Wednesday, June 9, 2010

OSPF EIGRP BGP dual point mutual redistribution - Part 2

In part 1 of the post, we solved feedback on OSPF side, but issue in the opposite direction still exists. OSPF routes redistributed into EIGRP on R1, R2 learns from R1/EIGRP and prefers it (since we have lowered EIGRP EXT AD to fix issue 1). Because R2 now prefers the feedback route, it will not redistribute these OSPF routes to EIGRP.

First, between R1 and R2, stop EIGRP advertising routes redistributed from OSPF.
router eigrp 1
  distance eigrp 90 100
  distribute-list route-map Deny_routes_from_OSPF in

route-map Deny_routes_from_OSPF deny 10
  match tag [ospf tags]

As best practice, also use tag to stop the feedback to redistribution points, by blocking OSPF originated routes from redistributing back into OSPF.
route-map redist_EIGRP-to-OSPF deny 10
 match tag [ospf tags]
route-map redist_EIGRP-to-OSPF permit 20

What about routes learned from MPLS? Those are typically remote offices, which we don’t want to be redistributed from OSPF.

See an MPLS remote network EIGRP side gets it from BGP as EIGRP EXT. OSPF side also gets it from BGP. Could redistributing router prefer EIGRP (since we set lower AD)? The router has two paths (left and right) to go out to the WAN to reach the remote site. We would want it to use the “local” MPLS exit point, in this case is through OSPF.

This is where EIGRP metric becomes important. Note EIGRP must have at least default metric for redistribution to happen. It is learning the same route from both BGP and OSPF. At the point of redistribution, the metric assigned to routes redistributed from OSPF to EIGRP should be lower than EIGRP metric going the other way (typically by setting lower bandwidth on WAN links). As a result, the redistribution router prefers the locally redistributed routes from OSPF.

Redistribution-rtr-1#sh ip eigrp top
EIGRP-IPv4 (AS 100): Topology default(0) entry for
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 4352
Routing Descriptor Blocks:, from Redistributed, Send flag is 0x0
Composite metric is (4352/0), Route is External
Vector metric:
Minimum bandwidth is 625000 Kbit
Total delay is 10 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 0
External data:
Originating router is (this system)
AS number of route is 1
External protocol is OSPF, external metric is 7
Administrator tag is 200 (0x000000C8)
But MPLS does cause additional feedback issue, which we will cover in part 3.

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