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 256 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 - 172.16.255.252) with 2 host addresses on each subnet.

DANGER

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

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

In the following example, packets for network 10.0.0.0 from Router 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 Router B to send traffic destined for network 10.0.0.0 via the alternate path--through Router D.

Router(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 EIGRP routing in autonomous system 1. The network command specifies EIGRP routing on networks 192.0.2.16 and 172.16.0.0. The first distance router configuration command sets the default administrative distance to 255, which instructs the router to ignore all routing updates from routers 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 router with the address 172.16.1.3.

Router(config)# router eigrp 1 
Router(config-router)# network 192.0.2.16 
Router(config-router)# network 172.16.0.0 
Router(config-router)# distance 255 
Router(config-router)# distance eigrp 80 100 
Router(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 router with the address 192.0.2.1 an administrative distance of 100 and all other routers on subnet 192.0.2.0 an administrative distance of 200:

Router(config-router)# distance 100 192.0.2.1 0.0.0.0 
Router(config-router)# distance 200 192.0.2.0 0.0.0.255 

However, if you reverse the order of these two commands, all routers on subnet 192.0.2.0 are assigned an administrative distance of 200, including the router at address 192.0.2.1:

Router(config-router)# distance 200 192.0.2.0 0.0.0.255 
Router(config-router)# distance 100 192.0.2.1 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 learned IP routes is 90. Preference is given to these IP routes rather than routes with the default administrative distance value of 110.

Router(config)# router isis
 
Router(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 EIGRP process. Any redistributed static routes should be sourced by a single router to minimize the likelihood of creating a routing loop.

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

Each EIGRP routing process provides routing information to only one autonomous system. The Cisco IOS software must run a separate EIGRP process and maintain a separate routing database for each autonomous system that the software services. However, you can transfer routing information among routing databases.

In the following example, 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:

Router(config)# router eigrp 1 
Router(config-router)# network 10.0.0.0 
Router(config-router)# exit 
Router(config)# router eigrp 101 
Router(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):

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

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

Router(config)# access-list 3 permit 192.168.7.0 
Router(config)# ! 
Router(config)# router eigrp 1 
Router(config-router)# redistribute eigrp 101 
Router(config-router)# distribute-list 3 out eigrp 101 
Router(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 how a router in autonomous system 1 is configured to run both RIP and EIGRP. The example advertises EIGRP-derived routes using RIP and assigns the EIGRP-derived routes a RIP metric of 10.

Router(config)# router rip 
Router(config-router)# default-metric 10 
Router(config-router)# redistribute eigrp 1 
Router(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 below. The following example shows how to redistribute all OSPF routes into EIGRP:

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

The following example shows how to redistribute RIP routes, with a hop count equal to 1, into OSPF. These routes will be redistributed into OSPF as external LSAs with a metric of 5, metric-type of type 1, and a tag equal to 1.

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

The following example shows how to redistribute OSPF learned routes with tag 7 as a RIP metric of 15:

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

The following example shows how to redistribute OSPF intra-area and inter-area routes with next-hop routers on serial interface 0/0 into BGP with an INTER_AS metric of 5:

Router(config)# router bgp 50000 
Router(config-router)# redistribute ospf 1 route-map 10 
Router(config-router)# exit 
Router(config)# route-map 10 permit 
Router(config-route-map)# match route-type internal 
Router(config-route-map)# match interface serial 0 
Router(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.

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

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

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

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

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

In OSPF, hello packets are not sent on an interface that is specified as passive. Hence, the router will not be able to discover any neighbors, and none of the OSPF neighbors will be able to see the router on that network. In effect, this interface will appear as a stub network to the OSPF domain. This configuration is useful if you want to import routes associated with a connected network into the OSPF domain without any OSPF activity on that interface.

The passive-interface router configuration command is typically used when the wildcard specification on the network router configuration command configures more interfaces than is desirable. The following configuration causes OSPF to run on all subnets of 172.18.0.0:

Router(config)# interface Ethernet 0/0 
Router(config-if)# ip address 172.18.1.1 255.255.255.0 
Router(config-if)# exit 
Router(config)# interface Ethernet 1/0 
Router(config-if)# ip address 172.18.2.1 255.255.255.0 
Router(config-if)# exit 
Router(config)# interface Ethernet 2/0 
Router(config-if)# ip address 172.18.3.1 255.255.255.0 
Router(config-if)# exit 
Router(config)# router ospf 1 
Router(config-router)# network 172.18.0.0 0.0.255.255 area 0 

Router(config-router)# exit 

If you do not want OSPF to run on 172.18.3.0, enter the following commands:

Router(config)# router ospf 1 
Router(config-router)# network 172.18.0.0 0.0.255.255 area 0 
Router(config-router)# passive-interface Ethernet 2 
Router(config-router)# exit 

The following example provides two sources with equal access to two different service providers. Packets that arrive on asynchronous interface 1/0/0 from the source 10.1.1.1 are sent to the device at 172.16.6.6 if the device has no explicit route for the destination of the packet. Packets that arrive from the source 172.17.2.2 are sent to the device at 192.168.7.7 if the device has no explicit route for the destination of the packet. All other packets for which the device has no explicit route to the destination are discarded.

Device(config)# access-list 1 permit ip 10.1.1.1 
Device(config)# access-list 2 permit ip 172.17.2.2 
Device(config)# interface async 1/0/0 
Device(config-if)# ip policy route-map equal-access 
Device(config-if)# exit 
Device(config)# route-map equal-access permit 10 
Device(config-route-map)# match ip address 1 
Device(config-route-map)# set ip default next-hop 172.16.6.6 
Device(config-route-map)# exit 
Device(config)# route-map equal-access permit 20 
Device(config-route-map)# match ip address 2 
Device(config-route-map)# set ip default next-hop 192.168.7.7 
Device(config-route-map)# exit 
Device(config)# route-map equal-access permit 30 
Device(config-route-map)# set default interface null 0 
Device(config-route-map)# exit 

The following example shows how to configure policy routing with Cisco Express Forwarding. The route is configured to verify that the next hop 10.0.0.8 of the route map named test is a Cisco Discovery Protocol neighbor before the device tries to policy-route to it.

Device(config)# ip cef 
Device(config)# interface GigabitEthernet 0/0/1 
Device(config-if)# ip route-cache flow 
Device(config-if)# ip policy route-map test 
Device(config-if)# exit 
Device(config)# route-map test permit 10 
Device(config-route-map)# match ip address 1 
Device(config-route-map)# set ip precedence priority 
Device(config-route-map)# set ip next-hop 10.0.0.8 
Device(config-route-map)# set ip next-hop verify-availability 
Device(config-route-map)# exit 
Device(config)# route-map test permit 20 
Device(config-route-map)# match ip address 101 
Device(config-route-map)# set interface Ethernet 0/0/3 
Device(config-route-map)# set ip tos max-throughput 
Device(config-route-map)# exit
 

The following example shows how to create route maps to match access lists, Border Gateway Protocol (BGP) community lists, and BGP autonomous system paths, and apply IP precedence to routes learned from neighbors.

In the figure below, Device A learns routes from autonomous system 10 and autonomous system 60. The quality of service (QoS) policy is applied to all packets that match defined route maps. Any packets from Device A to autonomous system 10 or autonomous system 60 are sent the appropriate QoS policy, as the numbered steps in the figure indicate.

Device A Configuration

interface serial 5/0/0/1:0
ip address 10.28.38.2 255.255.255.0
bgp-policy destination ip-prec-map
no ip mroute-cache
no cdp enable
frame-relay interface-dlci 20 IETF
router bgp 30
 table-map precedence-map
 neighbor 10.20.20.1 remote-as 10
 neighbor 10.20.20.1 send-community
!
ip bgp-community new-format
!
! Match community 1 and set the IP precedence to priority
route-map precedence-map permit 10
 match community 1
 set ip precedence priority
!
! Match community 2 and set the IP precedence to immediate
route-map precedence-map permit 20
 match community 2
 set ip precedence immediate
!
! Match community 3 and set the IP precedence to flash
route-map precedence-map permit 30
 match community 3
 set ip precedence flash
!
! Match community 4 and set the IP precedence to flash-override
route-map precedence-map permit 40
 match community 4
 set ip precedence flash-override
!
! Match community 5 and set the IP precedence to critical
route-map precedence-map permit 50
 match community 5
 set ip precedence critical
!
! Match community 6 and set the IP precedence to internet
route-map precedence-map permit 60
 match community 6
 set ip precedence internet
!
! Match community 7 and set the IP precedence to network
route-map precedence-map permit 70
 match community 7
 set ip precedence network
!
! Match ip address access list 69 or match autonomous system path 1
! and set the IP precedence to critical
route-map precedence-map permit 75
 match ip address 69
 match as-path 1
 set ip precedence critical
!
! For everything else, set the IP precedence to routine
route-map precedence-map permit 80
 set ip precedence routine
!
! Define community lists 
ip community-list 1 permit 60:1
ip community-list 2 permit 60:2
ip community-list 3 permit 60:3
ip community-list 4 permit 60:4
ip community-list 5 permit 60:5
ip community-list 6 permit 60:6
ip community-list 7 permit 60:7
!
! Define the AS path
ip as-path access-list 1 permit ^10_60
!
! Define the access list
access-list 69 permit 10.69.0.0

Device B Configuration

router bgp 10
 neighbor 10.30.30.1 remote-as 30
 neighbor 10.30.30.1 send-community
 neighbor 10.30.30.1 route-map send_community out
!
ip bgp-community new-format
!
! Match prefix 10 and set community to 60:1
route-map send_community permit 10
 match ip address 10
 set community 60:1
!
! Match prefix 20 and set community to 60:2
route-map send_community permit 20
 match ip address 20
 set community 60:2
!
! Match prefix 30 and set community to 60:3
route-map send_community permit 30
 match ip address 30
 set community 60:3
!
! Match prefix 40 and set community to 60:4
route-map send_community permit 40
 match ip address 40
 set community 60:4
!
! Match prefix 50 and set community to 60:5
route-map send_community permit 50
 match ip address 50
 set community 60:5
!
! Match prefix 60 and set community to 60:6
route-map send_community permit 60
 match ip address 60
 set community 60:6
!
! Match prefix 70 and set community to 60:7
route-map send_community permit 70
 match ip address 70
 set community 60:7
!
! For all others, set community to 60:8
route-map send_community permit 80
 set community 60:8
!
! Define access lists
access-list 10 permit 10.61.0.0
access-list 20 permit 10.62.0.0
access-list 30 permit 10.63.0.0
access-list 40 permit 10.64.0.0
access-list 50 permit 10.65.0.0
access-list 60 permit 10.66.0.0
access-list 70 permit 10.67.0.0

The following example shows how to configure a key chain named kc1. In this example, the software will always accept and send ks1 as a valid key. The key ks2 will be accepted from 1:30 p.m. to 3:30 p.m. and be 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 router.

Router(config)# interface Ethernet 0/0 
Router(config-if)# ip rip authentication key-chain kc1 
Router(config-if)# ip rip authentication mode md5 
Router(config-if)# exit 
Router(config)# router rip
Router(config-router)# network 172.19.0.0 
Router(config-router)# version 2 
Router(config-router)# exit 
Router(config)# key chain kc1 
Router(config-keychain)# key 1 
Router(config-keychain-key)# key-string ks1 
Router(config-keychain-key)# key 2 
Router(config-keychain-key)# key-string ks2 
Router(config-keychain-key)# accept-lifetime 13:30:00 Jan 25 2005 duration 7200 
Router(config-keychain-key)# send-lifetime 14:00:00 Jan 25 2005 duration 3600  
Router(config-keychain-key)# key 3 
Router(config-keychain-key)# key-string ks3 
Router(config-keychain-key)# accept-lifetime 14:30:00 Jan 25 2005 duration 7200 
Router(config-keychain-key)# send-lifetime 15:00:00 Jan 25 2005 duration 3600 
Router(config-keychain-key)# exit