deny ACE with nexthop.

  • The following interfaces are not supported: loopback, interflex, and L2.

  • The ABF feature configuration on A9K-SIP-700 is not supported.

  • ABF nexthop packets received by A9K-SIP-700 are dropped.


Note


There is one exception to this. In case of IP to TAG, the label is imposed by the ingress LC (based on ABF nexthop), and the packet crosses the fabric as a tag packet. These packets are handled by A9K-SIP-700 without any issue.



  • Packets punted in the ingress direction from the NPU to the LC CPU are not subjected to ABF treatment due to lack of ABF support in the slow path.

  • IP packet(s) needing fragmentation are not subjected to ABF. The packet is forwarded in the traditional way. Fragmented packets recieved are handled by ABF.

  • The nexthop functionality is not supported in VRF. Nexthop is looked at in the global table only.

Information About Implementing Access Lists and Prefix Lists

To implement access lists and prefix lists, you must understand the following concepts:

Access Lists and Prefix Lists Feature Highlights

This section lists the feature highlights for access lists and prefix lists.


  • Cisco IOS XR software provides the ability to clear counters for an access list or prefix list using a specific sequence number.

  • Cisco IOS XR software provides the ability to copy the contents of an existing access list or prefix list to another access list or prefix list.

  • Cisco IOS XR software allows users to apply sequence numbers to permit or deny statements and to resequence, add, or remove such statements from a named access list or prefix list.


    Note


    Resequencing is only for IPv4 prefix lists.


  • Cisco IOS XR software does not differentiate between standard and extended access lists. Standard access list support is provided for backward compatibility.

Purpose of IP Access Lists

Access lists perform packet filtering to control which packets move through the network and where. Such controls help to limit network traffic and restrict the access of users and devices to the network. Access lists have many uses, and therefore many commands accept a reference to an access list in their command syntax. Access lists can be used to do the following:


  • Filter incoming packets on an interface.

  • Filter outgoing packets on an interface.

  • Restrict the contents of routing updates.

  • Limit debug output based on an address or protocol.

  • Control vty access.

  • Identify or classify traffic for advanced features, such as congestion avoidance, congestion management, and priority and custom queueing.

How an IP Access List Works

An access list is a sequential list consisting of permit and deny statements that apply to IP addresses and possibly upper-layer IP protocols. The access list has a name by which it is referenced. Many software commands accept an access list as part of their syntax.

An access list can be configured and named, but it is not in effect until the access list is referenced by a command that accepts an access list. Multiple commands can reference the same access list. An access list can control traffic arriving at the router or leaving the router, but not traffic originating at the router.

IP Access List Process and Rules

Use the following process and rules when configuring an IP access list:


  • The software tests the source or destination address or the protocol of each packet being filtered against the conditions in the access list, one condition (permit or deny statement) at a time.

  • If a packet does not match an access list statement, the packet is then tested against the next statement in the list.

  • If a packet and an access list statement match, the remaining statements in the list are skipped and the packet is permitted or denied as specified in the matched statement. The first entry that the packet matches determines whether the software permits or denies the packet. That is, after the first match, no subsequent entries are considered.

  • If the access list denies the address or protocol, the software discards the packet and returns an Internet Control Message Protocol (ICMP) Host Unreachable message. ICMP is configurable in the Cisco IOS XR software.

  • If no conditions match, the software drops the packet because each access list ends with an unwritten or implicit deny statement. That is, if the packet has not been permitted or denied by the time it was tested against each statement, it is denied.

  • The access list should contain at least one permit statement or else all packets are denied.

  • Because the software stops testing conditions after the first match, the order of the conditions is critical. The same permit or deny statements specified in a different order could result in a packet being passed under one circumstance and denied in another circumstance.

  • If an access list is referenced by name in a command, but the access list does not exist, all packets pass.

  • Only one access list per interface, per protocol, per direction is allowed.

  • Inbound access lists process packets arriving at the router. Incoming packets are processed before being routed to an outbound interface. An inbound access list is efficient because it saves the overhead of routing lookups if the packet is to be discarded because it is denied by the filtering tests. If the packet is permitted by the tests, it is then processed for routing. For inbound lists, permit means continue to process the packet after receiving it on an inbound interface; deny means discard the packet.

  • Outbound access lists process packets before they leave the router. Incoming packets are routed to the outbound interface and then processed through the outbound access list. For outbound lists, permit means send it to the output buffer; deny means discard the packet.

  • An access list can not be removed if that access list is being applied by an access group in use. To remove an access list, remove the access group that is referencing the access list and then remove the access list.

  • An access list must exist before you can use the ipv4 access group command.

Helpful Hints for Creating IP Access Lists

Consider the following when creating an IP access list:


  • Create the access list before applying it to an interface.

  • Organize your access list so that more specific references in a network or subnet appear before more general ones.

  • To make the purpose of individual statements more easily understood at a glance, you can write a helpful remark before or after any statement.

Source and Destination Addresses

Source address and destination addresses are two of the most typical fields in an IP packet on which to base an access list. Specify source addresses to control packets from certain networking devices or hosts. Specify destination addresses to control packets being sent to certain networking devices or hosts.

Wildcard Mask and Implicit Wildcard Mask

Address filtering uses wildcard masking to indicate whether the software checks or ignores corresponding IP address bits when comparing the address bits in an access-list entry to a packet being submitted to the access list. By carefully setting wildcard masks, an administrator can select a single or several IP addresses for permit or deny tests.

Wildcard masking for IP address bits uses the number 1 and the number 0 to specify how the software treats the corresponding IP address bits. A wildcard mask is sometimes referred to as an inverted mask, because a 1 and 0 mean the opposite of what they mean in a subnet (network) mask.


  • A wildcard mask bit 0 means check the corresponding bit value.

  • A wildcard mask bit 1 means ignore that corresponding bit value.

You do not have to supply a wildcard mask with a source or destination address in an access list statement. If you use the host keyword, the software assumes a wildcard mask of 0.0.0.0.

Unlike subnet masks, which require contiguous bits indicating network and subnet to be ones, wildcard masks allow noncontiguous bits in the mask. For IPv6 access lists, only contiguous bits are supported.

You can also use CIDR format (/x) in place of wildcard bits. For example, the address 1.2.3.4 0.255.255.255 corresponds to 1.2.3.4/8.

Transport Layer Information

You can filter packets on the basis of transport layer information, such as whether the packet is a TCP, UDP, ICMP, or IGMP packet.

IP Access List Entry Sequence Numbering

The ability to apply sequence numbers to IP access-list entries simplifies access list changes. Prior to this feature, there was no way to specify the position of an entry within an access list. If a user wanted to insert an entry (statement) in the middle of an existing list, all the entries after the desired position had to be removed, then the new entry was added, and then all the removed entries had to be reentered. This method was cumbersome and error prone.

The IP Access List Entry Sequence Numbering feature allows users to add sequence numbers to access-list entries and resequence them. When you add a new entry, you choose the sequence number so that it is in a desired position in the access list. If necessary, entries currently in the access list can be resequenced to create room to insert the new entry.

Sequence Numbering Behavior

The following details the sequence numbering behavior:


  • If entries with no sequence numbers are applied, the first entry is assigned a sequence number of 10, and successive entries are incremented by 10. The maximum sequence number is 2147483646. If the generated sequence number exceeds this maximum number, the following message displays:

Exceeded maximum sequence number.

  • If you provide an entry without a sequence number, it is assigned a sequence number that is 10 greater than the last sequence number in that access list and is placed at the end of the list.

  • ACL entries can be added without affecting traffic flow and hardware performance.

  • If a new access list is entered from global configuration mode, then sequence numbers for that access list are generated automatically.

  • Distributed support is provided so that the sequence numbers of entries in the route processor (RP) and line card (LC) are synchronized at all times.

  • This feature works with named standard and extended IP access lists. Because the name of an access list can be designated as a number, numbers are acceptable.

IP Access List Logging Messages

Cisco IOS XR software can provide logging messages about packets permitted or denied by a standard IP access list. That is, any packet that matches the access list causes an informational logging message about the packet to be sent to the console. The level of messages logged to the console is controlled by the logging console command in global configuration mode.

The first packet that triggers the access list causes an immediate logging message, and subsequent packets are collected over 5-minute intervals before they are displayed or logged. The logging message includes the access list number, whether the packet was permitted or denied, the source IP address of the packet, and the number of packets from that source permitted or denied in the prior 5-minute interval.

However, you can use the { ipv4 | ipv6 } access-list log-update threshold command to set the number of packets that, when they match an access list (and are permitted or denied), cause the system to generate a log message. You might do this to receive log messages more frequently than at 5-minute intervals.


Caution


If you set the update-number argument to 1, a log message is sent right away, rather than caching it; every packet that matches an access list causes a log message. A setting of 1 is not recommended because the volume of log messages could overwhelm the system.


Even if you use the { ipv4 | ipv6} access-list log-update threshold command, the 5-minute timer remains in effect, so each cache is emptied at the end of 5 minutes, regardless of the number of messages in each cache. Regardless of when the log message is sent, the cache is flushed and the count reset to 0 for that message the same way it is when a threshold is not specified.


Note


The logging facility might drop some logging message packets if there are too many to be handled or if more than one logging message is handled in 1 second. This behavior prevents the router from using excessive CPU cycles because of too many logging packets. Therefore, the logging facility should not be used as a billing tool or as an accurate source of the number of matches to an access list.


Extended Access Lists with Fragment Control

Prior to this feature, nonfragmented packets and the initial fragment of a packet were processed by IP extended access lists (if such an access list was applied), but noninitial fragments were permitted by default. The IP Extended Access Lists with Fragment Control feature now allows more granularity of control over noninitial packets. You can specify whether the system examines noninitial IP fragments of packets when applying an IP extended access list.

Because noninitial fragments contain only Layer 3 information, access-list entries containing only Layer 3 information can be and now are applied to noninitial fragments. The fragment has all the information the system requires to filter, so the entry is applied to the fragments.

This feature adds the optional fragments keyword to the following IP access list commands: deny (IPv4) , permit (IPv4) , deny (IPv6) , permit (IPv6). By specifying the fragments keyword in an access-list entry, that particular access-list entry applies only to noninitial fragments of packets; the fragment is either permitted or denied accordingly.

The behavior of access-list entries regarding the presence or absence of the fragments keyword can be summarized as follows:

If the Access-List Entry has...

Then...

...no fragments keyword and all of the access-list entry information matches

For an access-list entry containing only Layer 3 information:


  • The entry is applied to nonfragmented packets, initial fragments, and noninitial fragments.

For an access-list entry containing Layer 3 and Layer 4 information:


  • The entry is applied to nonfragmented packets and initial fragments.
    • If the entry matches and is a permit statement, the packet or fragment is permitted.

    • If the entry matches and is a deny statement, the packet or fragment is denied.

  • The entry is also applied to noninitial fragments in the following manner. Because noninitial fragments contain only Layer 3 information, only the Layer 3 portion of an access-list entry can be applied. If the Layer 3 portion of the access-list entry matches, and
    • If the entry is a permit statement, the noninitial fragment is permitted.

    • If the entry is a deny statement, the next access-list entry is processed.

    Note   

    Note that the deny statements are handled differently for noninitial fragments versus nonfragmented or initial fragments.

...the fragments keyword and all of the access-list entry information matches

The access-list entry is applied only to noninitial fragments.

Note   

The fragments keyword cannot be configured for an access-list entry that contains any Layer 4 information.

You should not add the fragments keyword to every access-list entry, because the first fragment of the IP packet is considered a nonfragment and is treated independently of the subsequent fragments. Because an initial fragment will not match an access list permit or deny entry that contains the fragments keyword, the packet is compared to the next access list entry until it is either permitted or denied by an access list entry that does not contain the fragments keyword. Therefore, you may need two access list entries for every deny entry. The first deny entry of the pair will not include the fragments keyword, and applies to the initial fragment. The second deny entry of the pair will include the fragments keyword and applies to the subsequent fragments. In the cases where there are multiple deny access list entries for the same host but with different Layer 4 ports, a single deny access-list entry with the fragments keyword for that host is all that has to be added. Thus all the fragments of a packet are handled in the same manner by the access list.

Packet fragments of IP datagrams are considered individual packets and each fragment counts individually as a packet in access-list accounting and access-list violation counts.


Note


The fragments keyword cannot solve all cases involving access lists and IP fragments.


Policy Routing

Fragmentation and the fragment control feature affect policy routing if the policy routing is based on the match ip address command and the access list had entries that match on Layer 4 through Layer 7 information. It is possible that noninitial fragments pass the access list and are policy routed, even if the first fragment was not policy routed or the reverse.

By using the fragments keyword in access-list entries as described earlier, a better match between the action taken for initial and noninitial fragments can be made and it is more likely policy routing will occur as intended.

Comments About Entries in Access Lists

You can include comments (remarks) about entries in any named IP access list using the remark access list configuration command. The remarks make the access list easier for the network administrator to understand and scan. Each remark line is limited to 255 characters.

The remark can go before or after a permit or deny statement. You should be consistent about where you put the remark so it is clear which remark describes which permit or deny statement. For example, it would be confusing to have some remarks before the associated permit or deny statements and some remarks after the associated statements. Remarks can be sequenced.

Remember to apply the access list to an interface or terminal line after the access list is created. See the“Applying Access Lists” section for more information.

Access Control List Counters

In Cisco IOS XR software, ACL counters are maintained both in hardware and software. Hardware counters are used for packet filtering applications such as when an access group is applied on an interface. Software counters are used by all the applications mainly involving software packet processing.

Packet filtering makes use of 64-bit hardware counters per ACE. If the same access group is applied on interfaces that are on the same line card in a given direction, the hardware counters for the ACL are shared between two interfaces.

To display the hardware counters for a given access group, use the show access-lists ipv4 [access-list-name hardware {ingress | egress} [interface type interface-path-id] {location node-id}] command in EXEC mode.

To clear the hardware counters, use the clear access-list ipv4 access-list-name [hardware {ingress | egress} [interface type interface-path-id] {location node-id}] command in EXEC mode.

Hardware counting is not enabled by default for IPv4 ACLs because of a small performance penalty. To enable hardware counting, use the ipv4 access-group access-list-name {ingress | egress} [hardware-count] command in interface configuration mode. This command can be used as desired, and counting is enabled only on the specified interface.

Software counters are updated for the packets processed in software, for example, exception packets punted to the LC CPU for processing, or ACL used by routing protocols, and so on. The counters that are maintained are an aggregate of all the software applications using that ACL. To display software-only ACL counters, use the show access-lists ipv4 access-list-name [sequence number] command in EXEC mode.

All the above information is true for IPv6, except that hardware counting is always enabled; there is no hardware-count option in the IPv6 access-group command-line interface (CLI).

BGP Filtering Using Prefix Lists

Prefix lists can be used as an alternative to access lists in many BGP route filtering commands. The advantages of using prefix lists are as follows:


  • Significant performance improvement in loading and route lookup of large lists.

  • Incremental updates are supported.

  • More user friendly CLI. The CLI for using access lists to filter BGP updates is difficult to understand and use because it uses the packet filtering format.

  • Greater flexibility.

Before using a prefix list in a command, you must set up a prefix list, and you may want to assign sequence numbers to the entries in the prefix list.

How the System Filters Traffic by Prefix List

Filtering by prefix list involves matching the prefixes of routes with those listed in the prefix list. When there is a match, the route is used. More specifically, whether a prefix is permitted or denied is based upon the following rules:


  • An empty prefix list permits all prefixes.

  • An implicit deny is assumed if a given prefix does not match any entries of a prefix list.

  • When multiple entries of a prefix list match a given prefix, the longest, most specific match is chosen.

Sequence numbers are generated automatically unless you disable this automatic generation. If you disable the automatic generation of sequence numbers, you must specify the sequence number for each entry using the sequence-number argument of the permit and deny commands in either IPv4 or IPv6 prefix list configuration command. Use the no form of the permit or deny command with the sequence-number argument to remove a prefix-list entry.

The show commands include the sequence numbers in their output.

Information About Implementing ACL-based Forwarding

To implement access lists and prefix lists, you must understand the following concepts:

ACL-based Forwarding Overview

Converged networks carry voice, video and data. Users may need to route certain traffic through specific paths instead of using the paths computed by routing protocols. A simple solution to achieve this, is by specifying the next-hop address in ACL configurations, so that the configured next-hop address from ACL is used for fowarding packet towards its destination instead of routing packet-based destination address lookup. This feature of using next-hop in ACL configurations for forwarding is called ACL Based Forwarding (ABF).

ACL-based forwarding enables you to choose service from multiple providers for broadcast TV over IP, IP telephony, data, and so on, which provides a cafeteria-like access to the Internet. Service providers can divert user traffic to various content providers.

ABF-OT

To provide flexibility to the user to select the suitable nexthop, the ABF functionality is enhanced to interact with object-tracking (OT), which impacts:


  • Tracking prefix in CEF

  • Tracking the line-state protocol

  • IPSLA (IP Service Level Agreement)

IPSLA support for Object tracking

The OT-module interacts with the IPSLA-module to get reachability information. With IPSLA, the routers perform periodic measurements

How to Implement Access Lists and Prefix Lists

IPv6 ACL support is available on the Cisco ASR 9000 SIP 700 linecard and the ASR 9000 Ethernet linecards. The relevant scale is:


  • ACL enabled interfaces - 1000 (500 in each direction); for ASR 9000 Ethernet linecards- 4000

  • Unique ACLs - 512 (with 5 ACEs each); for ASR 9000 Ethernet linecards- 2000

  • Maximum ACEs per ACL - 8000 (for ASR 9000 Ethernet lincards, ACEs could be 16000, 8000, 4000- based on the LC model)

  • IPv6 ACL log will also be supported.

This section contains the following procedures:

Configuring Extended Access Lists

This task configures an extended IPv4 or IPv6 access list.

SUMMARY STEPS

1.    configure

2.    {ipv4 | ipv6} access-list name

3.    [ sequence-number ] remark remark

4.    Do one of the following:

  • [ sequence-number]{permit | deny} source source-wildcard destination destination-wildcard [precedence precedence] [dscp dscp] [fragments] [packet-length operator packet-length value] [log | log-input]
  • [ sequence-number ] {permit | deny} protocol {source-ipv6-prefix/prefix-length | any | host source-ipv6-address} [operator {port | protocol-port}] {destination-ipv6-prefix/prefix-length | any | host destination-ipv6-address} [operator {port | protocol-port}] [dscp value] [routing] [authen] [destopts] [fragments] [packet-length operator packet-length value] [log | log-input]

5.    Repeat Step 4 as necessary, adding statements by sequence number where you planned. Use the no sequence-number command to delete an entry.

6.    Use one of the following commands:

  • end
  • commit

7.    show access-lists {ipv4 | ipv6} [access-list-name hardware {ingress | egress} [interface type interface-path-id] {sequence number | location node-id} | summary [access-list-name] | access-list-name [sequence-number] | maximum [detail] [usage {pfilter location node-id}]]


DETAILED STEPS
  Command or Action Purpose
Step 1 configure


Example:
RP/0/RSP0/CPU0:router# configure
 

Enters global configuration mode.

 
Step 2 {ipv4 | ipv6} access-list name


Example:
RP/0/RSP0/CPU0:router(config)# ipv4 access-list acl_1

or

RP/0/RSP0/CPU0:router(config)# ipv6 access-list acl_2
 

Enters either IPv4 or IPv6 access list configuration mode and configures the named access list.

 
Step 3 [ sequence-number ] remark remark


Example:
RP/0/RSP0/CPU0:router(config-ipv4-acl)# 10 remark Do not allow user1 to telnet out
 

(Optional) Allows you to comment about a permit or deny statement in a named access list.


  • The remark can be up to 255 characters; anything longer is truncated.

  • Remarks can be configured before or after permit or deny statements, but their location should be consistent.

 
Step 4 Do one of the following:
  • [ sequence-number]{permit | deny} source source-wildcard destination destination-wildcard [precedence precedence] [dscp dscp] [fragments] [packet-length operator packet-length value] [log | log-input]
  • [ sequence-number ] {permit | deny} protocol {source-ipv6-prefix/prefix-length | any | host source-ipv6-address} [operator {port | protocol-port}] {destination-ipv6-prefix/prefix-length | any | host destination-ipv6-address} [operator {port | protocol-port}] [dscp value] [routing] [authen] [destopts] [fragments] [packet-length operator packet-length value] [log | log-input]


Example:
RP/0/RSP0/CPU0:router(config-ipv4-acl)# 10 permit 172.16.0.0 0.0.255.255
RP/0/RSP0/CPU0:router(config-ipv4-acl)# 20 deny 192.168.34.0 0.0.0.255

or

RP/0/RSP0/CPU0:router(config-ipv6-acl)# 20 permit icmp any any
RP/0/RSP0/CPU0:router(config-ipv6-acl)# 30 deny tcp any any gt 5000
 

Specifies one or more conditions allowed or denied in IPv4 access list acl_1.


  • The optional log keyword causes an information logging message about the packet that matches the entry to be sent to the console.

  • The optional log-input keyword provides the same function as the log keyword, except that the logging message also includes the input interface.

or

Specifies one or more conditions allowed or denied in IPv6 access list acl_2.


  • Refer to the deny (IPv6) and permit (IPv6) commands for more information on filtering IPv6 traffic based on based on IPv6 option headers and optional, upper-layer protocol type information.

Note   

Every IPv6 address list has two implicit permits used for neighbor advertisement and solicitation: Implicit Neighbor Discovery–Neighbor Advertisement (NDNA) permit, and Implicit Neighbor Discovery–Neighbor Solicitation (NDNS) permit.

Note   

Every IPv6 access list has an implicit deny ipv6 any any statement as its last match condition. An IPv6 access list must contain at least one entry for the implicit deny ipv6 any any statement to take effect.

 
Step 5 Repeat Step 4 as necessary, adding statements by sequence number where you planned. Use the no sequence-number command to delete an entry. 

Allows you to revise an access list.

 
Step 6 Use one of the following commands:
  • end
  • commit


Example:
RP/0/RSP0/CPU0:router(config)# end

or

RP/0/RSP0/CPU0:router(config)# commit
 

Saves configuration changes.


  • When you issue the end command, the system prompts you to commit changes:

    Uncommitted changes found, commit them
    before exiting(yes/no/cancel)? [cancel]:
    

    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

 
Step 7 show access-lists {ipv4 | ipv6} [access-list-name hardware {ingress | egress} [interface type interface-path-id] {sequence number | location node-id} | summary [access-list-name] | access-list-name [sequence-number] | maximum [detail] [usage {pfilter location node-id}]]


Example:
RP/0/RSP0/CPU0:router# show access-lists ipv4 acl_1
 

(Optional) Displays the contents of current IPv4 or IPv6 access lists.


  • Use the access-list-name argument to display the contents of a specific access list.

  • Use the hardware , ingress or egress , and location or sequence keywords to display the access-list hardware contents and counters for all interfaces that use the specified access list in a given direction (ingress or egress). The access group for an interface must be configured using the ipv4 access-group command for access-list hardware counters to be enabled.

  • Use the summary keyword to display a summary of all current IPv4 or IPv6 access-lists.

  • Use the interface keyword to display interface statistics.

 
What to Do Next

After creating an access list, you must apply it to a line or interface. See the Applying Access Lists section for information about how to apply an access list.

ACL commit fails while adding and removing unique Access List Entries (ACE). This happens due to the absence of an assigned manager process. The user has to exit the config-ipv4-acl mode to configuration mode and re-enter the config-ipv4-acl mode before adding the first ACE.

Applying Access Lists

After you create an access list, you must reference the access list to make it work. Access lists can be applied on either outbound or inbound interfaces. This section describes guidelines on how to accomplish this task for both terminal lines and network interfaces.

Set identical restrictions on all the virtual terminal lines, because a user can attempt to connect to any of them.

For inbound access lists, after receiving a packet, Cisco IOS XR software checks the source address of the packet against the access list. If the access list permits the address, the software continues to process the packet. If the access list rejects the address, the software discards the packet and returns an ICMP host unreachable message. The ICMP message is configurable.

For outbound access lists, after receiving and routing a packet to a controlled interface, the software checks the source address of the packet against the access list. If the access list permits the address, the software sends the packet. If the access list rejects the address, the software discards the packet and returns an ICMP host unreachable message.

When you apply an access list that has not yet been defined to an interface, the software acts as if the access list has not been applied to the interface and accepts all packets. Note this behavior if you use undefined access lists as a means of security in your network.

Controlling Access to an Interface

This task applies an access list to an interface to restrict access to that interface.

Access lists can be applied on either outbound or inbound interfaces.

SUMMARY STEPS

1.    configure

2.    interface type interface-path-id

3.    Do one of the following:

  • ipv4 access-group access-list-name {ingress | egress} [hardware-count] [interface-statistics]
  • ipv6 access-group access-list-name {ingress | egress} [interface-statistics]

4.    Do one of the following:

  • end
  • commit


DETAILED STEPS
  Command or Action Purpose
Step 1 configure


Example:
RP/0/RSP0/CPU0:router# configure
 

Enters global configuration mode.

 
Step 2 interface type interface-path-id


Example:
RP/0/RSP0/CPU0:router(config)# interface gigabitethernet 0/2/0/2
 

Configures an interface and enters interface configuration mode.


  • The type argument specifies an interface type. For more information on interface types, use the question mark (?) online help function.

  • The instance argument specifies either a physical interface instance or a virtual instance.
    • The naming notation for a physical interface instance is rack/slot/module/port. The slash (/) between values is required as part of the notation.

    • The number range for a virtual interface instance varies depending on the interface type.

 
Step 3 Do one of the following:
  • ipv4 access-group access-list-name {ingress | egress} [hardware-count] [interface-statistics]
  • ipv6 access-group access-list-name {ingress | egress} [interface-statistics]


Example:
RP/0/RSP0/CPU0:router(config-if)# ipv4 access-group p-in-filter in

RP/0/RSP0/CPU0:router(config-if)# ipv4 access-group p-out-filter out
 

Controls access to an interface.


  • Use the access-list-name argument to specify a particular IPv4 or IPv6 access list.

  • Use the in keyword to filter on inbound packets or the out keyword to filter on outbound packets.

  • Use the hardware-count keyword to enable hardware counters for the IPv4 access group.
    • Hardware counters are automatically enabled for IPv6 access groups.

  • Use the interface-statistics keyword to specify per-interface statistics in the hardware.

This example applies filters on packets inbound and outbound from GigabitEthernet interface 0/2/0/2.

 
Step 4 Do one of the following:
  • end
  • commit


Example:
RP/0/RSP0/CPU0:router(config-if)# end

or

RP/0/RSP0/CPU0:router(config-if)# commit
 

Saves configuration changes.


  • When you issue the end command, the system prompts you to commit changes:

    		Uncommitted changes found, commit them before exiting(yes/no/cancel)?[cancel]:
    		
    		

    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

 

Controlling Access to a Line

This task applies an access list to a line to control access to that line.

SUMMARY STEPS

1.    configure

2.    line {aux | console | default | template template-name}

3.    access-class list-name{ingress | egress}

4.    Use one of the following commands:

  • end
  • commit


DETAILED STEPS
  Command or Action Purpose
Step 1 configure


Example:
RP/0/RSP0/CPU0:router# configure
 

Enters global configuration mode.

 
Step 2 line {aux | console | default | template template-name}


Example:
RP/0/RSP0/CPU0:router(config)# line default
 

Specifies either the auxiliary, console, default, or a user-defined line template and enters line template configuration mode.


  • Line templates are a collection of attributes used to configure and manage physical terminal line connections (the console and auxiliary ports) and vty connections. The following templates are available in Cisco IOS XR software:
    • Aux line template—The line template that applies to the auxiliary line.

    • Console line template— The line template that applies to the console line.

    • Default line template—The default line template that applies to a physical and virtual terminal lines.

    • User-defined line templates—User-defined line templates that can be applied to a range of virtual terminal lines.

 
Step 3 access-class list-name{ingress | egress}


Example:
RP/0/RSP0/CPU0:router(config-line)# access-class acl_2 out
 

Restricts incoming and outgoing connections using an IPv4 or IPv6 access list.


  • In the example, outgoing connections for the default line template are filtered using the IPv6 access list acl_2.

 
Step 4 Use one of the following commands:
  • end
  • commit


Example:
RP/0/RSP0/CPU0:router(config)# end

or

RP/0/RSP0/CPU0:router(config)# commit
 

Saves configuration changes.


  • When you issue the end command, the system prompts you to commit changes:

    Uncommitted changes found, commit them
    before exiting(yes/no/cancel)? [cancel]:
    

    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

 

Configuring Prefix Lists

This task configures an IPv4 or IPv6 prefix list.

SUMMARY STEPS

1.    configure

2.    {ipv4 | ipv6} prefix-list name

3.    [ sequence-number ] remark remark

4.    [ sequence-number] {permit | deny} network/length [ge value] [le value] [eq value]

5.    Repeat Step 4 as necessary. Use the no sequence-number command to delete an entry.

6.    Do one of the following:

  • end
  • commit

7.    Do one of the following:

  • show prefix-list ipv4 [name] [sequence-number]
  • show prefix-list ipv6 [name] [sequence-number] [summary]

8.    clear {ipv4 | ipv6} prefix-list name [sequence-number]


DETAILED STEPS
  Command or Action Purpose
Step 1 configure


Example:
RP/0/RSP0/CPU0:router# configure
 

Enters global configuration mode.

 
Step 2 {ipv4 | ipv6} prefix-list name


Example:
RP/0/RSP0/CPU0:router(config)# ipv4 prefix-list pfx_1

or

RP/0/RSP0/CPU0:router(config)# ipv6 prefix-list pfx_2
 

Enters either IPv4 or IPv6 prefix list configuration mode and configures the named prefix list.


  • To create a prefix list, you must enter at least one permit or deny clause.

  • Use the no {ipv4 | ipv6} prefix-list name command to remove all entries in a prefix list.

 
Step 3 [ sequence-number ] remark remark


Example:
RP/0/RSP0/CPU0:router(config-ipv4_pfx)# 10 remark Deny all routes with a prefix of 10/8
		
RP/0/RSP0/CPU0:router(config-ipv4_pfx)# 20 deny 10.0.0.0/8 le 32
 

(Optional) Allows you to comment about the following permit or deny statement in a named prefix list.


  • The remark can be up to 255 characters; anything longer is truncated.

  • Remarks can be configured before or after permit or deny statements, but their location should be consistent.

 
Step 4 [ sequence-number] {permit | deny} network/length [ge value] [le value] [eq value]


Example:
RP/0/RSP0/CPU0:router(config-ipv6_pfx)# 20 deny 128.0.0.0/8 eq 24
 

Specifies one or more conditions allowed or denied in the named prefix list.


  • This example denies all prefixes matching /24 in 128.0.0.0/8 in prefix list pfx_2.

 
Step 5 Repeat Step 4 as necessary. Use the no sequence-number command to delete an entry. 

Allows you to revise a prefix list.

 
Step 6 Do one of the following:
  • end
  • commit


Example:
RP/0/RSP0/CPU0:router(config-ipv6_pfx)# end

or

RP/0/RSP0/CPU0:router(config-ipv6_pfx)# commit
 

Saves configuration changes.


  • When you issue the end command, the system prompts you to commit changes:

    		Uncommitted changes found, commit them before exiting(yes/no/cancel)?[cancel]:
    		
    		

    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

 
Step 7 Do one of the following:
  • show prefix-list ipv4 [name] [sequence-number]
  • show prefix-list ipv6 [name] [sequence-number] [summary]


Example:
RP/0/RSP0/CPU0:router# show prefix-list ipv4 pfx_1

or

RP/0/RSP0/CPU0:router# show prefix-list ipv6 pfx_2 summary
 

(Optional) Displays the contents of current IPv4 or IPv6 prefix lists.


  • Use the name argument to display the contents of a specific prefix list.

  • Use the sequence-number argument to specify the sequence number of the prefix-list entry.

  • Use the summary keyword to display summary output of prefix-list contents.

 
Step 8 clear {ipv4 | ipv6} prefix-list name [sequence-number]


Example:
RP/0/RSP0/CPU0:router# clear prefix-list ipv4 pfx_1 30
 

(Optional) Clears the hit count on an IPv4 or IPv6 prefix list.

Note   

The hit count is a value indicating the number of matches to a specific prefix-list entry.

 

Configuring Standard Access Lists

This task configures a standard IPv4 access list.

Standard access lists use source addresses for matching operations.

SUMMARY STEPS

1.    configure

2.    ipv4 access-list name

3.    [ sequence-number ] remark remark

4.    [ sequence-number ] {permit | deny} source [source-wildcard] [log | log-input]

5.    Repeat Step 4 as necessary, adding statements by sequence number where you planned. Use the no sequence-number command to delete an entry.

6.    Do one of the following:

  • end
  • commit

7.    show access-lists [ipv4 | ipv6] [access-list-name hardware {ingress | egress} [interface type interface-path-id] {sequence number | location node-id} | summary [access-list-name] | access-list-name [sequence-number] | maximum [detail] [usage {pfilter location node-id}]]


DETAILED STEPS
  Command or Action Purpose
Step 1 configure


Example:
RP/0/RSP0/CPU0:router# configure
 

Enters global configuration mode.

 
Step 2 ipv4 access-list name


Example:
RP/0/RSP0/CPU0:router# ipv4 access-list acl_1
 

Enters IPv4 access list configuration mode and configures access list acl_1.

 
Step 3 [ sequence-number ] remark remark


Example:
RP/0/RSP0/CPU0:router(config-ipv4-acl)# 10 remark Do not allow user1 to telnet out
 

(Optional) Allows you to comment about the following permit or deny statement in a named access list.


  • The remark can be up to 255 characters; anything longer is truncated.

  • Remarks can be configured before or after permit or deny statements, but their location should be consistent.

 
Step 4 [ sequence-number ] {permit | deny} source [source-wildcard] [log | log-input]


Example:
RP/0/RSP0/CPU0:router(config-ipv4-acl)# 20 permit 172.16.0.0 0.0.255.255

or

RRP/0/RSP0/CPU0:routerrouter(config-ipv4-acl)# 30 deny 192.168.34.0 0.0.0.255
 

Specifies one or more conditions allowed or denied, which determines whether the packet is passed or dropped.


  • Use the source argument to specify the number of network or host from which the packet is being sent.

  • Use the optional source-wildcard argument to specify the wildcard bits to be applied to the source.

  • The optional log keyword causes an information logging message about the packet that matches the entry to be sent to the console.

  • The optional log-input keyword provides the same function as the log keyword, except that the logging message also includes the input interface.

 
Step 5 Repeat Step 4 as necessary, adding statements by sequence number where you planned. Use the no sequence-number command to delete an entry. 

Allows you to revise an access list.

 
Step 6 Do one of the following:
  • end
  • commit


Example:
RP/0/RSP0/CPU0:router(config-ipv4-acl)# end

or

RP/0/RSP0/CPU0:router(config-ipv4-acl)# commit
 

Saves configuration changes.


  • When you issue the end command, the system prompts you to commit changes:

    		Uncommitted changes found, commit them before exiting(yes/no/cancel)?[cancel]:
    		
    		

    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

 
Step 7 show access-lists [ipv4 | ipv6] [access-list-name hardware {ingress | egress} [interface type interface-path-id] {sequence number | location node-id} | summary [access-list-name] | access-list-name [sequence-number] | maximum [detail] [usage {pfilter location node-id}]]


Example:
RP/0/RSP0/CPU0:router# show access-lists ipv4 acl_1
 

(Optional) Displays the contents of the named IPv4 access list.


  • The contents of an IPv4 standard access list are displayed in extended access-list format.

 
What to Do Next

After creating a standard access list, you must apply it to a line or interface. See the Applying Access Lists” section for information about how to apply an access list.

Copying Access Lists

This task copies an IPv4 or IPv6 access list.

SUMMARY STEPS

1.    copy access-list {ipv4 | ipv6}source-acl destination-acl

2.    show access-lists {ipv4 | ipv6}[access-list-name hardware {ingress | egress} [interface type interface-path-id] {sequence number | location node-id} | summary [access-list-name] | access-list-name [sequence-number] | maximum [detail] [usage {pfilter location node-id}]]


DETAILED STEPS
  Command or Action Purpose
Step 1 copy access-list {ipv4 | ipv6}source-acl destination-acl


Example:
RP/0/RSP0/CPU0:router# copy ipv6 access-list list-1 list-2
 

Creates a copy of an existing IPv4 or IPv6 access list.


  • Use the source-acl argument to specify the name of the access list to be copied.

  • Use the destination-acl argument to specify where to copy the contents of the source access list.
    • The destination-acl argument must be a unique name; if the destination-acl argument name exists for an access list, the access list is not copied.

 
Step 2 show access-lists {ipv4 | ipv6}[access-list-name hardware {ingress | egress} [interface type interface-path-id] {sequence number | location node-id} | summary [access-list-name] | access-list-name [sequence-number] | maximum [detail] [usage {pfilter location node-id}]]


Example:
RP/0/RSP0/CPU0:router# show access-lists ipv4 list-2
 

(Optional) Displays the contents of a named IPv4 or IPv6 access list. For example, you can verify the output to see that the destination access list list-2 contains all the information from the source access list list-1.

 

Sequencing Access-List Entries and Revising the Access List

This task shows how to assign sequence numbers to entries in a named access list and how to add or delete an entry to or from an access list. It is assumed that a user wants to revise an access list. Resequencing an access list is optional.

SUMMARY STEPS

1.    resequence access-list {ipv4 | ipv6} name [base [increment]]

2.    configure

3.    {ipv4 | ipv6} access-list name

4.    Do one of the following:

  • [ sequence-number ] {permit | deny} source source-wildcard destination destination-wildcard [precedence precedence] [dscp dscp] [fragments] [packet-length operator packet-length value] [log | log-input]
  • [ sequence-number ] {permit | deny} protocol {source-ipv6-prefix/prefix-length | any | host source-ipv6-address} [operator {port | protocol-port}] {destination-ipv6-prefix/prefix-length | any | host destination-ipv6-address} [operator {port | protocol-port}] [dscp value] [routing] [authen] [destopts] [fragments] [packet-length operator packet-length value] [log | log-input]

5.    Repeat Step 4 as necessary, adding statements by sequence number where you planned. Use the no sequence-number command to delete an entry.

6.    Do one of the following:

  • end
  • commit

7.    show access-lists [ipv4 | ipv6] [access-list-name hardware {ingress | egress} [interface type interface-path-id] {sequence number | location node-id} | summary [access-list-name] | access-list-name [sequence-number] | maximum [detail] [usage {pfilter location node-id}]]


DETAILED STEPS
  Command or Action Purpose
Step 1 resequence access-list {ipv4 | ipv6} name [base [increment]]


Example:
RP/0/RSP0/CPU0:router# resequence access-list ipv4 acl_3 20 15
 

(Optional) Resequences the specified IPv4 or IPv6 access list using the starting sequence number and the increment of sequence numbers.


  • This example resequences an IPv4 access list named acl_3. The starting sequence number is 20 and the increment is 15. If you do not select an increment, the default increment 10 is used.

 
Step 2 configure


Example:
RP/0/RSP0/CPU0:router# configure
 

Enters global configuration mode.

 
Step 3 {ipv4 | ipv6} access-list name


Example:
RP/0/RSP0/CPU0:router(config)# ipv4 access-list acl_1

or

RP/0/RSP0/CPU0:router(config)# ipv6 access-list acl_2
 

Enters either IPv4 or IPv6 access list configuration mode and configures the named access list.

 
Step 4 Do one of the following:
  • [ sequence-number ] {permit | deny} source source-wildcard destination destination-wildcard [precedence precedence] [dscp dscp] [fragments] [packet-length operator packet-length value] [log | log-input]
  • [ sequence-number ] {permit | deny} protocol {source-ipv6-prefix/prefix-length | any | host source-ipv6-address} [operator {port | protocol-port}] {destination-ipv6-prefix/prefix-length | any | host destination-ipv6-address} [operator {port | protocol-port}] [dscp value] [routing] [authen] [destopts] [fragments] [packet-length operator packet-length value] [log | log-input]


Example:
RP/0/RSP0/CPU0:router(config-ipv4-acl)# 10 permit 172.16.0.0 0.0.255.255
RP/0/RSP0/CPU0:router(config-ipv4-acl)# 20 deny 192.168.34.0 0.0.0.255

or

RP/0/RSP0/CPU0:router(config-ipv6-acl)# 20 permit icmp any any
RP/0/RSP0/CPU0:router(config-ipv6-acl)# 30 deny tcp any any gt 5000
 

Specifies one or more conditions allowed or denied in IPv4 access list acl_1.


  • The optional log keyword causes an information logging message about the packet that matches the entry to be sent to the console.

  • The optional log-input keyword provides the same function as the log keyword, except that the logging message also includes the input interface.

  • This access list happens to use a permit statement first, but a deny statement could appear first, depending on the order of statements you need.

or

Specifies one or more conditions allowed or denied in IPv6 access list acl_2.


  • Refer to the permit (IPv6) and deny (IPv6) commands for more information on filtering IPv6 traffic based on IPv6 option headers and upper-layer protocols such as ICMP, TCP, and UDP.

Note   

Every IPv6 access list has an implicit deny ipv6 any any statement as its last match condition. An IPv6 access list must contain at least one entry for the implicit deny ipv6 any any statement to take effect.

 
Step 5 Repeat Step 4 as necessary, adding statements by sequence number where you planned. Use the no sequence-number command to delete an entry. 

Allows you to revise the access list.

 
Step 6 Do one of the following:
  • end
  • commit


Example:
RP/0/RSP0/CPU0:router(config-ipv4-acl)# end

or

RP/0/RSP0/CPU0:router(config-ipv4-acl)# commit
 

Saves configuration changes.


  • When you issue the end command, the system prompts you to commit changes:

    		Uncommitted changes found, commit them before exiting(yes/no/cancel)?[cancel]:
    		
    		

    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

 
Step 7 show access-lists [ipv4 | ipv6] [access-list-name hardware {ingress | egress} [interface type interface-path-id] {sequence number | location node-id} | summary [access-list-name] | access-list-name [sequence-number] | maximum [detail] [usage {pfilter location node-id}]]


Example:
RP/0/RSP0/CPU0:router# show access-lists ipv4 acl_1
 

(Optional) Displays the contents of a named IPv4 or IPv6 access list.


  • Review the output to see that the access list includes the updated information.

 
What to Do Next

If your access list is not already applied to an interface or line or otherwise referenced, apply the access list. See the “Applying Access Lists” section for information about how to apply an access list.

Copying Prefix Lists

This task copies an IPv4 or IPv6 prefix list.

SUMMARY STEPS

1.    copy prefix-list {ipv4 | ipv6} source-name destination-name

2.    Do one of the following:

  • show prefix-list ipv4 [name] [sequence-number]
  • show prefix-list ipv6 [name] [sequence-number] [summary]


DETAILED STEPS
  Command or Action Purpose
Step 1 copy prefix-list {ipv4 | ipv6} source-name destination-name


Example:
RP/0/RSP0/CPU0:router# copy prefix-list ipv6 list_1 list_2
 

Creates a copy of an existing IPv4 or IPv6 prefix list.


  • Use the source-name argument to specify the name of the prefix list to be copied and the destination-name argument to specify where to copy the contents of the source prefix list.

  • The destination-name argument must be a unique name; if the destination-name argument name exists for a prefix list, the prefix list is not copied.

 
Step 2 Do one of the following:
  • show prefix-list ipv4 [name] [sequence-number]
  • show prefix-list ipv6 [name] [sequence-number] [summary]


Example:
RP/0/RSP0/CPU0:router# show prefix-list ipv6 list_2
 

(Optional) Displays the contents of current IPv4 or IPv6 prefix lists.


  • Review the output to see that prefix list list_2 includes the entries from list_1.

 

Sequencing Prefix List Entries and Revising the Prefix List

This task shows how to assign sequence numbers to entries in a named prefix list and how to add or delete an entry to or from a prefix list. It is assumed a user wants to revise a prefix list. Resequencing a prefix list is optional.

Before You Begin

Note


Resequencing IPv6 prefix lists is not supported.



SUMMARY STEPS

1.    resequence prefix-list ipv4 name [base [increment]]

2.    configure

3.    {ipv4 | ipv6} prefix-list name

4.    [ sequence-number ] {permit | deny} network/length [ge value] [le value] [eq value]

5.    Repeat Step 4 as necessary, adding statements by sequence number where you planned. Use the no sequence-number command to delete an entry.

6.    Do one of the following:

  • end
  • commit

7.    Do one of the following:

  • show prefix-list ipv4 [name] [sequence-number]
  • show prefix-list ipv6 [name] [sequence-number] [summary]


DETAILED STEPS
  Command or Action Purpose
Step 1 resequence prefix-list ipv4 name [base [increment]]


Example:
RP/0/RSP0/CPU0:router# resequence prefix-list ipv4 pfx_1 10 15
 

(Optional) Resequences the named IPv4 prefix list using the starting sequence number and the increment of sequence numbers.


  • This example resequences a prefix list named pfx_1. The starting sequence number is 10 and the increment is 15.

 
Step 2 configure


Example:
RP/0/RSP0/CPU0:router# configure
 

Enters global configuration mode.

 
Step 3 {ipv4 | ipv6} prefix-list name


Example:
RP/0/RSP0/CPU0:router(config)# ipv6 prefix-list pfx_2
 

Enters either IPv4 or IPv6 prefix list configuration mode and configures the named prefix list.

 
Step 4 [ sequence-number ] {permit | deny} network/length [ge value] [le value] [eq value]


Example:
RP/0/RSP0/CPU0:router(config-ipv6_pfx)# 15 deny 128.0.0.0/8 eq 24
 

Specifies one or more conditions allowed or denied in the named prefix list.

 
Step 5 Repeat Step 4 as necessary, adding statements by sequence number where you planned. Use the no sequence-number command to delete an entry. 

Allows you to revise the prefix list.

 
Step 6 Do one of the following:
  • end
  • commit


Example:
RP/0/RSP0/CPU0:router(config-ipv6_pfx)# end

or

RP/0/RSP0/CPU0:router(config-ipv6_pfx)# commit
 

Saves configuration changes.


  • When you issue the end command, the system prompts you to commit changes:

    		Uncommitted changes found, commit them before exiting(yes/no/cancel)?[cancel]:
    		
    		

    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

 
Step 7 Do one of the following:
  • show prefix-list ipv4 [name] [sequence-number]
  • show prefix-list ipv6 [name] [sequence-number] [summary]


Example:
RP/0/RSP0/CPU0:router# show prefix-list ipv6 pfx_2
 

(Optional) Displays the contents of current IPv4 or IPv6 prefix lists.


  • Review the output to see that prefix list pfx_2 includes all new information.

 

How to Implement ACL-based Forwarding

This section contains the following procedures:

Configuring ACL-based Forwarding with Security ACL

Perform this task to configure ACL-based forwarding with security ACL.

SUMMARY STEPS

1.    configure

2.    ipv4 access-list name

3.    [ sequence-number ] permit protocol source source-wildcard destination destination-wildcard [precedence precedence] [default nexthop [ipv4-address1] [ipv4-address2] [ipv4-address3]] [dscp dscp] [fragments] [packet-length operator packet-length value] [log | log-input] [nexthop[track track-name] [ipv4-address1] [ipv4-address2] [ipv4-address3]] [ttl ttl [value1 ... value2]]

4.    Do one of the following:

  • end
  • commit

5.    show access-list ipv4 [[access-list-name hardware {ingress | egress} [interface type interface-path-id] {sequence number | location node-id} | summary [access-list-name] | access-list-name [sequence-number] | maximum [detail] [usage {pfilter location node-id}]]


DETAILED STEPS
  Command or Action Purpose
Step 1 configure


Example:
RP/0/RSP0/CPU0:router# configure
 

Enters global configuration mode.

 
Step 2 ipv4 access-list name


Example:
RP/0/RSP0/CPU0:router(config)# ipv4 access-list security-abf-acl
 

Enters IPv4 access list configuration mode and configures the specified access list.

 
Step 3 [ sequence-number ] permit protocol source source-wildcard destination destination-wildcard [precedence precedence] [default nexthop [ipv4-address1] [ipv4-address2] [ipv4-address3]] [dscp dscp] [fragments] [packet-length operator packet-length value] [log | log-input] [nexthop[track track-name] [ipv4-address1] [ipv4-address2] [ipv4-address3]] [ttl ttl [value1 ... value2]]


Example:
RP/0/RSP0/CPU0:router(config-ipv4-acl)# 10 permit ipv4 10.0.0.0 0.255.255.255 any nexthop 50.1.1.2
RP/0/RSP0/CPU0:router(config-ipv4-acl)# 15 permit ipv4 30.2.1.0 0.0.0.255 any
RP/0/RSP0/CPU0:router(config-ipv4-acl)# 20 permit ipv4 30.2.0.0 0.0.255.255 any nexthop 40.1.1.2
RP/0/RSP0/CPU0:router(config-ipv4-acl)# 25 permit ipv4 any any
 

Sets the conditions for an IPv4 access list. The configuration example shows how to configure ACL-based forwarding with security ACL.


  • The nexthop keyword forwards the specified next hop for this entry.

 
Step 4 Do one of the following:
  • end
  • commit


Example:
RP/0/RSP0/CPU0:router(config-ipv4-acl)# end

or

RP/0/RSP0/CPU0:router(config-ipv4-acl)# commit
 

Saves configuration changes.


  • When you issue the end command, the system prompts you to commit changes:

    		Uncommitted changes found, commit them before exiting(yes/no/cancel)?[cancel]:
    		
    		

    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

 
Step 5 show access-list ipv4 [[access-list-name hardware {ingress | egress} [interface type interface-path-id] {sequence number | location node-id} | summary [access-list-name] | access-list-name [sequence-number] | maximum [detail] [usage {pfilter location node-id}]]


Example:
RP/0/RSP0/CPU0:router# show access-lists ipv4 security-abf-acl
 

Displays the information for ACL software.

 

Enabling track mode

SUMMARY STEPS

1.    configure

2.    track track-name

3.    Use one of the following commands:

  • end
  • commit


DETAILED STEPS
  Command or Action Purpose
Step 1 configure


Example:
RP/0/RSP0/CPU0:router# configure
 

Enters global configuration mode.

 
Step 2 track track-name


Example:
RP/0/RSP0/CPU0:router(config)# track t1
 

Enters track configuration mode.

 
Step 3 Use one of the following commands:
  • end
  • commit


Example:
RP/0/RSP0/CPU0:router(config)# end

or

RP/0/RSP0/CPU0:router(config)# commit
 

Saves configuration changes.


  • When you issue the end command, the system prompts you to commit changes:

    Uncommitted changes found, commit them
    before exiting(yes/no/cancel)? [cancel]:
    

    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

 

Configuring track type

There are different mechanisms to track the availability of the next-hop device. The tracking type can be of four types, using:


  • line protocol

  • list

  • route

  • IPSLA

Configuring tracking type (line protocol)

Line protocol is one of the object types the object tracker component can track. This object type provides an option for tracking state change notification from an interface. Based on the interface state change notification, it decides whether the track state should be UP or DOWN.

SUMMARY STEPS

1.    configure

2.    track track-name

3.    type line-protocol state interface type interface-path-id

4.    Use one of the following commands:

  • end
  • commit


DETAILED STEPS
  Command or Action Purpose
Step 1 configure


Example:
RP/0/RSP0/CPU0:router# configure
 

Enters global configuration mode.

 
Step 2 track track-name


Example:
RP/0/RSP0/CPU0:router(config)# track t1
 

Enters track configuration mode.

 
Step 3 type line-protocol state interface type interface-path-id


Example:
RP/0/RSP0/CPU0:router(config-track)# type line-protocol state interface tengige 0/4/4/0
 

Sets the interface which needs to be tracked for state change notifications.

 
Step 4 Use one of the following commands:
  • end
  • commit


Example:
RP/0/RSP0/CPU0:router(config)# end

or

RP/0/RSP0/CPU0:router(config)# commit
 

Saves configuration changes.


  • When you issue the end command, the system prompts you to commit changes:

    Uncommitted changes found, commit them
    before exiting(yes/no/cancel)? [cancel]:
    

    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

 

Configuring track type (list)

List is a boolen object type. Boolean refers to the capability of performing a boolean AND or boolean OR operation on combinations of different object types supported by object tracker.

SUMMARY STEPS

1.    configure

2.    track track-name

3.    type list boolean and

4.    Use one of the following commands:

  • end
  • commit


DETAILED STEPS
  Command or Action Purpose
Step 1 configure


Example:
RP/0/RSP0/CPU0:router# configure
 

Enters global configuration mode.

 
Step 2 track track-name


Example:
RP/0/RSP0/CPU0:router(config)# track t1
 

Enters track configuration mode.

 
Step 3 type list boolean and


Example:
RP/0/RSP0/CPU0:router(config-track)# type list boolean and
 

Sets the list of track objects on which boolean AND or boolean OR operations could be performed.

 
Step 4 Use one of the following commands:
  • end
  • commit


Example:
RP/0/RSP0/CPU0:router(config)# end

or

RP/0/RSP0/CPU0:router(config)# commit
 

Saves configuration changes.


  • When you issue the end command, the system prompts you to commit changes:

    Uncommitted changes found, commit them
    before exiting(yes/no/cancel)? [cancel]:
    

    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

 

Configuring tracking type (route)

Route is a route object type. The object tracker tracks the fib notification to determine the route reachability and the track state.

SUMMARY STEPS

1.    configure

2.    track track-name

3.    type route reachability

4.    Use one of the following commands:

  • end
  • commit


DETAILED STEPS
  Command or Action Purpose
Step 1 configure


Example:
RP/0/RSP0/CPU0:router# configure
 

Enters global configuration mode.

 
Step 2 track track-name


Example:
RP/0/RSP0/CPU0:router(config)# track t1
 

Enters track configuration mode.

 
Step 3 type route reachability


Example:
RP/0/RSP0/CPU0:router(config-track)# type route reachability
 

Sets the route on which reachability state needs to be learnt dynamically.

 
Step 4 Use one of the following commands:
  • end
  • commit


Example:
RP/0/RSP0/CPU0:router(config)# end

or

RP/0/RSP0/CPU0:router(config)# commit
 

Saves configuration changes.


  • When you issue the end command, the system prompts you to commit changes:

    Uncommitted changes found, commit them
    before exiting(yes/no/cancel)? [cancel]:
    

    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

 

Configuring tracking type (rtr)

IPSLA is an ipsla object type. The object tracker tracks the return code of ipsla operation to determine the track state changes.

SUMMARY STEPS

1.    configure

2.    track track-name

3.    type rtr ipsla operation id reachability

4.    Use one of the following commands:

  • end
  • commit


DETAILED STEPS
  Command or Action Purpose
Step 1 configure


Example:
RP/0/RSP0/CPU0:router# configure
 

Enters global configuration mode.

 
Step 2 track track-name


Example:
RP/0/RSP0/CPU0:router(config)# track t1
 

Enters track configuration mode.

 
Step 3 type rtr ipsla operation id reachability


Example:
RP/0/RSP0/CPU0:routertype rtr 100 reachability
 

Sets the ipsla operation id which needs to be tracked for reachability.

 
Step 4 Use one of the following commands:
  • end
  • commit


Example:
RP/0/RSP0/CPU0:router(config)# end

or

RP/0/RSP0/CPU0:router(config)# commit
 

Saves configuration changes.


  • When you issue the end command, the system prompts you to commit changes:

    Uncommitted changes found, commit them
    before exiting(yes/no/cancel)? [cancel]:
    

    • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

    • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

    • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

  • Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

 

Configuration Examples for Implementing Access Lists and Prefix Lists

This section provides the following configuration examples:

Resequencing Entries in an Access List: Example

The following example shows access-list resequencing. The starting value in the resequenced access list is 10, and increment value is  20. The subsequent entries are ordered based on the increment values that users provide, and the range is from 1 to 2147483646.

When an entry with no sequence number is entered, by default it has a sequence number of 10 more than the last entry in the access list.

ipv4 access-list acl_1
10 permit ip host 10.3.3.3 host 172.16.5.34
20 permit icmp any any
30 permit tcp any host 10.3.3.3
40 permit ip host 10.4.4.4 any
60 permit ip host 172.16.2.2 host 10.3.3.12
70 permit ip host 10.3.3.3 any log
80 permit tcp host 10.3.3.3 host 10.1.2.2
100 permit ip any any

configure
	ipv4 access-list acl_1
	end
resequence ipv4 access-list acl_1 10 20 

ipv4 access-list acl_1
10 permit ip host 10.3.3.3 host 172.16.5.34
30 permit icmp any any
50 permit tcp any host 10.3.3.3
70 permit ip host 10.4.4.4 any
90 permit ip host 172.16.2.2 host 10.3.3.12
110 permit ip host 10.3.3.3 any log
130 permit tcp host 10.3.3.3 host 10.1.2.2
150 permit ip any any 
ipv4 access-list acl_1
10 permit ip host 10.3.3.3 host 172.16.5.34
20 permit icmp any any
30 permit tcp any host 10.3.3.3
40 permit ip host 10.4.4.4 any
60 permit ip host 172.16.2.2 host 10.3.3.12
70 permit ip host 10.3.3.3 any log
80 permit tcp host 10.3.3.3 host 10.1.2.2
100 permit ip any any

configure
ipv6 access-list acl_1
end
resequence ipv6 access-list acl_1 10 20 

ipv4 access-list acl_1
10 permit ip host 10.3.3.3 host 172.16.5.34
30 permit icmp any any
50 permit tcp any host 10.3.3.3
70 permit ip host 10.4.4.4 any
90 Dynamic test permit ip any any
110 permit ip host 172.16.2.2 host 10.3.3.12
130 permit ip host 10.3.3.3 any log
150 permit tcp host 10.3.3.3 host 10.1.2.2
170 permit ip host 10.3.3.3 any
190 permit ip any any

Adding Entries with Sequence Numbers: Example

In the following example, an new entry is added to IPv4 access list acl_5.

ipv4 access-list acl_5
 2 permit ipv4 host 10.4.4.2 any
 5 permit ipv4 host 10.0.0.44 any
 10 permit ipv4 host 10.0.0.1 any
 20 permit ipv4 host 10.0.0.2 any
configure
ipv4 access-list acl_5
 15 permit 10.5.5.5 0.0.0.255
end
ipv4 access-list acl_5
 2 permit ipv4 host 10.4.4.2 any
 5 permit ipv4 host 10.0.0.44 any
 10 permit ipv4 host 10.0.0.1 any
 15 permit ipv4 10.5.5.5 0.0.0.255 any
 20 permit ipv4 host 10.0.0.2 any

Adding Entries Without Sequence Numbers: Example

The following example shows how an entry with no specified sequence number is added to the end of an access list. When an entry is added without a sequence number, it is automatically given a sequence number that puts it at the end of the access list. Because the default increment is 10, the entry will have a sequence number 10 higher than the last entry in the existing access list.

configure
ipv4 access-list acl_10
permit 10
.1.1.1 0.0.0.255
permit 10
.2.2.2 0.0.0.255
permit 10
.3.3.3 0.0.0.255
end

ipv4 access-list acl_10
 10 permit ip 10
.1.1.0 0.0.0.255 any
 20 permit ip 10
.2.2.0 0.0.0.255 any
 30 permit ip 10
.3.3.0 0.0.0.255 any

configure
ipv4 access-list acl_10
permit 10
.4.4.4 0.0.0.255
end

ipv4 access-list acl_10
 10 permit ip 10
.1.1.0 0.0.0.255 any
 20 permit ip 10
.2.2.0 0.0.0.255 any
 30 permit ip 10
.3.3.0 0.0.0.255 any
 40 permit ip 10
.4.4.0 0.0.0.255 any

Additional References

The following sections provide references related to implementing access lists and prefix lists.

Related Documents

Related Topic

Document Title

Access list commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples

Access List Commands module in Cisco ASR 9000 Series Aggregation Services Router IP Addresses and Services Command Reference

Prefix list commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples

Prefix List Commands module in Cisco ASR 9000 Series Aggregation Services Router IP Addresses and Services Command Reference

Terminal services commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples

Terminal Services Commands module in Cisco ASR 9000 Series Aggregation Services Router System Management Command Reference

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