Monday, January 11, 2010
Tuning Route Redistribution using AD
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!