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Using route tags with IGPs outside redistribution [7:86568] posted 03/30/2004
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I'm struggling with a problem that _ought_ to be soluble. All the 
pieces seem to be present in IOS, it's a perfectly straighforward 
thing to do with BGP, but I can't seem to find -- if it exists -- the 
command in IOS that links the pieces.

Let me abstract for a moment.  A BGP community is an identifier for a 
group of external routes. It is reasonable, and consistent with the 
RFC definitions in such things as RIPv2, that an IGP route tag can be 
considered an identifier for a group of internal routes.  You can, 
indeed, set tag on a ip route statement.

In BGP, I might receive a route on eBGP speaker R1, and set a 
community on that route. Subsequently, on a different router, I can 
have a route map on a neighbor subcommand, that, for example, does a 
match-community and changes the output interface according to the 
match. This works fine.

In IGPs, it works fine to connect a route map, with set/match tag, to 
the routing process with a redistribute subcommand. I have not been 
able to find a way to link such a route map to an IGP process 
_without_ involving redistribution.  If the IGP implementations 
worked at all like BGP, it should be possible to put a route map on a 
neighbor statement, but I haven't been able to find documentation of 
this. Indeed, ip route has a "tag" keyword.

Every example I've seen, however, puts multiple route-maps, tied to 
different redistribte statements, on the router that has the 
redistributing processes on it, and the route map that does the tag 
manipulation gets invoked by the redistribute statement.

What am I missing? Surely I can link a route map containing a set tag 
inside a process, without having to have a redistribute statement. If 
this can't be done (and also have route maps -- maybe policy routing? 
-- check the tag inside the domain), you lose the ability to have 
useful fine-grained routing policy inside an IGP.

If so, do I code the reference to the route map?  Have I missed some 
weird command that puts it under a router major process without an 
associated redistribute command? That's the way _I'd_ implement it.

Maybe there isn't a way to do it, but that certainly limits the 
flexibility of IGPs.




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