Subject: DVMRP aggregates.

From: Shahzad Ali <[email-masked]>
Date: Mon, 12 Jul 1999 12:36:42 -0400 (EDT)

Hi,

I had a question regarding aggregates in DVMRP.  When an aggregate is
configured on an interface, the aggregating router starts advertising this
aggregate instead of the more specific routes.  That means that downstream
routers will remove all cache entries based on the more specific routes
that it had alongwith any prunes that it had sent upstream.  But the
aggregator at this point, still has the prune active and so traffic from a
network would not be able to reach the downstream unless the prunes are
grafted back.  

Similar things happen when an aggregate is removed from an interface.
Downstreams will remove all cache entries together with any prunes that
they had sent for these entries.  But the aggregator will still have the
prunes active that it sent for all more specific routes contained within
the aggregate unless these prunes are grafted back.

Example:

Assume an aggreagte of default is configured on RTE B:

	      	Subnet X (sending traffic for group G)
		 |
		 |
		RTE A 
		 |
		 |
	    	RTE B (default advertised to RTE C)
		 |     (prune sent upstream for X)
		 |
		RTE C (Prune sent upstream for 0/0 and group G)

Now, the aggregate is removed:

	      	Subnet X (sending traffic for group G)
		 |
		 |
		RTE A 
		 |
		 |
	    	RTE B (Still has a prune sent upstream for X)
		 |
		 |
		RTE C (doesnt see 0/0 anymore, removes all cache entries
			originating from 0/0 alongwith their prunes)

	RTE C is blackholed.

Is it appropraite in this case for RTE B to send a graft for X upstream?
Or am I missing something?

Cheers.
-Shahzad Ali
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Software Engineer, Ericcson IP Infrastructure.
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