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).