Route tagging question

[Karthik Janakiraman]

Hello,

  I am confused with the route tagging between routing protocols. When i was going through different documents, i am not able to understand the tagging information. The configuration shows some tag number when redistributing between the routing protocols. Is it arbitary number picked in random? How this tag is helpful in preventing the loops, since most of the documents have the below format

route-map rip_to_eigrp deny 10

match tag 88

  Route-map statement to deny any routes that have a tag of "88"

           from being redistributed into EIGRP

route-map rip_to_eigrp permit 20

set tag 77

How do you know that the routes learned via rip will have tag of 88 and other non EIGRP routes as 77, unless there are some IP address matching the tag or am i missing some thing. Can any one help me explain or provide me with the links which has good referecence?

Thanks

[Giuseppe Larosa]

Hello Karthik,

route tagging is a useful tool to perform mutual redistribution between two routing protocols specially if multiple border routers are used for mutual redistribution between two routing domains.

The idea is to tag routes differently depending on their original routing domain to avoid re-injection of routing information on different redistribution points (nodes) to original routing domain.

The tag  values to be used are design choices

The logic is the following:

when redistributing RIP routes into EIGRP skip all routes with route-tag = 88 because that value means RIP routes that actually have been imported from the same EIGRP domain ( on the other border node or simply some time ago)

of remaining RIP routes accept them but tag them with value 77.

The design objectives are achieved by using a second route-map that performs a dual action and it is used for importing EIGRP routes into RIP.

route-map red-eigrp-into-rip deny 10

match tag 77

route-map red-eigrp-into-rip permit 20

set tag 88

router rip

redistribute eigrp 100 route-map red-eigrp-into-rip

! seed metric

default-metric 5

!

router eigrp 100

! seed metric required for successful redistribution into EIGRP

default-metric 10000 100 1 255 1500

redistribute rip route-map  rip_to_eigrp

!

the power of route tags is that they automatically adapt to changes: for example if a new prefix is added to EIGRP domain no changes in configuration are needed simply the new route will exist as not tagged route in EIGRP routing domain and as as a RIP route with tag 88 in the RIP routing domain.

Also the same configuration can be applied to both ASBR border routers.

Hope to help

Giuseppe