Today I went through a quick study session and lab that shows how administrative distance can cause suboptimal routing when using route redistribution. I tested this with a lab that was redistributing RIP and OSPF both ways. Since OSPF has the default AD of 110 and RIP has an AD of 120, the ASBR routers redistributing chose the OSPF routes even when the RIP routes were closer. To modify this I had to create an access-list with all the possible RIP routes and assign it the AD of 125 on the two ASBR routers and inject the access-list into the OSPF routes. Now when the ASBR's received a RIP route from OSPF (due to redistribution) it automatically had an AD value of 125 which was larger than the default RIP AD 120. The RIP routes were installed into the ASBR's routing table due to having a lower AD value of 120!
Instead of configuring an access-list you can modifyt the AD of the OSPF external route to be higher than RIP :)
ReplyDeleteuse
ro 1
distance ospf external 175
You're exactly right, a lot simplier the K.I.S.S. method!I went with the access-list route just to gain a little understanding of how you can fine tune routes within your network
ReplyDeleteHow did the ASBRs which were advertising RIP routes into OSPF get the same routes back in on the same interface?
ReplyDeleteI have tried creating a routing loop in GNS3 using RIP and OSPF (using only 1 ASBR) but I couldn't create a routing loop!
Am I mising something - I guess I am - I just dont know what it is..