OSPF Question

[nickgodfrey]

xx4510xx

xx3750Axx xx3750Bxx

xx3560Axx xx3560Bxx xx3560Cxx

I have three 3560 which each have a separate trunk port connected to two 3750?s.

The three 3560?s are in different vlans.

The 3750?s are both connected via layer 3 links to the 4510.

I am using HSRP on the 3750?s.

3560a and 3560b are sending data via 3750a

3560c is sending data via 3750b.

There is no link between the two 3750?s. All works without any problem. I can pull the active trunk links out and the data goes via the alternate 3750.

Now I introduce OSPF and use area0 for both 3750?s and the 4510. Again all seems to work ok but looking at the routing table on the either of the 3750?s I see lots of possible routes to the other 3750?s routed link back to the 4510 all with the same cost. I am thinking that this is happening because OSPF updates are being sent across each 3560 switch to the other 3750.

My questions is. Even though all seems to work ok is this the correct state to leave these switches or have I made a complete hash of things  I did think of using acl?s to prevent OSPF messages traversing the 3560?s

[Aaron Harrison]

Hi Nick

Your theory sounds good to me - what you want to do is limit the interfaces that the 3750s can set up adjacencies over.

If the only links you want to peer over are the 3750 to 4510 links, the what I would do is go into your router ospf process and enter

passive-interface default

no passive-interface

This stops hellos etc being sent out of the other interfaces except the one specified with no-passive interface.

The result should be that each 3750 will peer with the 4510, but not with each other.

In a design like this you wouldn't want them to peer accross the access layer 3560s as this could lead to transit traffic heading accross the access layer at some point.

Regards

Aaron

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[nickgodfrey]

Aaron

Thats just what i was looking for.

Many thanks for your help

Regards

Nick