Basic IP Routing

The following example uses two different subnet masks for the class B network address of 172.16.0.0. A subnet mask of /24 is used for LAN interfaces. The /24 mask allows 265 subnets with 254 host IP addresses on each subnet. The final subnet of the range of possible subnets using a /24 mask (172.16.255.0) is reserved for use on point-to-point interfaces and assigned a longer mask of /30. The use of a /30 mask on 172.16.255.0 creates 64 subnets (172.16.255.0 to 172.16.255.252) with 2 host addresses on each subnet.

Caution: To ensure unambiguous routing, you must not assign 172.16.255.0/24 to a LAN interface in your network.

Device(config)# interface GigabitEthernet 0/0/0
Device(config-if)# ip address 172.16.1.1 255.255.255.0
Device(config-if)# ! 8 bits of host address space reserved for GigabitEthernet interfaces 
Device(config-if)# exit
Device(config)# interface Serial 0/0/0
Device(config-if)# ip address 172.16.255.5 255.255.255.252
Device(config-if)# ! 2 bits of address space reserved for point-to-point serial interfaces
Device(config-if)# exit
Device(config)# router rip 
Device(config-router)# network 172.16.0.0
Device(config-router)# ! Specifies the network directly connected to the device 

In the following example, packets for network 10.0.0.0 from Device B (where the static route is installed) will be routed through 172.18.3.4 if a route with an administrative distance less than 110 is not available. The figure below illustrates this example. The route learned by a protocol with an administrative distance of less than 110 might cause Device B to send traffic destined for network 10.0.0.0 via the alternate path through Device D.

Device(config)# ip route 10.0.0.0 255.0.0.0 172.18.3.4 110

In the following example, the router eigrp global configuration command configures Enhanced Interior Gateway Routing Protocol (EIGRP) routing in autonomous system 1. The network command configuration specifies EIGRP routing on networks 192.168.7.0 and 172.16.0.0. The first distance router configuration command sets the default administrative distance to 255, which instructs the device to ignore all routing updates from devices for which an explicit distance has not been set. The second distance command sets the administrative distance to 80 for internal EIGRP routes and to 100 for external EIGRP routes. The third distance command sets the administrative distance to 120 for the device with the address 172.16.1.3.

Device(config)# router eigrp 1 
Device(config-router)# network 192.168.7.0 
Device(config-router)# network 172.16.0.0 
Device(config-router)# distance 255 
Device(config-router)# distance eigrp 80 100 
Device(config-router)# distance 120 172.16.1.3 0.0.0.0 

Note	The distance eigrp command must be used to set the administrative distance for EIGRP-derived routes.

The following example assigns the device with the address 192.168.7.18 an administrative distance of 100 and all other devices on subnet 192.168.7.0 an administrative distance of 200:

Device(config-router)# distance 100 192.168.7.18 0.0.0.0 
Device(config-router)# distance 200 192.168.7.0 0.0.0.255 

However, if you reverse the order of these two commands, all devices on subnet 192.168.7.0 are assigned an administrative distance of 200, including the device at address 192.168.7.18:

Device(config-router)# distance 200 192.168.7.0 0.0.0.255 
Device(config-router)# distance 100 192.168.7.18 0.0.0.0 

Note	Assigning administrative distances can be used to solve unique problems. However, administrative distances should be applied carefully and consistently to avoid the creation of routing loops or other network failures.

In the following example, the distance value for IP routes learned is 90. Preference is given to these IP routes rather than routes with the default administrative distance value of 110.

Device(config)# router isis 
Device(config-router)# distance 90 ip 

In the example that follows, three static routes are specified, two of which are to be advertised. The static routes are created by specifying the redistribute static router configuration command and then specifying an access list that allows only those two networks to be passed to the Enhanced Interior Gateway Routing Protocol (EIGRP) process. Any redistributed static routes should be sourced by a single device to minimize the likelihood of creating a routing loop.

Device(config)# ip route 192.168.2.0 255.255.255.0 192.168.7.65 
Device(config)# ip route 192.168.5.0 255.255.255.0 192.168.7.65 
Device(config)# ip route 10.10.10.0 255.255.255.0 10.20.1.2 
Device(config)# ! 
Device(config)# access-list 3 permit 192.168.2.0 0.0.255.255 
Device(config)# access-list 3 permit 192.168.5.0 0.0.255.255 
Device(config)# access-list 3 permit 10.10.10.0 0.0.0.255 
Device(config)# ! 
Device(config)# router eigrp 1 
Device(config-router)# network 192.168.0.0 
Device(config-router)# network 10.10.10.0 
Device(config-router)# redistribute static metric 10000 100 255 1 1500
Device(config-router)# distribute-list 3 out static

Each Enhanced Interior Gateway Routing Protocol (EIGRP) routing process provides routing information to only one autonomous system. The Cisco software must run a separate EIGRP process and maintain a separate routing database for each autonomous system that it services. However, you can transfer routing information between these routing databases.

In the following configuration, network 10.0.0.0 is configured under EIGRP autonomous system 1 and network 192.168.7.0 is configured under EIGRP autonomous system 101:

Device(config)# router eigrp 1 
Device(config-router)# network 10.0.0.0 
Device(config-router)# exit 
Device(config)# router eigrp 101 
Device(config-router)# network 192.168.7.0 

In the following example, routes from the 192.168.7.0 network are redistributed into autonomous system 1 (without passing any other routing information from autonomous system 101):

Device(config)# access-list 3 permit 192.168.7.0 
Device(config)# ! 
Device(config)# route-map 101-to-1 permit 10 
Device(config-route-map)# match ip address 3 
Device(config-route-map)# set metric 10000 100 1 255 1500 
Device(config-route-map)# exit 
Device(config)# router eigrp 1 
Device(config-router)# redistribute eigrp 101 route-map 101-to-1 
Device(config-router)#! 

The following example is an alternative way to redistribute routes from the 192.168.7.0 network into autonomous system 1. Unlike the previous configuration, this method does not allow you to set the metric for redistributed routes.

Device(config)# access-list 3 permit 192.168.7.0 
Device(config)# ! 
Device(config)# router eigrp 1 
Device(config-router)# redistribute eigrp 101 
Device(config-router)# distribute-list 3 out eigrp 101 
Device(config-router)# !

Consider a WAN at a university that uses the Routing Information Protocol (RIP) as an interior routing protocol. Assume that the university wants to connect its WAN to regional network 172.16.0.0, which uses the Enhanced Interior Gateway Routing Protocol (EIGRP) as the routing protocol. The goal in this case is to advertise the networks in the university network to devices in the regional network.

Mutual redistribution is configured between EIGRP and RIP in the following example:

Device(config)# access-list 10 permit 172.16.0.0 
Device(config)# ! 
Device(config)# router eigrp 1 
Device(config-router)# network 172.16.0.0 
Device(config-router)# redistribute rip metric 10000 100 255 1 1500 
Device(config-router)# default-metric 10
Device(config-router)# distribute-list 10 out rip 
Device(config-router)# exit
Device(config)# router rip 
Device(config-router)# redistribute eigrp 1 
Device(config-router)# ! 

In this example, an EIGRP routing process is started. The network router configuration command specifies that network 172.16.0.0 (the regional network) is to send and receive EIGRP routing information. The redistribute router configuration command specifies that RIP-derived routing information be advertised in routing updates. The default-metric router configuration command assigns an EIGRP metric to all RIP-derived routes. The distribute-list router configuration command instructs the Cisco software to use access list 10 (not defined in this example) to limit the entries in each outgoing update. The access list prevents unauthorized advertising of university routes to the regional network.

In the following example, mutual redistribution is configured between the Enhanced Interior Gateway Routing Protocol (EIGRP) and the Border Gateway Protocol (BGP).

Routes from EIGRP routing process 101 are injected into BGP autonomous system 50000. A filter is configured to ensure that the correct routes are advertised, in this case, three networks. Routes from BGP autonomous system 50000 are injected into EIGRP routing process 101. The same filter is used.

Device(config)# ! All networks that should be advertised from R1 are controlled with ACLs: 
Device(config)# access-list 1 permit 172.18.0.0 0.0.255.255 
Device(config)# access-list 1 permit 172.16.0.0 0.0.255.255 
Device(config)# access-list 1 permit 172.25.0.0 0.0.255.255
Device(config)# ! Configuration for router R1:
Device(config)# router bgp 50000 
Device(config-router)# network 172.18.0.0 
Device(config-router)# network 172.16.0.0 
Device(config-router)# neighbor 192.168.10.1 remote-as 2 
Device(config-router)# neighbor 192.168.10.15 remote-as 1 
Device(config-router)# neighbor 192.168.10.24 remote-as 3 
Device(config-router)# redistribute eigrp 101 
Device(config-router)# distribute-list 1 out eigrp 101 
Device(config-router)# exit 
Device(config)# router eigrp 101 
Device(config-router)# network 172.25.0.0
Device(config-router)# redistribute bgp 50000 
Device(config-router)# distribute-list 1 out bgp 50000 
Device(config-router)# ! 

Caution

BGP should be redistributed into an Interior Gateway Protocol (IGP) when there are no other suitable options. Redistribution from BGP into any IGP should be applied with proper filtering by using distribute lists, IP prefix lists, and route map statements to limit the number of prefixes.

OSPF typically requires coordination among many internal devices, area border routers (ABRs), and Autonomous System Boundary Routers (ASBRs). At a minimum, OSPF-based devices can be configured with all default parameter values, with no authentication, and with interfaces assigned to areas.

This section provides the following configuration examples:

• The first example shows simple configurations illustrating basic OSPF commands.
• The second example shows configurations for an internal device, ABR, and ASBR within a single, arbitrarily assigned OSPF autonomous system.
• The third example illustrates a more complex configuration and the application of various tools available for controlling OSPF-based routing environments.

The following example shows a device in autonomous system 1 that is configured to run both the Routing Information Protocol (RIP) and the Enhanced Interior Gateway Routing Protocol (EIGRP). The example advertises EIGRP-derived routes using RIP and assigns the EIGRP-derived routes a RIP metric of 10.

Device(config)# router rip 
Device(config-router)# redistribute eigrp 1 
Device(config-router)# default-metric 10 
Device(config-router)# exit 

The examples in this section illustrate the use of redistribution, with and without route maps. Examples from both the IP and Connectionless Network Service (CLNS) routing protocols are given. The following example redistributes all Open Shortest Path First (OSPF) routes into the Enhanced Interior Gateway Routing Protocol (EIGRP):

Device(config)# router eigrp 1 
Device(config-router)# redistribute ospf 101 
Device(config-router)# exit 

The following example redistributes Routing Information Protocol (RIP) routes with a hop count equal to 1 into OSPF. These routes will be redistributed into OSPF as external link state advertisements (LSAs) with a metric of 5, metric a type of type 1, and a tag equal to 1.

Device(config)# router ospf 1 
Device(config-router)# redistribute rip route-map rip-to-ospf 
Device(config-router)# exit 
Device(config)# route-map rip-to-ospf permit 
Device(config-route-map)# match metric 1 
Device(config-route-map)# set metric 5 
Device(config-route-map)# set metric-type type 1 
Device(config-route-map)# set tag 1 
Device(config-route-map)# exit 

The following example redistributes OSPF learned routes with tag 7 as a RIP metric of 15:

Device(config)# router rip 
Device(config-router)# redistribute ospf 1 route-map 5 
Device(config-router)# exit 
Device(config)# route-map 5 permit 
Device(config-route-map)# match tag 7 
Device(config-route-map)# set metric 15 

The following example redistributes OSPF intra-area and interarea routes with next hop devices on serial interface 0/0/0 into the Border Gateway Protocol (BGP) with an INTER_AS metric of 5:

Device(config)# router bgp 50000 
Device(config-router)# redistribute ospf 1 route-map 10 
Device(config-router)# exit 
Device(config)# route-map 10 permit 
Device(config-route-map)# match route-type internal 
Device(config-route-map)# match interface serial 0/0/0 
Device(config-route-map)# set metric 5 

The following example redistributes two types of routes into the integrated IS-IS routing table (supporting both IP and CLNS). The first type is OSPF external IP routes with tag 5; these routes are inserted into Level 2 IS-IS link-state packets (LSPs) with a metric of 5. The second type is ISO-IGRP derived CLNS prefix routes that match CLNS access list 2000; these routes will be redistributed into IS-IS as Level 2 LSPs with a metric of 30.

Device(config)# router isis 
Device(config-router)# redistribute ospf 1 route-map 2 
Device(config-router)# redistribute iso-igrp nsfnet route-map 3 

Device(config-router)# exit 
Device(config)# route-map 2 permit 
Device(config-route-map)# match route-type external 
Device(config-route-map)# match tag 5 
Device(config-route-map)# set metric 5 
Device(config-route-map)# set level level-2 
Device(config-route-map)# exit 
Device(config)# route-map 3 permit 
Device(config-route-map)# match address 2000 
Device(config-route-map)# set metric 30 
Device(config-route-map)# exit 

With the following configuration, OSPF external routes with tags 1, 2, 3, and 5 are redistributed into RIP with metrics of 1, 1, 5, and 5, respectively. The OSPF routes with a tag of 4 are not redistributed.

Device(config)# router rip 
Device(config-router)# redistribute ospf 101 route-map 1 
Device(config-router)# exit 
Device(config)# route-map 1 permit 
Device(config-route-map)# match tag 1 2 
Device(config-route-map)# set metric 1 
Device(config-route-map)# exit 
Device(config)# route-map 1 permit 
Device(config-route-map)# match tag 3 
Device(config-route-map)# set metric 5 
Device(config-route-map)# exit 
Device(config)# route-map 1 deny 
Device(config-route-map)# match tag 4 
Device(config-route-map)# exit 
Device(config)# route map 1 permit 
Device(config-route-map)# match tag 5 
Device(config-route-map)# set metric 5 
Device(config-route-map)# exit 

Given the following configuration, a RIP learned route for network 172.18.0.0 and an ISO-IGRP learned route with prefix 49.0001.0002 will be redistributed into an IS-IS Level 2 LSP with a metric of 5:

Device(config)# router isis 
Device(config-router)# redistribute rip route-map 1 
Device(config-router)# redistribute iso-igrp remote route-map 1 
Device(config-router)# exit 
Device(config)# route-map 1 permit 
Device(config-route-map)# match ip address 1 
Device(config-route-map)# match clns address 2 
Device(config-route-map)# set metric 5 
Device(config-route-map)# set level level-2 
Device(config-route-map)# exit 
Device(config)# access-list 1 permit 172.18.0.0 0.0.255.255 
Device(config)# clns filter-set 2 permit 49.0001.0002... 

The following configuration example illustrates how a route map is referenced by the default-information router configuration command. This type of reference is called conditional default origination. OSPF will originate the default route (network 0.0.0.0) with a type 2 metric of 5 if 172.20.0.0 is in the routing table.

Device(config)# route-map ospf-default permit 
Device(config-route-map)# match ip address 1 
Device(config-route-map)# set metric 5 
Device(config-route-map)# set metric-type type-2 
Device(config-route-map)# exit 
Device(config)# access-list 1 172.20.0.0 0.0.255.255 
Device(config)# router ospf 101 
Device(config-router)# default-information originate route-map ospf-default 

The following example configures a key chain named chain1. In this example, the software always accepts and sends key1 as a valid key. The key key2 is accepted from 1:30 p.m. to 3:30 p.m. and is sent from 2:00 p.m. to 3:00 p.m. The overlap allows for migration of keys or discrepancy in the set time of the device. Likewise, the key key3 immediately follows key2, and there is 30-minutes on each side to handle time-of-day differences.

Device(config)# interface GigabitEthernet 0/0/0
Device(config-if)# ip rip authentication key-chain chain1 
Device(config-if)# ip rip authentication mode md5 
Device(config-if)# exit 
Device(config)# router rip
Device(config-router)# network 172.19.0.0 
Device(config-router)# version 2 
Device(config-router)# exit 
Device(config)# key chain chain1 
Device(config-keychain)# key 1 
Device(config-keychain-key)# key-string key1 
Device(config-keychain-key)# exit
Device(config-keychain)# key 2 
Device(config-keychain-key)# key-string key2 
Device(config-keychain-key)# accept-lifetime 13:30:00 Jan 25 2005 duration 7200 
Device(config-keychain-key)# send-lifetime 14:00:00 Jan 25 2005 duration 3600  
Device(config-keychain-key)# exit
Device(config-keychain)# key 3 
Device(config-keychain-key)# key-string key3 
Device(config-keychain-key)# accept-lifetime 14:30:00 Jan 25 2005 duration 7200 
Device(config-keychain-key)# send-lifetime 15:00:00 Jan 25 2005 duration 3600 
Device(config-keychain-key)# end 

The following example configures a key chain named chain1:

Device(config)# key chain chain1 
Device(config-keychain)# key 1 
Device(config-keychain-key)# key-string key1 
Device(config-keychain-key)# exit
Device(config-keychain)# key 2 
Device(config-keychain-key)# key-string key2 
Device(config-keychain-key)# accept-lifetime 00:00:00 Dec 5 2004 23:59:59 Dec 5 2005 
Device(config-keychain-key)# send-lifetime 06:00:00 Dec 5 2004 18:00:00 Dec 5 2005 
Device(config-keychain-key)# exit 
Device(config-keychain)# exit 
Device(config)# interface GigabitEthernet 0/0/0 
Device(config-if)# ip address 172.19.104.75 255.255.255.0 secondary 172.19.232.147 255.255.255.240 
Device(config-if)# ip rip authentication key-chain chain1 
Device(config-if)# media-type 10BaseT 
Device(config-if)# exit 
Device(config)# interface GigabitEthernet 1/0/0
Device(config-if)# no ip address 
Device(config-if)# shutdown 
Device(config-if)# media-type 10BaseT 
Device(config-if)# exit 
Device(config)# interface Fddi 0 
Device(config-if)# ip address 10.1.1.1 255.255.255.0 
Device(config-if)# no keepalive 
Device(config-if)# exit 
Device(config)# interface Fddi 1/0/0
Device(config-if)# ip address 172.16.1.1 255.255.255.0 
Device(config-if)# ip rip send version 1 
Device(config-if)# ip rip receive version 1 
Device(config-if)# no keepalive 
Device(config-if)# exit 
Device(config)# router rip 
Device(config-router)# version 2 
Device(config-router)# network 172.19.0.0 
Device(config-router)# network 10.0.0.0 
Device(config-router)# network 172.16.0.0