You want to tag specific routes to prevent routing loops during mutual redistributing between routing protocols.
You can tag external routes in OSPF by using the redistribute command with the tag keyword:
Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#router ospf 55 Router1(config-router)#redistribute eigrp 11 metric-type 1 subnets tag 67 Router1(config-router)#end Router1#
Route tagging in OSPF is similar to route tagging in RIP Version 2 and EIGRP, which we discussed in Chapter 6 and Chapter 7. Just like those protocols, OSPF doesn't directly use the route tags. But they are useful when distributing routes into foreign routing protocols.
In the example configuration, Router1 is an ASBR that connects to a network that uses EIGRP process number 11. We have configured this router so that it redistributes these EIGRP routes into OSPF as external Type 1 routes with a tag value of 67:
Router5#show ip route 10.2.2.0
Routing entry for 10.2.2.0/30
Known via "ospf 87", distance 110, metric 45
Tag 67, type extern 1
Redistributing via ospf 87
Last update from 172.25.1.5 on Ethernet0, 00:07:14 ago
Routing Descriptor Blocks:
* 172.25.1.5, from 172.25.25.1, 00:07:14 ago, via Ethernet0
Route metric is 45, traffic share count is 1
Router5#
The tags become useful when you redistribute the tagged routes into another network. For example, the following configuration shows how we might redistribute this group of external routes into RIP, but no internal OSPF routes. This sort of configuration is useful if you want to allow the RIP and EIGRP external networks to talk to one another through your OSPF network, but prevent them from seeing your own routing tables:
Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#router rip Router1(config-router)#version 2 Router1(config-router)#redistribute ospf 87 route-map TAGGEDROUTES Router1(config-router)#exit Router1(config)#route-map TAGGEDROUTES permit 10 Router1(config-route-map)#match tag 67 Router1(config-route-map)#exit Router1(config)#route-map TAGGEDROUTES deny 20 Router1(config-route-map)#end Router1#
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