Note update tagged “1979” is sent due to better next hop FD. This is an example of network with the feedback issue. As long as east coast has a feasible successor (one with metric lower than current best FD), then this route will be advertised to west coast, with EIGRP distance of 100, thus preventing redistribution. On EIGRP to OSPF redistribution point, the tag filter does not stop feedback from a route learned via BGP. The data center running EIGRP will also learn the same route from MPLS cloud as a BGP route, in this case not tagged. Why is there a third issue with BGP? Because the same route is advertised out of OSPF to MPLS via BGP. Recall from part 2, tag 25 is used to indicate routes originated in OSPF, and prevented from feed back from EIGRP back to OSPF. BGP is used as the dynamic routing protocol through the MPLS cloud. In the illustration, MPLS provides the WAN backup path for direct facilities (OSPF-EIGRP). The business requirement may be such that certain traffic should take one type of link as its primary path, while still having a backup path in case of failure. In a resilient architecture, there are multiple paths to the same destination. In a typical enterprise WAN environment, carrier MPLS can also be used to carry traffic between sites and data centers. We use administrative distance to control preference, and use tags to prevent loops. In part 1 and part 2 of the post, we mainly focused on OSPF and EIGRP mutual redistribution.
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