OSPF_ Discard routes

[feroz syed]

i have directly connected three routers scenario, (R3---R2---R1),router 3 advertise it's loopback and direclty connected int via ospf and same thing on r1 and r2.

R2 is ABR here, R3 advertise 3.3.3.3/32 loopback and 192.168.23.0 network to R2, R2  want to send summariz 3.3.3.3/32 network as 3.0.0.0/8 to R3. aslo r2 don't doesn't add a null0 interface. we can configure No Discard Routes on R2 to avoid installing null interface But why & what is the reason for avoid installing null interface.

Any help with this would be greatly appreciated

[daniel.dib]

Hi,

Installing a discard route can keep the router from using shorter prefixes such as the default route to reach prefixes.

See this thread which explains the behavior:

https://supportforums.cisco.com/thread/2253071

Daniel Dib

CCIE #37149

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[feroz syed]

i already came from that link, But Removing Null0 will create routing loops in network.

[daniel.dib · Accepted solution]

It could create routing loops. Cisco does not prevent oneself from shooting themself in the foot. As with any commands you need to know what it does and what are the consequences of enabling it.

The discard route is not any concern unless you have networks part of the summary that are reachable via a lesser prefix such as default route.

To prevent from this situation you should not summarize a larger prefix than the networks that are behind the summary. That would prevent the discard route from causing any issues.

Daniel Dib
CCIE #37149

Please rate helpful posts.