Table Of Contents
Enable ICMP Protocol Unreachable Messages
Enable ICMP Mask Reply Messages
Configure Simplex Ethernet Interfaces
Limit the Source of DRP Queries
Configure Authentication of DRP Queries and Responses
Create Standard and Extended Access Lists Using Numbers
Create Standard and Extended Access Lists Using Names
Specify IP Extended Access Lists with Fragment Control
Benefits of Fragment Control in an IP Extended Access List
Apply the Access List to an Interface or Terminal Line
Configure the Hot Standby Router Protocol
Enable Hot Standby Router Protocol
Configure Hot Standby Router Protocol Group Attributes
Change the Hot Standby Router Protocol MAC Refresh Interval
Configure IP Precedence Accounting
Configure Performance Parameters
Set the TCP Connection Attempt Time
Enable TCP Selective Acknowledgment
Set the TCP Outgoing Queue Size
Monitor and Maintain the IP Network
Clear Caches, Tables, and Databases
Monitor and Maintain the DRP Server Agent
Clear the Access List Counters
Display System and Network Statistics
IP Services Configuration Examples
Simplex Ethernet Interfaces Example
Implicit Masks in Access Lists Examples
IP Extended Access List with Fragment Control Example
Hot Standby Router Protocol MAC Refresh Interval Examples
No Switch or Learning Bridge Present Example
Switch or Learning Bridge Present Example
Configuring IP Services
This chapter describes how to configure optional IP services. For a complete description of the IP services commands in this chapter, refer to the "IP Services Commands" chapter of the Network Protocols Command Reference, Part 1. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.
IP Services Task List
To configure optional IP services, complete any of the tasks in the following sections:
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Configure the Hot Standby Router Protocol
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Remember that not all the tasks in these sections are required. The tasks you must perform will depend on your network and your needs.
At the end of this chapter, the examples in the "IP Services Configuration Examples" section illustrate how you might configure your network using IP.
Manage IP Connections
The IP suite offers a number of services that control and manage IP connections. ICMP provides many of these services. ICMP messages are sent by routers or access servers to hosts or other routers when a problem is discovered with the Internet header. For detailed information on ICMP, see RFC 792.
To manage various aspects of IP connections, perform the appropriate tasks in the following sections:
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See the "ICMP Services Example" section at the end of this chapter for examples of ICMP services.
Enable ICMP Protocol Unreachable Messages
If the Cisco IOS software receives a nonbroadcast packet destined for itself that uses an unknown protocol, it sends an ICMP Protocol Unreachable message back to the source. Similarly, if the software receives a packet that it is unable to deliver to the ultimate destination because it knows of no route to the destination address, it sends an ICMP Host Unreachable message to the source. This feature is enabled by default.
You can enable this service if it has been disabled by using the following command in interface configuration mode:
ip unreachables
Enable the sending of ICMP Protocol Unreachable and Host Unreachable messages.
To limit the rate that ICMP destination unreachable messages are generated, use the following command in global configuration mode:
ip icmp rate-limit unreachable [df] milliseconds
Limit the rate that ICMP destination unreachable messages are generated.
Enable ICMP Redirect Messages
Routes are sometimes less than optimal. For example, it is possible for the router to be forced to resend a packet through the same interface on which it was received. If this happens, the Cisco IOS software sends an ICMP Redirect message to the packet's originator telling it that it is on a subnet directly connected to the receiving device, and that it must forward the packet to another system on the same subnet. The software does this because the originating host presumably could have sent that packet to the next hop without involving this device at all. The Redirect message instructs the sender to remove the receiving device from the route and substitute a specified device representing a more direct path. This feature is enabled by default. However, when Hot Standby Router Protocol is configured on an interface, ICMP Redirect messages are disabled by default for the interface.
You can enable the sending of ICMP Redirect messages if this feature was disabled by performing the following task in interface configuration mode:
Enable ICMP Mask Reply Messages
Occasionally, network devices must know the subnet mask for a particular subnetwork in the internetwork. To achieve this information, such devices can send ICMP Mask Request messages. These messages are responded to by ICMP Mask Reply messages from devices that have the requested information. The Cisco IOS software can respond to ICMP Mask Request messages if this function is enabled.
To enable the sending of ICMP Mask Reply messages, use the following command in interface configuration mode:
Understand Path MTU Discovery
The Cisco IOS software supports the IP Path MTU Discovery mechanism, as defined in RFC 1191. IP Path MTU Discovery allows a host to dynamically discover and cope with differences in the maximum allowable maximum transmission unit (MTU) size of the various links along the path. Sometimes a router is unable to forward a datagram because it requires fragmentation (the packet is larger than the MTU you set for the interface with the ip mtu command), but the "don't fragment" (DF) bit is set. The Cisco IOS software sends a message to the sending host, alerting it to the problem. The host will have to fragment packets for the destination so that they fit the smallest packet size of all the links along the path. This technique is shown in Figure 15.
Figure 15 IP Path MTU Discovery
IP Path MTU Discovery is useful when a link in a network goes down, forcing the use of another, different MTU-sized link (and different routers). As shown in , suppose a router is sending IP packets over a network where the MTU in the first router is set to 1,500 bytes, but the second router is set to 512 bytes. If the datagram's "Don't fragment" bit is set, the datagram would be dropped because the 512-byte router is unable to forward it. All packets larger than 512 bytes are dropped in this case. The second router returns an ICMP Destination Unreachable message to the source of the datagram with its Code field indicating, "Fragmentation needed and DF set." To support IP Path MTU Discovery, it would also include the MTU of the next-hop network link in the low-order bits of an unused header field.
IP Path MTU Discovery is also useful when a connection is first being established and the sender has no information at all about the intervening links. It is always advisable to use the largest MTU that the links will bear; the larger the MTU, the fewer packets the host must send.
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The Cisco 7000 and Cisco 4000 routers support fast switching of IP packets between Ethernet and FDDI interfaces. When packets are being sent from FDDI to Ethernet interfaces and you are not using IP Path MTU Discovery, FDDI packets with data lengths larger than 1500 bytes will be fragmented into multiple Ethernet packets. This will slow performance. If the majority of your traffic travels off the FDDI ring, you might want to either lower the MTU size on your host FDDI interfaces to 1,500 bytes or run IP Path MTU Discovery on your hosts.
Because the CTR card does not support the switching of frames larger than 4,472 bytes, some interoperability problems may occur if CTR cards are intermixed with other Token Ring cards on the same network. You can minimize this by setting lower (and the same) IP maximum packet sizes for all devices on the network with the ip mtu interface command.
To enable Path MTU Discovery for connections initiated by the router (when the router is acting as a host), see the section "Enable TCP Path MTU Discovery" later in this chapter.
Set the MTU Packet Size
All interfaces have a default MTU packet size. You can adjust the IP MTU size so that if an IP packet exceeds the MTU set for an interface, the Cisco IOS software will fragment it.
Changing the MTU value (with the mtu interface configuration command) can affect the IP MTU value. If the current IP MTU value is the same as the MTU value, and you change the MTU value, the IP MTU value will be modified automatically to match the new MTU. However, the reverse is not true; changing the IP MTU value has no effect on the value for the mtu interface configuration command.
Also, all devices on a physical medium must have the same protocol MTU in order to operate.
To set the MTU packet size for a specified interface, use the following command in interface configuration mode:
Enable IP Source Routing
The Cisco IOS software examines IP header options on every packet. It supports the IP header options Strict Source Route, Loose Source Route, Record Route, and Time Stamp, which are defined in RFC 791. If the software finds a packet with one of these options enabled, it performs the appropriate action. If it finds a packet with an invalid option, it sends an ICMP Parameter Problem message to the source of the packet and discards the packet.
IP provides a provision that allows the source IP host to specify a route through the IP network. This provision is known as source routing. Source routing is specified as an option in the IP header. If source routing is specified, the software forwards the packet according to the specified source route. This feature is employed when you want to force a packet to take a certain route through the network. The default is to perform source routing.
You can enable IP source-route header options if they have been disabled by using the following command in global configuration mode:
Configure Simplex Ethernet Interfaces
You can configure simplex Ethernet interfaces. This feature is useful for setting up dynamic IP routing over a simplex circuit (a circuit that receives only or transmits only). When a route is learned on a receive-only interface, the interface designated as the source of the route is converted to the interface you specify. When packets are routed out this specified interface, they are sent to the IP address of the source of the routing update. To reach this IP address on a transmit-only Ethernet link, a static ARP entry mapping this IP address to the hardware address of the other end of the link is required.
To assign a transmit interface to a receive-only interface, use the following command in interface configuration mode:
transmit-interface type number
Assign a transmit interface to a receive-only interface.
See the "Simplex Ethernet Interfaces Example" section at the end of this chapter for an example of configuring a simplex Ethernet interface.
Configure a DRP Server Agent
The Director Response Protocol (DRP) is a simple User Datagram Protocol (UDP)-based application developed by Cisco Systems. It enables Cisco's DistributedDirector product to query routers (DRP Server Agents) in the field for Border Gateway Protocol (BGP) and Interior Gateway Protocol (IGP) routing table metrics between distributed servers and clients. DistributedDirector, a separate standalone product, uses DRP to transparently redirect end-user service requests to the topologically closest responsive server. DRP enables DistributedDirector to provide dynamic, scalable, and "network intelligent" Internet traffic load distribution between multiple geographically dispersed servers.
DRP Server Agents are border routers (or peers to border routers) that support the geographically distributed servers for which DistributedDirector service distribution is desired. Note that, because DistributedDirector makes decisions based on BGP and IGP information, all DRP Server Agents must have access to full BGP and IGP routing tables.
Refer to the Cisco DistributedDirector 2501 Installation and Configuration Guide or the Cisco DistributedDirector 4700-M Installation and Configuration Guide for information on how to configure DistributedDirector.
Perform the tasks in the following sections to configure and maintain the DRP Server Agent. The first task is required; the remaining tasks are optional.
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To monitor and maintain the DRP Server Agent, see the section "Monitor and Maintain the DRP Server Agent" later in this chapter.
For an example of configuring a DRP Server Agent, see the section "DRP Server Agent Example" at the end of this chapter.
Enable the DRP Server Agent
The DRP Server Agent is disabled by default. To enable it, use the following command in global configuration mode:
Limit the Source of DRP Queries
As a security measure, you can limit the source of valid DRP queries. If a standard IP access list is applied to the interface, the Server Agent will respond only to DRP queries originating from an IP address in the list. If no access list is configured, the server agent will answer all queries.
If both an access group and a key chain (described in the next section) have been configured, both security mechanisms must allow access before a request is processed.
To limit the source of valid DRP queries, use the following command in global configuration mode:
ip drp access-group access-list-number
Control the sources of valid DRP queries by applying a standard IP access list.
Configure Authentication of DRP Queries and Responses
Another available security measure is to configure the DRP Server Agent to authenticate DRP queries and responses. You define a key chain, identify the keys that belong to the key chain, and specify how long each key is valid. To do so, use the following commands beginning in global configuration mode:
When configuring your key chains and keys, keep these points in mind:
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Filter IP Packets
Packet filtering helps control packet movement through the network. Such control can help limit network traffic and restrict network use by certain users or devices. To permit or deny packets from crossing specified interfaces, we provide access lists.
You can use access lists in the following ways:
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This section summarizes how to create IP access lists and how to apply them.
See the "IP Services Configuration Examples" section at the end of this chapter for examples of configuring IP access lists.
An access list is a sequential collection of permit and deny conditions that apply to IP addresses. The Cisco IOS software tests addresses against the conditions in an access list one by one. The first match determines whether the software accepts or rejects the address. Because the software stops testing conditions after the first match, the order of the conditions is critical. If no conditions match, the software rejects the address.
The two main tasks involved in using access lists are as follows:
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These and other tasks are described in this section and are labeled as required or optional. Either the first or second task is required, depending on whether you identify your access list with a number or a name.
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Create Standard and Extended Access Lists Using Numbers
CautionRelease 11.1 and later releases introduced substantial changes to IP access lists. These extensions are backward compatible; migrating from a release earlier than Release 11.1 to the current image will convert your access lists automatically. However, previous releases are not upwardly compatible with these changes. Thus, if you save an access list with the current image and then use older software, the resulting access list will not be interpreted correctly. This could cause you severe security problems. Save your old configuration file before booting Release 11.1-or-later images.
The software supports the following styles of access lists for IP:
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To create a standard access list, use one of the following commands in global configuration mode:
The Cisco IOS software can now provide logging messages about packets permitted or denied by a standard IP access list. That is, any packet that matches the access list will cause 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. This capability was previously only available in extended IP access lists.
The first packet that triggers the access list causes a logging message right away, 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.
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For an example of a standard IP access list using logs, see the example "Numbered Access List Examples" at the end of this chapter.
To create an extended access list, use one of the following commands in global configuration mode:
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After an access list is created initially, any subsequent additions (possibly entered from the terminal) are placed at the end of the list. In other words, you cannot selectively add or remove access list command lines from a specific access list.
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After creating an access list, you must apply it to a line or interface, as shown in the section "Apply the Access List to an Interface or Terminal Line" later in this chapter.
See the "Implicit Masks in Access Lists Examples" section at the end of this chapter for examples of implicit masks.
Create Standard and Extended Access Lists Using Names
CautionNamed access lists will not be recognized by any software release prior to Cisco IOS Release 11.2.
You can identify IP access lists with an alphanumeric string (a name) rather than a number. Named access lists allow you to configure more IP access lists in a router than if you were to use numbered access lists. If you identify your access list with a name rather than a number, the mode and command syntax are slightly different. Currently, only packet and route filters can use a named list.
Consider the following before configuring named access lists:
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To create a standard access list, use the following commands beginning in global configuration mode:
To create an extended access list, use the following commands beginning in global configuration mode:
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After you initially create an access list, you place any subsequent additions (possibly entered from the terminal) at the end of the list. In other words, you cannot selectively add access list command lines to a specific access list. However, you can use no permit and no deny commands to remove entries from a named access list.
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After creating an access list, you must apply it to a line or interface, as shown in the following section, "Apply the Access List to an Interface or Terminal Line."
See the "Named Access List Example" section at the end of this chapter for an example of a named access list.
Specify IP Extended Access Lists with Fragment Control
This section describes the functionality added to IP extended named and numbered access lists. You can now specify whether the system examines noninitial IP fragments of packets when applying an IP extended access list.
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.
Because noninitial fragments contain only Layer 3 information, access-list entries containing only Layer 3 information can and now are applied to noninitial fragments. The fragment has all the information the system needs to filter, so the entry is applied to the fragments.
This feature adds the optional fragments keyword to four IP access list commands [access-list (IP extended), deny (IP), dynamic, and permit (IP)]. 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:
Be aware that you should not simply 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. 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, and so on, 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 needs to be added. Thus all the fragments of a packet are handled in the same manner by the access list.
The fragments keyword can be applied to dynamic access lists also.
Packet fragments of IP datagrams are considered individual packets and each counts individually as a packet in access list accounting and access list violation counts.
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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 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.
Benefits of Fragment Control in an IP Extended Access List
If the fragments keyword is used in additional IP access list entries that deny fragments, the fragment control feature provides the following benefits:
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You are able to block more of the traffic you intended to block, not just the initial fragment of such packets. The unwanted fragments no longer linger at the receiver until the reassembly timeout is reached because they are blocked before being sent to the receiver. Blocking a greater portion of unwanted traffic improves security and reduces the risk from potential hackers.
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By blocking unwanted noninitial fragments of packets, you are not paying for traffic you intended to block.
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By blocking unwanted noninitial fragments of packets from ever reaching the receiver, that destination does not have to store the fragments until the reassembly timeout period is reached.
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The noninitial fragments will be handled in the same way as the initial fragment, which is what you would expect. There are fewer unexpected policy routing results and fewer fragment of packets being routed when they should not be.
For an example of fragment control in an IP extended access list, see the IP Extended Access List with Fragment Control Example.
Apply the Access List to an Interface or Terminal Line
After you create an access list, you can apply it to one or more interfaces. Access lists can be applied on either outbound or inbound interfaces. The following two tables show how to accomplish this task for both terminal lines and network interfaces. Remember the following:
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Use the following command in line configuration mode. Only numbered access lists can be applied to lines. Set identical restrictions on all the virtual terminal lines, because a user can attempt to connect to any of them.
access-class access-list-number {in | out}
Restrict incoming and outgoing connections between a particular virtual terminal line (into a device) and the addresses in an access list.
Use the following command in interface configuration mode:
ip access-group {access-list-number | name} {in | out}
Control access to an interface.
For inbound access lists, after receiving a packet, the Cisco IOS 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.
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 transmits 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 will act as if the access list has not been applied to the interface and will accept all packets. Remember this behavior if you use undefined access lists as a means of security in your network.
Configure the Hot Standby Router Protocol
The Hot Standby Router Protocol provides high network availability because it routes IP traffic from hosts on Ethernet, FDDI, or Token Ring networks without relying on the availability of any single router.
This feature is useful for hosts that do not support a router discovery protocol (such as IRDP) and do not have the functionality to switch to a new router when their selected router reloads or loses power. Because existing TCP sessions can survive the failover, this protocol also provides a more transparent means of recovery for hosts that dynamically select a next hop for routing IP traffic.
When the Hot Standby Router Protocol is configured on a network segment, it provides a virtual MAC address and an IP address that is shared among routers in a group of routers that is running the Hot Standby Router Protocol. One of these devices is selected by the protocol to be the active router. The active router receives and routes packets destined for the group's MAC address. For n routers running the Hot Standby Router Protocol, there are n + 1 IP and MAC addresses assigned.
The Hot Standby Router Protocol detects when the designated active router fails, at which point a selected standby router assumes control of the Hot Standby group's MAC and IP addresses. A new standby router is also selected at that time.
Devices that are running the Hot Standby Router Protocol send and receive multicast UDP-based hello packets to detect router failure and to designate active and standby routers.
When the Hot Standby Router Protocol is configured on an interface, ICMP Redirect messages are disabled by default for the interface.
You can configure multiple Hot Standby groups on an interface, thereby making fuller use of the redundant routers and load sharing. To do so, specify a group number for each Hot Standby command you configure for the interface.
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The Hot Standby Router Protocol is supported over Inter-Switch Link (ISL) encapsulation. Refer to the "Configuring Routing between VLANs with ISL Encapsulation" chapter in the Cisco IOS Switching Services Configuration Guide.
Enable Hot Standby Router Protocol
To enable the Hot Standby Router Protocol on an interface, use the following command in interface configuration mode:
standby [group-number] ip [ip-address [secondary]]
Enable the Hot Standby Router Protocol.
Configure Hot Standby Router Protocol Group Attributes
To configure other Hot Standby group attributes that affect how the local router participates in the Hot Standby Router Protocol, perform one or more of the following tasks in interface configuration mode:
Change the Hot Standby Router Protocol MAC Refresh Interval
When Hot Standby Router Protocol (HSRP) runs over FDDI, you can change the interval at which a packet is sent to refresh the MAC cache on learning bridges or switches. HSRP hello packets use the burned-in address (BIA) instead of the MAC virtual address. Refresh packets keep the switch's or learning bridge's MAC cache current.
You can change the refresh interval on FDDI rings to a longer or shorter interval, thereby using bandwidth more efficiently. You can prevent the sending of any MAC refresh packets if you don't need them (if you have FDDI but do not have a learning bridge or switch).
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This feature is supported on these platforms:
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By default, a packet is sent every 10 seconds to refresh the MAC cache on learning bridges or switches. To change the interval, use the following command in interface configuration mode:
For examples of this feature, see the section "Hot Standby Router Protocol MAC Refresh Interval Examples" at the end of this chapter.
Configure IP Accounting
Our IP accounting support provides basic IP accounting functions. By enabling IP accounting, users can see the number of bytes and packets switched through the Cisco IOS software on a source and destination IP address basis. Only transit IP traffic is measured and only on an outbound basis; traffic generated by the software or terminating in the software is not included in the accounting statistics. To maintain accurate accounting totals, the software maintains two accounting databases: an active and a checkpointed database.
Our IP accounting support also provides information identifying IP traffic that fails IP access lists. Identifying IP source addresses that violate IP access lists alerts you to possible attempts to breach security. The data also indicates that you should verify IP access list configurations. To make this feature available to users, you must enable IP accounting of access list violations using the ip accounting access-violations command. Users can then display the number of bytes and packets from a single source that attempted to breach security against the access list for the source destination pair. By default, IP accounting displays the number of packets that have passed access lists and were routed.
To enable IP accounting, use one of the following commands for each interface in interface configuration mode:
ip accounting
Enable basic IP accounting.
ip accounting access-violations
Enable IP accounting with the ability to identify IP traffic that fails IP access lists.
To configure other IP accounting functions, use one or more of the following commands in global configuration mode:
To display IP access violations for a specific IP accounting database, use the following command in EXEC mode:
show ip accounting [checkpoint] access-violations
Display IP access-violation information.
To display IP access violations, you must give the access-violations keyword on the command. If you do not specify the keyword, the command defaults to displaying the number of packets that have passed access lists and were routed. The access violations output displays the number of the access list failed by the last packet for the source and destination pair. The number of packets reveals how aggressive the attack is upon a specific destination.
Use the EXEC command show ip accounting to display the active accounting database, and traffic coming from a remote site and transiting through a router. To display the checkpointed database, use the show ip accounting checkpoint EXEC command. The clear ip accounting EXEC command clears the active database and creates the checkpointed database.
Configure IP MAC Accounting
The MAC address accounting functionality provides accounting information for IP traffic based on the source and destination MAC addresses on LAN interfaces. MAC accounting calculates the total packet and byte counts for a LAN interface that receives or sends IP packets to or from a unique MAC address. It also records a timestamp for the last packet received or sent. For example, with IP MAC accounting, you can determine how much traffic is being sent to and/or received from various peers at NAPS/peering points. IP MAC accounting is supported on Ethernet, FastEthernet, and FDDI interfaces and supports Cisco Express Forwarding (CEF), distributed CEF (dCEF), flow, and optimum switching.
To configure the interface for IP accounting based on the MAC address, perform the following steps beginning in global configuration:
To remove IP accounting based on the MAC address from the interface, use the no ip accounting mac-address command.
Use the EXEC command show interface mac to display MAC accounting information for interfaces configured for MAC accounting.
Configure IP Precedence Accounting
The precedence accounting feature provides accounting information for IP traffic based on the precedence on any interface. This feature calculates the total packet and byte counts for an interface that receives or sends IP packets and sorts the results based on IP precedence. This feature is supported on all interfaces and subinterfaces and supports CEF, dCEF, flow, and optimum switching.
To configure the interface for IP accounting based on IP precedence, perform the following steps beginning in global configuration mode
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To remove IP accounting based on IP precedence from the interface, use the no ip accounting precedence command.
Use the EXEC command show interface precedence to display precedence accounting information for interfaces configured for precedence accounting.
Configure Performance Parameters
To tune IP performance, complete any of the tasks in the following sections. To configure various switching options, refer to the "Cisco IOS Switching Paths" chapter in the Cisco IOS Switching Services Configuration Guide.
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Compress TCP Packet Headers
You can compress the headers of your TCP/IP packets in order to reduce their size, thereby increasing performance. Header compression is particularly useful on networks with a large percentage of small packets (such as those supporting many Telnet connections). This feature only compresses the TCP header, so it has no effect on UDP packets or other protocol headers. The TCP header compression technique, described fully in RFC 1144, is supported on serial lines using HDLC or PPP encapsulation. You must enable compression on both ends of a serial connection.
You can optionally specify outgoing packets to be compressed only if TCP incoming packets on the same interface are compressed. If you do not specify this option, the Cisco IOS software will compress all traffic. The default is no compression.
You also can specify the total number of header compression connections that can exist on an interface. You should configure one connection for each TCP connection through the specified interface.
To enable compression, use either of the following optional commands in interface configuration mode:
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Set the TCP Connection Attempt Time
You can set the amount of time the Cisco IOS software will wait to attempt to establish a TCP connection. In previous versions of software, the system would wait a fixed 30 seconds when attempting to establish a connection. This amount of time is not sufficient in networks that have dial-up asynchronous connections (such as a network consisting of dial-on-demand links that are implemented over modems) because it will affect your ability to Telnet over the link (from the router) if the link must be brought up.
Because the connection attempt time is a host parameter, it does not pertain to traffic going through the device, just to traffic originated at the device.
To set the TCP connection attempt time, use the following command in global configuration mode:
ip tcp synwait-time seconds
Set the amount of time the Cisco IOS software will wait to attempt to establish a TCP connection.
Enable TCP Path MTU Discovery
Path MTU Discovery is a method for maximizing the use of available bandwidth in the network between the end points of a TCP connection, and is described in RFC 1191. By default, this feature is disabled. Existing connections are not affected when this feature is turned on or off. To enable Path MTU Discovery, use the following command in global configuration mode:
ip tcp path-mtu-discovery [age-timer {minutes | infinite}]
Enable Path MTU Discovery.
Customers using TCP connections to move bulk data between systems on distinct subnets would benefit most by enabling this feature. This might include customers using RSRB with TCP encapsulation, STUN, X.25 Remote Switching (also known as XOT or X.25 over TCP), and some protocol translation configurations.
The ip tcp path-mtu-discovery command is to enable Path MTU Discovery for connections initiated by the router when it is acting as a host. For a discussion of how the Cisco IOS software supports Path MTU Discovery when the device is acting as a router, see the section "Understand Path MTU Discovery" earlier in this chapter.
The age-timer is a time interval for how often TCP should re-estimate the Path MTU with a larger maximum segment size (MSS). The default path MTU Discovery age-timer is 10 minutes; its maximum is 30 minutes. You can turn off the age-timer by setting it to infinite.
Enable TCP Selective Acknowledgment
The TCP selective acknowledgment feature improves performance in the event that multiple packets are lost from one TCP window of data.
Prior to this feature, with the limited information available from cumulative acknowledgments, a TCP sender could learn about only one lost packet per round trip time. An aggressive sender could choose to retransmit packets early, but such retransmitted segments might have already been successfully received.
The TCP selective acknowledgment mechanism helps improve performance. The receiving TCP host returns selective acknowledgment packets to the sender, informing the sender of data that has been received. In other words, the receiver can acknowledge packets received out of order. The sender can then retransmit only the missing data segments (instead of everything since the first missing packet).
Prior to selective acknowledgment, if TCP lost packets 4 and 7 out of an 8-packet window, TCP would receive acknowledgment of only packets 1, 2, and 3. Packets 4 through 8 would have to be resent. With selective acknowledgment, TCP receives acknowledgment of packets 1, 2, 3, 5, 6, and 8. Only packets 4 and 7 have to be resent.
Refer to RFC 2018 for more detailed information on TCP selective acknowledgment.
The feature is used only when multiple packets are dropped within one TCP window. There is no performance impact when the feature is enabled but not used. To enable TCP selective acknowledgment, use the following command in global configuration mode.
Enable TCP Timestamp
The TCP timestamp option provides better TCP round-trip time measurements. Because the timestamps are always sent and echoed in both directions and the timestamp value in the header is always changing, TCP header compression will not compress the outgoing packet. To allow TCP header compression over a serial link, the TCP timestamp option is disabled.
Refer to RFC 1323 for more detailed information on TCP timestamp.
To enable TCP timestamp, use the following command in global configuration mode.
If you want to use TCP header compression over a serial line, TCP timestamp and TCP selective acknowledgment must be disabled. Both features are disabled by default. To disable TCP selective acknowledgment once it is enabled, refer to the "Enable TCP Selective Acknowledgment" section earlier in this chapter.
Set the TCP Maximum Read Size
By default, for Telnet and rlogin, the maximum number of characters that TCP reads from the input queue at once is a very large number (the largest possible 32-bit positive number). We do not recommend that you change this value. However, you could change that value by using the following command in global configuration mode.
Set the TCP Window Size
The default TCP window size is 2144 bytes. We recommend you keep the default value, unless you know your router is sending large packets (greater than 536 bytes). To change the default window size, use the following command in global configuration mode.
Set the TCP Outgoing Queue Size
The default TCP outgoing queue size per connection is 5 segments if the connection has a TTY associated with it (like a Telnet connection). If there is no TTY connection associated with it, the default queue size is 20 segments. To change the 5-segment default value, use the following command in global configuration mode.
Configure IP over WANs
You can configure IP over X.25, SMDS, Frame Relay, and DDR networks. To do this for X.25, SMDS, or Frame Relay, configure the address mappings as described in the appropriate chapters of the Wide-Area Networking Configuration Guide. For DDR, refer to the Dial Solutions Configuration Guide.
Monitor and Maintain the IP Network
To monitor and maintain your network, perform the tasks in the following sections:
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Clear Caches, Tables, and Databases
You can remove all contents of a particular cache, table, or database. Clearing a cache, table, or database can become necessary when the contents of the particular structure have become or are suspected to be invalid.
The following table lists the tasks associated with clearing caches, tables, and databases. Use the following commands as needed in EXEC mode:
clear ip accounting [checkpoint]
Clear the active IP accounting or checkpointed database when IP accounting is enabled.
clear tcp statistics
Clear TCP statistics.
Monitor and Maintain the DRP Server Agent
To monitor and maintain the DRP Server Agent, use the following commands in EXEC mode:
clear ip drp
Clear statistics being collected on DRP requests and responses.
show ip drp
Display information about the DRP Server Agent.
Clear the Access List Counters
The system counts how many packets pass each line of an access list; the counters are displayed by the show access-lists command. You can clear the counters of an access list by using the following command in EXEC mode:
