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Catalyst 3550 Multilayer Switch Software Configuration Guide, 12.1(13)EA1
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Index
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Preface
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Overview
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Using the Command-Line Interface
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Getting Started with CMS
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Assigning the Switch IP Address and Default Gateway
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Configuring IE2100 CNS Agents
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Clustering Switches
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Administering the Switch
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Configuring Switch-Based Authentication
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Configuring 802.1X Port-Based Authentication
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Configuring Interface Characteristics
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Configuring VLANs
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Configuring VTP
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Configuring Voice VLAN
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Configuring 802.1Q and Layer 2 Protocol Tunneling
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Configuring STP
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Configuring RSTP and MSTP
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Configuring Optional Spanning-Tree Features
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Configuring the DHCP Option 82 for Subscriber Identification
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Configuring IGMP Snooping and MVR
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Configuring Port-Based Traffic Control
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Configuring CDP
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Configuring UDLD
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Configuring SPAN and RSPAN
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Configuring RMON
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Configuring System Message Logging
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Configuring SNMP
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Configuring Network Security with ACLs
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Configuring QoS
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Configuring EtherChannels
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Configuring IP Unicast Routing
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Configuring HSRP
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Configuring Web Cache Services By Using WCCP
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Configuring IP Multicast Routing
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Configuring MSDP
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Configuring Fallback Bridging
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Troubleshooting
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Supported MIBs
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Working with the IOS File System, Configuration Files, and Software Images
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Unsupported CLI Commands for Release 12.1(13)EA1
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Table Of Contents
Configuring IP Unicast Routing
Configuring IP Addressing on Layer 3 Interfaces
Default Addressing Configuration
Assigning IP Addresses to Network Interfaces
Configuring Address Resolution Methods
Routing Assistance When IP Routing is Disabled
ICMP Router Discovery Protocol (IRDP)
Configuring Broadcast Packet Handling
Enabling Directed Broadcast-to-Physical Broadcast Translation
Forwarding UDP Broadcast Packets and Protocols
Establishing an IP Broadcast Address
Monitoring and Maintaining IP Addressing
Configuring Basic RIP Parameters
Configuring RIP Authentication
Configuring Summary Addresses and Split Horizon
Understanding Load Balancing and Traffic Distribution Control
Configuring Basic IGRP Parameters
Configuring Basic OSPF Parameters
Configuring OSPF Area Parameters
Configuring Other OSPF Parameters
Configuring a Loopback Interface
Configuring Basic EIGRP Parameters
Configuring EIGRP Route Authentication
Monitoring and Maintaining EIGRP
Managing Routing Policy Changes
Configuring BGP Decision Attributes
Configuring BGP Filtering with Route Maps
Configuring BGP Filtering by Neighbor
Configuring Prefix Lists for BGP Filtering
Configuring BGP Community Filtering
Configuring BGP Neighbors and Peer Groups
Configuring Aggregate Addresses
Configuring a Routing Domain Confederation
Configuring BGP Route Reflectors
Monitoring and Maintaining BGP
Default Multi-VRF CE Configuration
Multi-VRF CE Configuration Guidelines
Configuring a VPN Routing Session
Configuring BGP PE to CE Routing Sessions
Multi-VRF CE Configuration Example
Displaying Multi-VRF CE Status
Configuring Protocol-Independent Features
Configuring Cisco Express Forwarding
Configuring the Number of Equal-Cost Routing Paths
Configuring Static Unicast Routes
Specifying Default Routes and Networks
Using Route Maps to Redistribute Routing Information
Configuring Policy-Based Routing
Controlling Advertising and Processing in Routing Updates
Filtering Sources of Routing Information
Monitoring and Maintaining the IP Network
Configuring IP Unicast Routing
This chapter describes how to configure IP unicast routing on your Catalyst 3550 multilayer switch. Beginning with Cisco IOS Release 12.1(11)EA1, basic routing functions, including static unicast routing and the Routing Information Protocol (RIP), are available with both the standard multilayer software image (SMI) and the enhanced multilayer software image (EMI). To use advanced routing features and other routing protocols, or for all routing support prior to Release 12.1(11)EA1, you must have the enhanced multilayer software image installed on your switch.
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For more detailed IP unicast configuration information, refer to the Cisco IOS IP and IP Routing Configuration Guide for Release 12.1. For complete syntax and usage information for the commands used in this chapter, refer to the Cisco IOS IP and IP Routing Command Reference for Release 12.1.
This chapter consists of these sections:
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Understanding Routing
In some network environments, VLANs are associated with individual networks or subnetworks. In an IP network, each subnetwork is mapped to an individual VLAN. Configuring VLANs helps control the size of the broadcast domain and keeps local traffic local. However, network devices in different VLANs cannot communicate with one another without a Layer 3 device (router) to route traffic between the VLAN, referred to as inter-VLAN routing. You configure one or more routers to route traffic to the appropriate destination VLAN.
Figure 30-1 shows a basic routing topology. Switch A is in VLAN 10, and Switch B is in VLAN 20. The router has an interface in each VLAN.
Figure 30-1 Routing Topology Example
When Host A in VLAN 10 needs to communicate with Host B in VLAN 10, it sends a packet addressed to that host. Switch A forwards the packet directly to Host B, without sending it to the router.
When Host A sends a packet to Host C in VLAN 20, Switch A forwards the packet to the router, which receives the traffic on the VLAN 10 interface. The router checks the routing table, determines the correct outgoing interface, and forwards the packet on the VLAN 20 interface to Switch B. Switch B receives the packet and forwards it to Host C.
Routers can perform unicast routing in three different ways:
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Default routing refers to sending traffic with a destination unknown to the router to a default outlet or destination.
Static unicast routing forwards packets from predetermined ports through a single path into and out of a network. Static routing is secure and uses little bandwidth, but does not automatically respond to changes in the network, such as link failures, and therefore, might result in unreachable destinations. As networks grow, static routing becomes a labor-intensive liability.
Dynamic routing protocols are used by routers to dynamically calculate the best route for forwarding traffic. There are two types of dynamic routing protocols:
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Distance-vector protocols supported by the Catalyst 3550 switch are Routing Information Protocol (RIP), which uses a single distance metric (cost) to determine the best path; Interior Gateway Routing Protocol (IGRP), which uses a series of metrics; and Border Gateway Protocol (BGP), which adds a path vector mechanism. The switch also supports the Open Shortest Path First (OSPF) link-state protocol and Enhanced IGRP (EIGRP), which adds some link-state routing features to traditional IGRP to improve efficiency.
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Steps for Configuring Routing
By default, IP routing is disabled on the Catalyst 3550 switch, and you must enable it before routing can take place. For detailed IP routing configuration information, refer to the Cisco IOS IP and IP Routing Configuration Guide for Release 12.1.
In the following procedures, the specified interface must be one of these Layer 3 interfaces:
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All Layer 3 interfaces must have IP addresses assigned to them. See the "Assigning IP Addresses to Network Interfaces" section.
Configuring routing consists of several main procedures:
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Configuring IP Addressing on Layer 3 Interfaces
A required task for configuring IP routing is to assign IP addresses to Layer 3 network interfaces to enable the interfaces and allow communication with the hosts on those interfaces that use IP. These sections describe how to configure various IP addressing features. Assigning IP addresses to the interface is required; the other procedures are optional.
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Default Addressing Configuration
Table 30-1 shows the default addressing configuration.
Assigning IP Addresses to Network Interfaces
An IP address identifies a location to which IP packets can be sent. Some IP addresses are reserved for special uses and cannot be used for host, subnet, or network addresses. Table 30-2 lists ranges of IP addresses and shows which are reserved and which are available for use. RFC 1166, "Internet Numbers," contains the official description of IP addresses.
An interface can have one primary IP address. A mask identifies the bits that denote the network number in an IP address. When you use the mask to subnet a network, the mask is referred to as a subnet mask. To receive an assigned network number, contact your Internet service provider.
Beginning in privileged EXEC mode, follow these steps to assign an IP address and a network mask to a Layer 3 interface:
Use the no ip address interface configuration command to remove an IP address or to disable IP processing.
This example shows how to configure an IP address on and enable Gigabit Ethernet interface 0/10:
Switch# configure terminalEnter configuration commands, one per line. End with CNTL/Z.Switch(config)# interface gigabitethernet0/10Switch(config-if)# no switchportSwitch(config-if)# ip address 10.1.2.3 255.255.0.0Switch(config-if)# no shutdownUse of Subnet Zero
Subnetting with a subnet address of zero is strongly discouraged because of the problems that can arise if a network and a subnet have the same addresses. For example, if network 131.108.0.0 is subnetted as 255.255.255.0, subnet zero would be written as 131.108.0.0, which is the same as the network address.
You can use the all ones subnet (131.108.255.0) and even though it is discouraged, you can enable the use of subnet zero if you need the entire subnet space for your IP address.
Beginning in privileged EXEC mode, follow these steps to enable subnet zero:
Use the no ip subnet-zero global configuration command to restore the default and disable the use of subnet zero.
Classless Routing
By default, classless routing behavior is enabled on the switch when it is configured to route. With classless routing, if a router receives packets for a subnet of a network with no default route, the router forwards the packet to the best supernet route. A supernet consists of contiguous blocks of Class C address spaces used to simulate a single, larger address space and is designed to relieve the pressure on the rapidly depleting Class B address space.
In Figure 30-2, classless routing is enabled. When the host sends a packet to 120.20.4.1, instead of discarding the packet, the router forwards it to the best supernet route. If you disable classless routing and a router receives packets destined for a subnet of a network with no network default route, the router discards the packet.
Figure 30-2 IP Classless Routing
In Figure 30-3, the router in network 128.20.0.0 is connected to subnets 128.20.1.0, 128.20.2.0, and 128.20.3.0. If the host sends a packet to 120.20.4.1, because there is no network default route, the router discards the packet.
Figure 30-3 No IP Classless Routing
To prevent the switch from forwarding packets destined for unrecognized subnets to the best supernet route possible, you can disable classless routing behavior.
Beginning in privileged EXEC mode, follow these steps to disable classless routing:
To restore the default and have the switch forward packets destined for a subnet of a network with no network default route to the best supernet route possible, use the ip classless global configuration command.
Configuring Address Resolution Methods
You can control interface-specific handling of IP by using address resolution. A device using IP can have both a local address or MAC address, which uniquely defines the device on its local segment or LAN, and a network address, which identifies the network to which the device belongs. The local address or MAC address is known as a data link address because it is contained in the data link layer (Layer 2) section of the packet header and is read by data link (Layer 2) devices. To communicate with a device on Ethernet, the software must determine the MAC address of the device. The process of determining the MAC address from an IP address is called address resolution. The process of determining the IP address from the MAC address is called reverse address resolution.
The switch can use these forms of address resolution:
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Catalyst 3550 switches also use the Reverse Address Resolution Protocol (RARP), which functions the same as ARP does, except that the RARP packets request an IP address instead of a local MAC address. Using RARP requires a RARP server on the same network segment as the router interface. Use the ip rarp-server address interface configuration command to identify the server.
For more information on RARP, refer to the Cisco IOS Configuration Fundamentals Configuration Guide for Release 12.1.
You can perform these tasks to configure address resolution:
Define a Static ARP Cache
ARP and other address resolution protocols provide dynamic mapping between IP addresses and MAC addresses. Because most hosts support dynamic address resolution, you usually do not need to specify static ARP cache entries. If you must define a static ARP cache entry, you can do so globally, which installs a permanent entry in the ARP cache that the switch uses to translate IP addresses into MAC addresses. Optionally, you can also specify that the switch respond to ARP requests as if it were the owner of the specified IP address. If you do not want the ARP entry to be permanent, you can specify a timeout period for the ARP entry.
Beginning in privileged EXEC mode, follow these steps to provide static mapping between IP addresses and MAC addresses:
To remove an entry from the ARP cache, use the no arp ip-address hardware-address type global configuration command. To remove all nonstatic entries from the ARP cache, use the clear arp-cache privileged EXEC command.
Set ARP Encapsulation
By default, Ethernet ARP encapsulation (represented by the arpa keyword) is enabled on an IP interface. You can change the encapsulation methods to SNAP if required by your network.
Beginning in privileged EXEC mode, follow these steps to specify the ARP encapsulation type:
To disable an encapsulation type, use the no arp arpa or no arp snap interface configuration command.
Enable Proxy ARP
By default, the switch uses proxy ARP to help hosts determine MAC addresses of hosts on other networks or subnets.
Beginning in privileged EXEC mode, follow these steps to enable proxy ARP if it has been disabled:
To disable proxy ARP on the interface, use the no ip proxy-arp interface configuration command.
Routing Assistance When IP Routing is Disabled
These mechanisms allow the switch to learn about routes to other networks when it does not have IP routing enabled:
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Proxy ARP
Proxy ARP, the most common method for learning about other routes, enables an Ethernet host with no routing information to communicate with hosts on other networks or subnets. The host assumes that all hosts are on the same local Ethernet and that they can use ARP to determine their MAC addresses. If a switch receives an ARP request for a host that is not on the same network as the sender, the switch evaluates whether it has the best route to that host. If it does, it sends an ARP reply packet with its own Ethernet MAC address, and the host that sent the request sends the packet to the switch, which forwards it to the intended host. Proxy ARP treats all networks as if they are local and performs ARP requests for every IP address.
Proxy ARP is enabled by default. To enable it after it has been disabled, see the "Enable Proxy ARP" section. Proxy ARP works as long as other routers support it.
Default Gateway
Another method for locating routes is to define a default router or default gateway. All nonlocal packets are sent to this router, which either routes them appropriately or sends an IP Control Message Protocol (ICMP) redirect message back, defining which local router the host should use. The switch caches the redirect messages and forwards each packet as efficiently as possible. A limitation of this method is that there is no means of detecting when the default router has gone down or is unavailable.
Beginning in privileged EXEC mode, follow these steps to define a default gateway (router) when IP routing is disabled:
Use the no ip default-gateway global configuration command to disable this function.
This example shows how to set and verify a default gateway:
Switch(config)# ip default-gateway 10.1.5.59Switch(config)# endSwitch# show ip redirectDefault gateway is 10.1.5.59Host Gateway Last Use Total Uses InterfaceICMP redirect cache is emptyICMP Router Discovery Protocol (IRDP)
Router discovery allows the switch to dynamically learn about routes to other networks using IRDP. IRDP allows hosts to locate routers. When operating as a client, the switch generates router discovery packets. When operating as a host, the switch receives router discovery packets. The switch can also listen to Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP) routing updates and use this information to infer locations of routers. The switch does not actually store the routing tables sent by routing devices; it merely keeps track of which systems are sending the data. The advantage of using IRDP is that it allows each router to specify both a priority and the time after which a device is assumed to be down if no further packets are received.
Each device discovered becomes a candidate for the default router, and a new highest-priority router is selected when a higher priority router is discovered, when the current default router is declared down, or when a TCP connection is about to time out because of excessive retransmissions.
The only required task for IRDP routing on an interface is to enable IRDP processing on that interface. When enabled, the default parameters apply. You can optionally change any of these parameters.
Beginning in privileged EXEC mode, follow these steps to enable and configure IRDP on an interface:
If you change the maxadvertinterval value, the holdtime and minadvertinterval values also change, so it is important to first change the maxadvertinterval value, before manually changing either the holdtime or minadvertinterval values.
Use the no ip irdp interface configuration command to disable IRDP routing.
Configuring Broadcast Packet Handling
After configuring an IP interface address, you can enable routing and configure one or more routing protocols, or you can configure the way the switch responds to network broadcasts. A broadcast is a data packet destined for all hosts on a physical network. The switch supports two kinds of broadcasting:
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Routers provide some protection from broadcast storms by limiting their extent to the local cable. Bridges (including intelligent bridges), because they are Layer 2 devices, forward broadcasts to all network segments, thus propagating broadcast storms. The best solution to the broadcast storm problem is to use a single broadcast address scheme on a network. In most modern IP implementations, you can set the address to be used as the broadcast address. Many implementations, including the one in the Catalyst 3550 switch, support several addressing schemes for forwarding broadcast messages.
Perform the tasks in these sections to enable these schemes:
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Enabling Directed Broadcast-to-Physical Broadcast Translation
By default, IP directed broadcasts are dropped; they are not forwarded. Dropping IP-directed broadcasts makes routers less susceptible to denial-of-service attacks.
You can enable forwarding of IP-directed broadcasts on an interface where the broadcast becomes a physical (MAC-layer) broadcast. Only those protocols configured by using the ip forward-protocol global configuration command are forwarded.
You can specify an access list to control which broadcasts are forwarded. When an access list is specified, only those IP packets permitted by the access list are eligible to be translated from directed broadcasts to physical broadcasts. For more information on access lists, see "Configuring Network Security with ACLs."
Beginning in privileged EXEC mode, follow these steps to enable forwarding of IP-directed broadcasts on an interface:
Use the no ip directed-broadcast interface configuration command to disable translation of directed broadcast to physical broadcasts. Use the no ip forward-protocol global configuration command to remove a protocol or port.
Forwarding UDP Broadcast Packets and Protocols
User Datagram Protocol (UDP) is an IP host-to-host layer protocol, as is TCP. UDP provides a low-overhead, connectionless session between two end systems and does not provide for acknowledgment of received datagrams. Network hosts occasionally use UDP broadcasts to determine address, configuration, and name information. If such a host is on a network segment that does not include a server, UDP broadcasts are normally not forwarded. You can remedy this situation by configuring an interface on a router to forward certain classes of broadcasts to a helper address. You can use more than one helper address per interface.
You can specify a UDP destination port to control which UDP services are forwarded. You can specify multiple UDP protocols. You can also specify the Network Disk (ND) protocol, which is used by older diskless Sun workstations and the network security protocol SDNS.
By default, both UDP and ND forwarding are enabled if a helper address has been defined for an interface. The description for the ip forward-protocol interface configuration command in the Cisco IOS IP and IP Routing Command Reference for Release 12.1 lists the ports that are forwarded by default if you do not specify any UDP ports.
If you do not specify any UDP ports when you configure the forwarding of UDP broadcasts, you are configuring the router to act as a BOOTP forwarding agent. BOOTP packets carry Dynamic Host Configuration Protocol (DHCP) information.
Beginning in privileged EXEC mode, follow these steps to enable forwarding UDP broadcast packets on an interface and specify the destination address:
Use the no ip helper-address interface configuration command to disable the forwarding of broadcast packets to specific addresses. Use the no ip forward-protocol global configuration command to remove a protocol or port.
Establishing an IP Broadcast Address
The most popular IP broadcast address (and the default) is an address consisting of all ones (255.255.255.255). However, the switch can be configured to generate any form of IP broadcast address.
Beginning in privileged EXEC mode, follow these steps to set the IP broadcast address on an interface:
To restore the default IP broadcast address, use the no ip broadcast-address interface configuration command.
Flooding IP Broadcasts
You can allow IP broadcasts to be flooded throughout your internetwork in a controlled fashion by using the database created by the bridging STP. Using this feature also prevents loops. To support this capability, bridging must be configured on each interface that is to participate in the flooding. If bridging is not configured on an interface, it still can receive broadcasts. However, the interface never forwards broadcasts it receives, and the router never uses that interface to send broadcasts received on a different interface.
Packets that are forwarded to a single network address using the IP helper-address mechanism can be flooded. Only one copy of the packet is sent on each network segment.
To be considered for flooding, packets must meet these criteria. (Note that these are the same conditions used to consider packet forwarding using IP helper addresses.)
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A flooded UDP datagram is given the destination address specified with the ip broadcast-address interface configuration command on the output interface. The destination address can be set to any address. Thus, the destination address might change as the datagram propagates through the network. The source address is never changed. The TTL value is decremented.
When a flooded UDP datagram is sent out an interface (and the destination address possibly changed), the datagram is handed to the normal IP output routines and is, therefore, subject to access lists, if they are present on the output interface.
Beginning in privileged EXEC mode, follow these steps to use the bridging spanning-tree database to flood UDP datagrams:
Use the no ip forward-protocol spanning-tree global configuration command to disable the flooding of IP broadcasts.
In the Catalyst 3550 switch, the majority of packets are forwarded in hardware; most packets do not go through the switch CPU. For those packets that do go to the CPU, you can speed up spanning tree-based UDP flooding by a factor of about four to five times by using turbo-flooding. This feature is supported over Ethernet interfaces configured for ARP encapsulation.
Beginning in privileged EXEC mode, follow these steps to increase spanning-tree-based flooding:
To disable this feature, use the no ip forward-protocol turbo-flood global configuration command.
Monitoring and Maintaining IP Addressing
When the contents of a particular cache, table, or database have become or are suspected to be invalid, you can remove all its contents by using the clear privileged EXEC commands. Table 30-3 lists the commands for clearing contents.
You can display specific statistics, such as the contents of IP routing tables, caches, and databases; the reachability of nodes; and the routing path that packets are taking through the network. Table 30-4 lists the privileged EXEC commands for displaying IP statistics.
