Router's subinterfaces connected to a Layer-3 switch

[Farooq Muhammad]

Hi ,

Can any please help if i connect a Routers subinterfaces to a layer 3 switch's Trunk port . I know it normally works fine with a layer 2 switch or if i make layer 3 switch to act as layer 2.But the problem is i want one of the subinterface to form OSPF relation with one of SVI on this layer 3 switch where as another subinterface on router should be acting normally as a Gateway for users of a a particular vlan existing on the switch with no svi for it.

It should have worked fine but it isn't.Does it require some extra configuration or is it not workable this way.

Regards.

Farooq.

[Jon Marshall]

Farooq

Is there a reason you don't want to use the L3 switch for routing the vlans ?

If you don't then when you say it doesn't work what isn't working ie. does a neighborship not form, do routes not get exchanged etc.

Perhaps you could post the relevant configs from your switch and router ?

Jon

[Richard Burts · Accepted solution]

Farooq

If I am understanding your post correctly you have a layer 3 switch configured with a trunk port which is connected to a router. You have a VLAN on the switch that has no SVI on the switch and want the router to serve as gateway for that VLAN. You have another VLAN on the switch that does have an SVI and you want the SVI to form an OSPF neighbor relationship with the router. Assuming that the VLANs are correctly configured, that the trunk allows the VLANs, that OSPF is supported on the switch and is configured for that SVI, and that the router is correctly configured then it should work.

If it is not working I would check and verify that OSPF is supported on that switch. And then I would look for connectivity issues that might interfere. And then I would look for configuration issues. And I agree with Jon that we need a better understanding of what is not working.

HTH

Rick

[Farooq Muhammad]

Richards

You have exactly put together what i have configured and verified.Now the reason i am using Router to serve this particular vlan is BRAS .This router will act as BRAS for this particular vlan only and will form normal ospf relation with the switch on another subinterface with the SVI on the switch.OSPF does form routes are exchanged but now when i connect a normal user in DSL vlan ( BRAS not yet configured on the router because i want to verify it first for a normal vlan) the pc is not able to ping the G.way on the router subinterface and neither the router is able to ping the pc.I have verified subinterface as up and up. and ospf neighbour adj. full on the other subinterface. The switch port connected to pc is in this vlan 2 and is up.

Thanks for replies.

Farooq.

[Jon Marshall]

Farooq

Is the DSL vlan meant to be routed from the switch or the router ?

Perhaps if you could provide configs and explain exactly what should be able to connect to what we could help more.

Jon

[Richard Burts]

Farooq

Thanks for the additional information. It is helpful but still not enough to understand the full problem. It might be helpful if you would post the output of show ip interface brief and of show ip ospf interface from the router. And the output from the switch of show ip ospf interface, show ip interface brief, show interface status, and show vlan.

It would also be helpful if you would post the router config, especially the subinterface config and the config from the switch for the interfaces, the trunk, and the SVI.

HTH

Rick

[Farooq Muhammad]

I will show  all the output of the above commands when back to office .I understand all these but has any body used the router and layer 3 switch in such a scenario .Forget about for a moment the BRAS i will deal that. At present i am concerned with this scenario only what do you people say ? Shall this layer 3 switch and router work or has some one seen or labed it before ?

Farooq

[Richard Burts]

Farooq

I have not tried this particular combination that I can remember. So I do not speak from experience. But based on my understanding it should be possible to do what you describe between a router and a layer 3 switch - assuming that the routing and OSPF functions are supported on that particular model of switch and the particular version of software that it is running.

HTH

Rick

[Farooq Muhammad]

Thanks for your help.

I labbed it up it was working and will apply on weekend to my existing network. I think there was a mistake of putting the host in incorrect vlan which put me in a doubt about the correctness of the model.

Regards.

Farooq.

[Richard Burts]

Farooq

I am glad that you now see that it would work and that our suggestions about this were helpful. Thank you for using the rating system to mark this question as answered.

HTH

Rick