C3750G VRF Question

[DuncanM2008]

Hello,

I'm looking for some practical advice from people who have used the VRF-Lite functionality on Cisco 3750G's (WS-C3750G-24T-S), I've got a situation currently with a set of 3750's running inter-vlan routing for around 80/90 connected subnets (140+ gig ports).

I'm looking in the coming week at creating a new VRF and enabling OSPF for that VRF while leaving the existing routing arrangements in the Default IP Routing Table.

Has anyone done this before and experienced any issues?

I'm in a situation where I can't replicate the live config into a lab to test the impact / implications of enabling / creating VRF's so therefore I'm seeking the advice of people who will have done this before hopefully

I know the 3750's have a very small support for VRF's (24 if I recall) but I only plan on using 2 or 3 max currently.

Any advice?

Thanks,

[DuncanM2008]

Hello,

Sorry to bump this thread but I'm struggling to find any caveat's to the procedure, anyone else seen any issues with the above?

Thanks,

[Peter Paluch]

Hello Duncan,

I have used VRFs on multilayer switches a couple of times and I did not encounter any difficulties. Certainly, using 3 VRFs should not strain the resources of the switch in any way.

You should take care to observe the following:

• As each OSPF process will need a unique RID, it is best to set an unique RID for each OSPF process using the router-id command manually
• Use the capability vrf-lite command in each OSPF process running in a VRF. This command deactivates certain checks on LSA that are required by PE routers in MPLS networks but that could cause reachability issues in your network.

Apart from this, I am not aware of any other special precautions. Perhaps somebody else will add his/her own experiences here!

Best regards,

Peter

[DuncanM2008]

Hi Peter,

Perfect just the answer I was waiting for .

I'll make sure I force a RID using the OSPF router-id command.

If I'm only using VRF-Lite without any MPLS support i.e. (Not in a PE-CE model) therefore I'm just using to it to virtualise the routing table and run overlapping IP subnets do I need this feature?

Many Thanks for the response and your time.

[Peter Paluch]

Hi Duncan,

I am glad to have helped.

If I'm only using VRF-Lite without any MPLS support i.e. (Not in a PE-CE  model) therefore I'm just using to it to virtualise the routing table  and run overlapping IP subnets do I need this feature?

Yes, you do. In fact, you need to use the capability vrf-lite precisely because you are not doing PE-CE modes with MPLS. You see, without this command, your Cat3750 would consider an ABR between the VRF and the so-called MPLS Superbackbone. That would, among other things, cause it to ignore OSPF inter-area routes coming from directly attached non-backbone areas within the particular VRF.

To put it simply, in your network, it will do no harm, and may be actually necessary (to be absolutely sure, I would need to see the logical topology of your network including the area numbering).

Best regards,

Peter

[DuncanM2008]

Hi Peter,

That's fine I'll enable it as part of the changes I make to ensure I don't encounter any issues, currently OSPF will only run for Area 0 but that's not to say that we wont commision other areas in the near future.

It makes sense the more I read about and I'm glad you brought it up, something else valuable I've learned.

Hopefully should be no IA routes yet

Just O or O E2.

Many thanks!

[Peter Paluch]

Duncan,

It has been a pleasure!

Best regards,

Peter