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New Member

InterVLAN routing issues

The company that I work for has a linked 3750 switch stack that was only working as a Layer 2 switch and sent all Layer 3 to a external 3745 router via sub-interfaces for inter-VLAN routing. Everything worked fine, but a little slow. The subnets were 172.16.x.x/22, 172.17.x.x/22 and 172.18.x.x/22. I added two new subnets (172.16.128.x/24 and 172.16.130.x/24) on the 3745 for eventual migration to shorten the subnets and move servers off of the user VLAN.

This weekend I turned on IP routing on the 3750 switch stack, moved the VLAN IP gateway addresses to VLAN interfaces on the switch stack and got them off of the sub-interfaces on the router, turned on EIGRP, changed the link to the external 3745 to a Layer 3 routed interface and made a few other minor tweaks and route changes to get the network up and running.

But one thing that I am noticing is that the VLAN interfaces are not showing the traffic that should be flowing over those interfaces. Everything is working and no users have reported issues, but I still cannot see traffic on the VLAN interface. Any ideas?

3 REPLIES
Hall of Fame Super Bronze

Re: InterVLAN routing issues

Most of the traffic would be Layer3 fast-switched and wouldn't flow via the SVI.

It goes from ingress port to egress port.

New Member

Re: InterVLAN routing issues

If that's the case, other than looking at it on a port-by-port basis, how I would be able to do any traffic analysis. Also, there doesn't seem to be any speed difference on things like file transfers after the migration. I know the backplane of the switches would be faster than sending InterVLAN traffic off-switch to a router, but that doesn't seem to be the case here.

Is there something i'm missing?

Hall of Fame Super Bronze

Re: InterVLAN routing issues

Traffic analysis will have to be done per port if the ingress and egress port reside on the same switch.

If they reside on different switches, you will analyze the uplink ports.

Tools such as NetFlow are pretty good on these kind of analysis.

You should see speed improvement in a Layer3 switched environment,

if you don't - you've reached the physical port or application limits.

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