In a typical enterprise WAN environment, carrier MPLS can also be used to carry traffic between sites and data centers. In a resilient architecture, there are multiple paths to the same destination. 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 the illustration, MPLS provides the WAN backup path for direct facilities (OSPF-EIGRP). BGP is used as the dynamic routing protocol through the MPLS cloud.
Recall from part 2, tag 25 is used to indicate routes originated in OSPF, and prevented from feed back from EIGRP back to OSPF. Why is there a third issue with BGP? Because the same route is advertised out of OSPF to MPLS 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. On EIGRP to OSPF redistribution point, the tag filter does not stop feedback from a route learned via BGP. 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.
This is an example of network with the feedback issue. Note update tagged “1979” is sent due to better next hop FD. The end result is a network originated from west coast advertised out MPLS, became advertised back from east coast back on EIGRP, and preventing desired redistribution from OSPF into EIGRP.
East-RTR1#sh ip eigrp top 172.31.44.0 255.255.254.0
EIGRP-IPv4 (AS 100): Topology default(0) entry for 172.31.44.0/23
State is Passive, Query origin flag is 1, 2 Successor(s), FD is 30464
Routing Descriptor Blocks:
10.48.137.101 (GigabitEthernet0/0/1), from 10.48.137.101, Send flag is 0x0
Composite metric is (30464/30208), Route is External
Vector metric:
Minimum bandwidth is 625000 Kbit
Total delay is 1030 microseconds
Reliability is 255/255
Load is 31/255
Minimum MTU is 1500
Hop count is 3
External data:
Originating router is 168.147.152.166
AS number of route is 1
External protocol is OSPF, external metric is 20
Administrator tag is 250 (0x000000FA)
10.48.138.101 (GigabitEthernet0/0/2), from 10.48.138.101, Send flag is 0x0
Composite metric is (30464/30208), Route is External
Vector metric:
Minimum bandwidth is 625000 Kbit
Total delay is 1030 microseconds
Reliability is 255/255
Load is 2/255
Minimum MTU is 1500
Hop count is 3
External data:
Originating router is 168.147.152.166
AS number of route is 1
External protocol is OSPF, external metric is 20
Administrator tag is 250 (0x000000FA)
10.250.32.205 (GigabitEthernet0/2/0), from 10.250.32.205, Send flag is 0x0
Composite metric is (32768/7168), Route is External
Vector metric:
Minimum bandwidth is 625000 Kbit
Total delay is 1120 microseconds
Reliability is 255/255
Load is 2/255
Minimum MTU is 1500
Hop count is 3
External data:
Originating router is 10.250.248.1
AS number of route is 64601
External protocol is BGP, external metric is 0
Administrator tag is 1979 (0x0000369B)
The fix is by also setting tag (25) from routes coming from the west coast data center (identified by originating AS number) on the redistribution point from BGP to EIGRP. These tagged routes can be prevented from “feedback” on the EIGRP link with a route map. The route map can be applied on EIGRP interface distribute-list.
