You want to tag specific routes to prevent routing loops while mutually redistributing routes between two routing protocols.
This example shows how to tag external routes in EIGRP:
Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#ip route 0.0.0.0 0.0.0.0 172.25.1.1 Router1(config)#access-list 7 permit 0.0.0.0 Router1(config)#route-map TAGGING permit 10 Router1(config-route-map)#match ip address 7 Router1(config-route-map)#set tag 5 Router1(config-route-map)#exit Router1(config)#router eigrp 55 Router1(config-router)#redistribute static route-map TAGGING Router1(config-router)#end Router1#
The external route is static here, but the same technique applies to routes learned through other routing protocols.
You can only tag routes that EIGRP has learned from another routing protocol. As we saw when talking about route tags with RIP, EIGRP does not use these tags directly, it only distributes them. You would use these tags at network boundaries when redistributing routes into another routing process.
For instance, if our EIGRP network was being used as a transit network between two other routing protocols we could tag routes learned from the first external network. We could then redistribute only the first network's routes into the second external network. Similarly, we could redistribute only the second network's routes into the first network. In this way we could ensure that the external networks use the EIGRP network for only transit, and prevent them from reaching anything internal.
As discussed in Chapter 6, RIP Version 2 supports 16-bit tags, which gives it a range from 0 to 65,535. EIGRP and OSPF use 32-bit tags for a range from 0 to 4,294,967,295. These tags are purely internal, of course, so there is no interoperability problem. It is unlikely that you will need 4 billion tags, but this expanded range can be useful because you can map the 32-bit tags to IP addresses as a mnemonic for the external network information:
Router1#show ip eigrp topology 0.0.0.0
IP-EIGRP (AS 55): Topology entry for 0.0.0.0/0
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 28160
Routing Descriptor Blocks:
0.0.0.0, from Rstatic, Send flag is 0x0
Composite metric is (28160/0), Route is External
Vector metric:
Minimum bandwidth is 100000 Kbit
Total delay is 100 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 0
External data:
Originating router is 172.25.25.1 (this system)
AS number of route is 0
External protocol is Static, external metric is 0
Administrator tag is 5 (0x00000005)
Exterior flag is set
0.0.0.0 (Null0), from 0.0.0.0, Send flag is 0x0
Composite metric is (28160/0), Route is Internal
Vector metric:
Minimum bandwidth is 100000 Kbit
Total delay is 100 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 0
Exterior flag is set
Router1#
The following is a simple example in which we redistribute from EIGRP into OSPF only those routes that have a route tag value of 5. Presumably this tag was set at another network boundary:
Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#router ospf 11 Router1(config-router)#redistribute eigrp 55 route-map TAGOUT Router1(config-router)#exit Router1(config)#route-map TAGOUT permit 10 Router1(config-route-map)#match tag 5 Router1(config-route-map)#route-map TAGOUT deny 20 Router1(config-route-map)#end Router1#
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