BGP question

[acbennyma]

Any body know this concept, why if I don't add access-list. 129.213.1.0 will be advertised

http://www.cisco.com/en/US/tech/tk365/tk80/technologies_tech_note09186a00800c95bb.shtml#redistribution

Case Study 1

Redistribution

This will cause 129.213.1.0 to be originated by your AS. This is misleading because you are not the source of 129.213.1.0 but AS100 is.

So you would have to use filters to prevent that network from being sourced out by your AS.

The correct configuration would be:

RTC#

router eigrp 10

network 175.220.0.0

redistribute bgp 200

default-metric 1000 100 250 100 1500

router bgp 200

neighbor 1.1.1.1 remote-as 300

neighbor 1.1.1.1 distribute-list 1 out

redistribute eigrp 10

access-list 1 permit 175.220.0.0 0.0.255.255

[a.awan · Accepted solution]

In the case study you have mentioned BGP routes are being redistributed into EIGRP and then EIGRP is being redistributed back into BGP. We call this mutual redistribution and whenever you configure mutual redistribution it is always recommended to couple this with access-lists so you do not run into the issue of routes being redistributed back into the same protocol that is the source of learning those routes.

The author of the case study is highlighting the same issue. The prefix 129.213.1.0 is learned via BGP and is then redistributed into the IGP which is EIGRP in this case. The router is also configured to redistribute EIGRP back into BGP so that internal routes can be advertised via BGP. Since EIGRP is also carrying routes originally learnt via BGP this can cause those routes to be advertised to other BGP neighbors thereby making your AS look like the owner of someone else's prefixes. The use of an access-list while mutual redistribution is configured helps in mitigating this issue.

[Harold Ritter]

In this particular scenario though, the prefix 129.213.1.0/24 would not be redistributed back into BGP because BGP owns this route on RTC (protocol via which the route is installed in the RIB). Generally, mutual redistribution becomes dangerous when you have mutual redistribution points.

Hope this helps,

[Harold Ritter]

Sorry, I meant multiple redistribution points rather than mutual redistribution points.