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Catalyst IOS Layer 3 distribution & Layer 2 core

I have a simple network. Two core 6509 with Sup 720 and three Cat 4506 access/distribution with single gig trunks to both the core 6509s.

I'm running OSPF Layer 3 between Cat4506s and the core switches but just Layer 2 between the two core 6509s. When I fail the gig trunks (on the Sup720) between the two core 6509s or power down one of the core 6509s the whole network appears to go into a death spiral. Even the console port locks out on the Sup720.

Is there something fundamentally wrong with the design? Should I not use the Sup720 gig ports?

3 REPLIES

Re: Catalyst IOS Layer 3 distribution & Layer 2 core

From your problem description I deduced that you have both 6509's interconnected. (just Layer 2 between the two core 6509s.) Is this true? With L3 switching at the distribution layer, each core forms a separate networkpath (= ip network). They should therefore not be interconnected. L3-OSPF will take care of load balancing and failover.

There may be a spanning tree issue when they are connected and this might be what troubles you.

When you connect a 6509 only to the distribution layer, the spanning tree domain is equal to a single 6509 and you will have two separate spantree domains in the network. This avoids hickups due to root and/or topology changes.

Apart from this, it should not occur that the console locks up, you have a software issue as well.

Regards,

Leo

New Member

Re: Catalyst IOS Layer 3 distribution & Layer 2 core

Leo,

Your summation is correct. We have just removed the inter-core Gigabit connections from the Sup720 and inserted onto a core GBIC card and this time successfuly rerouted around the trunk failures. I am not happy about the layer 2 domain spanning the core chassis and would prefer that to be layer 3 or not to exist at all.

Thanks

Re: Catalyst IOS Layer 3 distribution & Layer 2 core

Julian,

In the setup that you have, there is no need for any link between the two 6509 core switches. It should work even better without them as you are excluding spantree issues. Hence my suggestion is that you remove these links completely.

Regards,

Leo

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