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Connecting routers to dual "core" switches

At the core of my network I have pair of Catalyst 6509s with Sup720s. These are configured with a number of VLANs spread across both switches, using VRRP for fail-over. Servers with dual NICs are connected to both switches, providing a reasonable degree of redundancy. Likewise, my "edge" switches have dual dot1q trunk uplinks to both switches. PVST is configured. The idea is that the loss of one switch won't bring the whole network down. So far, so good.

What I now want to do is connect some new 3845 routers in a similarly redundant way. I want to connect Gi0/0 to a port on Switch1, and Gi0/1 to Switch2, so that both switches have a direct connection to the router (which then handles a T3 link to another office, for example).

Initially I thought I could just configure a port on the 6509 with an IP address and make point to point links to the 3845, and let OSPF worry about the routing - but the Sup720 doesn't let me do that with a switch port. I considered making the links dot1q trunks, but then where would I set IP addresses on the router? At the subinterface level? When I try that, it complains about overlapping networks on different interfaces.

I'm looking for some sample configs or "best practice" advice, or specific pointers to documents on that might explain how I should go about this. Unfortunately searches (on or google) return just Too Much Information - it's like trying to drink from a fire hose. Any pointers would be appreciated!


Re: Connecting routers to dual "core" switches

Hi Mcreilly,

You should be able to assign an ip address on cat6k sup720 if you are running native ios on sup 720.

If you are running catos then you will not be able to do that and you can have it configured as trunk and connect to the router.Also I do not think that you need subinterfaces on router and trunk on switch because your cat6k with sup720 must be doing intervlan routing between vlans.

You can just connect it on some port on any vlan and same subnet ip address which you have it on msfc for that vlan you can assign on the router interface and anybody want to go out via t3 link will get routes on sup720 and move out via router vlan.

For suppose you do not want the router to be part of existing vlan you can create one vlan on cat6k sup720 and assign one port to that new vlan and connect the royter to that new vlan port and then create logical interface on msfc for that new vlan and assign an ip address range on that logical vlan and same subnet ip address range you can assign on router physical interface.

Any one from other vclan get routed on sup 720 msfc and will move out via the vlan on which you have connected the router.

because you have only one router you will not be able to maintain box level redundancy by which i mean if the router goes down t3 will be unreachable.



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