Monday, June 7, 2010

OSPF EIGRP BGP dual point mutual redistribution - Part 1

It is still a common design to use IGP in the enterprise core. For a large enterprise, the requirement to integrate OSPF with EIGRP may be the result of mergers and acquisitions.

Although it may look like a CCIE bootcamp lab, mutual redistribution can be even more challenging in a production environment, due to these factors:
-As it will be shown, dual router mutual redistribution can be very tricky
-"Backdoor" paths such as multiple MPLS WAN networks adds more complexity
-Having multiple data centers, and the need to route different traffic in different failure scenarios

As I worked through the issues in a very large enterprise environment, We have seen multiple issues having a chain reaction, making the symptoms very difficult to diagnose. Sometimes, fixing one issue may introduce new ones. Unless we have an absolute crisp grasp of the design, and a systematic approach, the chance of confusion is extremely high.

This is the first of a 3 part series which I thought would be worthwhile to share some basics, and provide a logical breakdown of the interacting issues into separate and manageable pieces.

The much simplified diagram shows dual mutual redistribution points (blue arrow). So what is the issue? with dual router mutual redistribution, feedback can occur in both directions, resulting in inconsistency on the two redistribution points, and sub-optimal routing, even potential routing loops.
Issue 1: EIGRP->OSPF (feedback from OSPF)

Only applies to EIGRP EXT routes, which has a higher AD (170) than OSPF (110). These routes are redistributed into OSPF via R1. R2 learns from R1/OSPF, and prefers it. Therefore R2 will not redistribute these routes from EIGRP to OSPF, resulting in only one path used. Any EIGRP EXT route that doesn’t exist in OSPF will show up on one of the routers as preferring OSPF due to lower admin distance, breaking EIGRP->OSPF redistribution on one router. This means all traffic will be directed to one side.

This issues is resolved by setting EIGRP EXT AD to lower than OSPF (distance eigrp 90 100).

Note there is a side effect, the redistribution router will always prefer EIGRP path (due to lower EIGRP AD). But within OSPF, routes redistributed from EIGRP will have a higher metric (set with redistribution route map), therefore there is no risk of disturbing preference within OSPF.
Setting OSPF EXT AD(distance ospf ext 200) is similar. Both solutions will introduce issue 2, which we will cover in part 2.

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