OSPF: Point-to-Multipoint & Blocking /32 Host Routes

Unanswered Question
Feb 12th, 2008

http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6586/ps6660/ps6808/prod_white_paper0900aecd8055c34e.pdf

This URL suggest that blocking /32 host routes in a point-to-multipoint environment is possible using filtering. I've tested it using the correct code but it's not successful.

I just want a sanity check - anyone?

"If you need to block the OSPF /32 routes, you can add the following on all hub and spoke routers:

router ospf <#>

...

distribute-list prefix-list Block-32 out (block OSPF/32 connected routes)

ip prefix-list Block-32 deny <tunnel-subnet> <mask> ge 32

ip prefix-list Block-32 permit any le 32

● Use Cisco IOS Software Release 12.4(9)T or later instead of 12.4(6)T. This is because we need to have a fix to NHRP so that it

won't be confused by the Cisco IOS OSPF behavior of installing /32 routes for the directly connected routers on the GRE tunnel

interface. Otherwise use the technique above to have OSPF block adding these /32 routes to the routing table if you need to run IOS

code between 12.4(6)T and 12.4(9)T."

tia

Ajaz

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Mohamed Sobair Tue, 02/12/2008 - 22:51

Hi Ajaz,

distribute-list should block the route from being installed in the routing table.

here is example:

router ospf x

distribute-list 10 in

access-list 10 deny x.x.x.x

access-list 10 permit any

HTH

Mohamed

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