Route Tags - Trying to understand them

[jimmiejoe]

As I study for my CCNP recert, I keep seeing the word "Tag" talked about in documents for multiple protocols. I'm making good use of the Cisco site for most questions, but the concept of tags eludes me.

Are route tags just something in the header that means something to the administrator or the protocol that can be changed? I see where it can be modified using route maps. What are tags, where do they come from, and what are they used for? I'm not looking for a granular explanation, but would like a good understanding.

If someone has a good explanation for tags, I would appreciate it. Even a web doc would be appreciated. I've been covering OSPF and EIGRP in my lab over the last two weeks, and this weekend I go to BGP, where I know I'll see tags.

Thanks in advance,

Jim

[c.lemaire]

Hi Jim,

Tags are mainly used when you redistribute two (or more) routing protocols into each other. Tags are used to avoid routing loops.

You'll find a very good example here : http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009487e.shtml

You may also use tags to help the administrator. For example, a customer of mine redistribute some static routes into OSPF and tag these routes with the OSPF area number. So a simple "show ip route" in the backbone area show the tag and the admin knows immediately in which area the route has been redistributed.

I hope this helps you,

Christophe

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