Interesting BGP/IGP interaction problem
I’ve stumbled across a really interesting BGP/IGP problem described by Jeremy Filliben that nicely illustrates the dangers of using more than one IGP in your network. You should read the original post for details, here’s a short summary:
- The same IP prefix is received by two BGP border routers (A and D) and sent to a third IBGP-only router (E).
- E can reach A via OSPF. It can reach D via EIGRP.
- E receives two BGP paths to the target IP prefix from A and D. They are identical, so the IGP metric (taken from the IP routing table) is used as the tie-breaker.
- EIGRP and OSPF metrics are totally incomparable and thus A (reachable via OSPF) is always preferred over D (reachable via EIGRP).
Lesson learned: use a single IGP in your AS (or at least in its BGP core).
"Autonomous System is a set of routers under a single technical administration, using an interior gateway protocol and common metrics to route packets within the AS"
However, I doubt everyone pays attention to this defintion until he sees the effect of incomparable metrics in practice :)
This router has 1 interface into OSPF routing protocol, another one into BGP mixed up with EIGRP. It would be a RIB-Failure because of the double-use of 2 IGP + BGP. But it is necesary.
I set up this lab on my GNS3 and it works though It shows a rib failure.
Was just reading the scenerio that u described above ....Have a little confusion ...You said "E" would learn that route from "A" via OSPF and "D" via EIGRP and to choose the best path it would use the METRIC ....Would it not user AD value prior to that ???? *DONT_KNOW*
i second Vandana's comments. Can someone clear on this one since EIGRP Route summary/Internal EIGRP AD is superior than OSPF.