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Friday, January 31, 2014

How is the cisco devices generate a default route in OSPF domain?


What are the different ways to generate a default route in OSPF domain and what are the configuration steps involved?

Cisco device uses “default-information originate” statement to generate a default route in OSPF domain. There are two ways to originate default route by using this command:
1) When you configure “default-information originate”  under OSPF process  without any argument after this statement, OSPF process will  first check,if any default route is already present in routing table. If a default route is already present in routing table via static or any dynamic protocol, OSPF originate default route. If default route does not exist in routing table, OSPF will not originate default route.
2)This behavior can be modified by adding "always" argument to the "default-infomation orginate" statement,which essentially skips the checking for a default route already being installed in the table.

This document discuss about conditional route origination in OSPF domain. Conditional default route in OSPF originate by using route-map in default-information command under router OSPF process. The route map configured in the default-information originates command check the existing IP prefixes in the IP routing table.

Configuration steps:

1) prefix-list or access-list: To originate default route only when necessary prefixes are present in routing table.First you need to match these prefixes by configuring prefix-list or access-list.
Example:

Prefix-list configuration:

Router (config)#ip prefix-list Default_route sequence 10 permit 192.168.1.0/24

Access-list configuration:

Router(config)#ip access-list standard Default_route
Router(config-std-nacl)# permit 192.168.1.0 0.0.0.255


2) Attach prefix-list or access-list in route map:

Example:

Router(config)#route-map Ospf_default permit 10
Router(config-route-map)#match ip address prefix-list Default_route
Router(config-route-map)#exit

3) Configure “default-information originate always route-map” statement:

Example:

Router(config)#router ospf 100
Router(config-router)#default-information originate always route-map Ospf_default 

Background:


In the below topology R1 is connected to two ISP using serial interface. The router R1 and R2 runs OSPF in Area 0, where R1 is advertising default route to R2 only when both serial links are up. Once both link goes down R1 should withdraw default route from OSPF domain.

Topology Diagram:

OSPF_CONDITION.jpg


Configuring R1:

R1(config)#ip prefix-list default_route sequence 10 permit 192.168.1.0/24
R1(config)#ip prefix-list default_route sequence 20 permit 192.168.2.0/24
R1(config)#route-map ospf_default permit 10
R1(config-route-map)#match ip add prefix-list default_route
R1(config-route-map)#exit
R1(config)#router ospf 100
R1(config-router)#default-information originate always route-map ospf_default
R1(config-router)#exit

R1 advertise a default route to OSPF domain, as both ISP links are up.

Verifying default route on R2

R2#sh ip ospf database | in 0.0.0.0
0.0.0.0         192.168.2.1     371         0x80000001 0x0018CA 100
R2#sh ip route ospf
O*E2 0.0.0.0/0 [110/1] via 10.1.1.1, 00:06:22, FastEthernet0/0

We need at least one prefix match in list present in R1’s routing table to advertise default route in OSPF domain.

Disabling one link towards ISP1:

R1(config)#int s0/0
R1(config-if)#sh
*Mar  1 00:17:09.971: %LINK-5-CHANGED: Interface Serial0/0, changed state to administratively down
*Mar  1 00:17:10.971: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to down

Verification on R2:

R2#sh ip route ospf
O*E2 0.0.0.0/0 [110/1] via 10.1.1.1, 00:10:30, FastEthernet0/0

Disabling remaining link towards ISP2:

Verification on R2:

R1(config-if)#int s0/1
R1(config-if)#sh
R1(config-if)#
*Mar  1 00:18:39.131: %SYS-5-CONFIG_I: Configured from console by console
*Mar  1 00:18:39.731: %LINK-5-CHANGED: Interface Serial0/1, changed state to administratively down


Turned on debug in R2 to get closer view of default route deletion

R2#debug ip routing
IP routing debugging is on
*Mar  1 00:19:25.799: RT: del 0.0.0.0 via 10.1.1.1, ospf metric [110/1]
*Mar  1 00:19:25.803: RT: delete network route to 0.0.0.0
*Mar  1 00:19:25.803: RT: NET-RED 0.0.0.0/0
*Mar  1 00:19:25.803: RT: NET-RED 0.0.0.0/0
R2#sh ip route ospf
R2#

From the above output it is clear that when the both links connected to ISP goes down, router R1 stop generation of default route to OSPF domain.

Citation - This blog post does not reflect original content from the author. Rather it summarizes content that are relevant to the topic from different sources in the web. The sources might include any online discussion boards, forums, websites and others.

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