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BGP Question NF78 ip route command (typo - please read) - ghjeffe - 04-21-2010

I don't understand why installing a static route to 192.168.12.0/30 would help RouterA advertise prefix 192.168.12.0/22 and if RouterA is running OSPF why wouldn't it have 192.168.12.0/22 in its routing table already anyway?


AS 65500 is not advertising prefix 192.168.12.0/22 to its provider. AS 65500 is running OSPF as its IGP. Which of the following additions to the config is most likely to solve the problem?

Correct Answer: RouterA(config)#ip route 192.168.12.0 255.255.255.252 null0


Thanks, Gary


Re: BGP Question NF78 ip route command - joshuad31 - 04-21-2010

BGP is very picky when it comes to taking network statements and advertising them.  The network statement must EXACTLY match the routes as they appear in the routing table propagated by the IGP (unless synchronization is turned off).  Perhaps Router B is not advertising a summary route but many different singular routes within the range of 192.168.12.0/22,  If this is the case then you would either need network statements for each one or you would need to create a summary on Router A.

We already have a network statement which should inject a route into BGP and advertise it out to the neighbor if it can be found in the routing table propagated by an IGP or a static route.  That is why this is the correct answer.  Without seeing the routing table I can't tell you why the OSPF routes are not being picked up by BGP but its most likely because Router B is not sending them as a summary but instead individual subnets within that range.  If that were true and you didn't want to create the static summary route then you would have to create network statements for each subnet as it appears in the routing table propagated by OSPF.


Re: BGP Question NF78 ip route command - ghjeffe - 04-21-2010

Thanks for your reply.  Understood about the picky network statement part.  I hadn't thought about the possibility of RouterA receiving a different prefix from RouterB.  Duh, on my part.

I'll assume though that the ip route 192.168.12.0 255.255.255.252 null0 command is a typo then and should read ip route 192.168.12.0 255.255.252.0 null0 since we are attempting to advertise 192.168.12.0/22 not 192.168.12.0/30.


Re: BGP Question NF78 ip route command - joshuad31 - 04-21-2010

Yes for sure that is a typo.  You may want to change the subject of the topic to "typo please correct: BGP Question NF78 ip route command" so that it will get the attention of the forum Admin.