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OSPF Across Trunks

Greetings!

I am beginning to implement OSPF into the network I work on. I have a network which consists of multiple 4500, 6500 series routers and 3560s.

Each switch has its own set of VLANs. In other words VLAN X on one switch is not the same as VLAN X on another switch.

I had envisioned connecting each switch via trunk links and then routing via the loopback addresses on each device. That didn't work.

Then I created a VLAN 100 on each switch. Each one was configured with an IP in the same subnet range (10.3.0.1 and 10.3.0.2). I added these addresses to the OSPF process in area 0.

It worked. However, this is not desired.

I have not seen many examples of 4500 layer 3 switches configured with OSPF. The examples I have seen show the interfaces configured with IPs.

ANy help or pointers would greatly help. If you need more info let me know.

Thanks!

3 REPLIES
Silver

OSPF Across Trunks

Rachid Chaoua wrote:

Greetings!

I am beginning to implement OSPF into the network I work on. I have a network which consists of multiple 4500, 6500 series routers and 3560s.

Each switch has its own set of VLANs. In other words VLAN X on one switch is not the same as VLAN X on another switch.

I had envisioned connecting each switch via trunk links and then routing via the loopback addresses on each device. That didn't work.

Then I created a VLAN 100 on each switch. Each one was configured with an IP in the same subnet range (10.3.0.1 and 10.3.0.2). I added these addresses to the OSPF process in area 0.

It worked. However, this is not desired.

I have not seen many examples of 4500 layer 3 switches configured with OSPF. The examples I have seen show the interfaces configured with IPs.

ANy help or pointers would greatly help. If you need more info let me know.

Thanks!

The way you describe your VLAn setup (using the same VLAN number on a different switch for a different purpose/subnet is what I summise from your description), the only way I could see of doing this would be to either

1) Use a small (/30) routed subnet on the connection between devices, put that subnet into area 0 and then have each individual device redistribute connected into OSPF via the area 0 link.

2) Setup a bunch of virtual links back to a "core" area 0 distributing your isolated networks.

There's no way you can use a trunk link for this the way you have VLAN's setup - trunks assume that VLAN X on one end of the line if VLAN X on the other end of the link - layer 2 conenctivity - and just transfers everything from layer 3 with it - so you'd have conflicting IP address ranges being broadcast into the VLAN's where you have similar numbers on different devices.

You *could* change your VLAN numbers so that each VLAN is unique, and then use trunk links - but that could be more work than you'd want to do (not to mention being more disruptive), depending on the number of devices you have.

Cheers.

New Member

OSPF Across Trunks

Thanks for the reply.

I inherited a network and am attempting to create a more hierarchical structure by setting up a core switching environment.

The switches I am installing are that core. Currently, I have a trunk port setup between one of the new switches and a pre-existing switch.

If it were me I would use VTP and be done with it. I would have everyone be in the same domain. It seems that is not an option  from what I've been told.

However, i am stuck trying to figure out a less than optimal way to handle this.

Silver

OSPF Across Trunks

Rachid Chaoua wrote:

Thanks for the reply.

I inherited a network and am attempting to create a more hierarchical structure by setting up a core switching environment.

The switches I am installing are that core. Currently, I have a trunk port setup between one of the new switches and a pre-existing switch.

If it were me I would use VTP and be done with it. I would have everyone be in the same domain. It seems that is not an option  from what I've been told.

However, i am stuck trying to figure out a less than optimal way to handle this.

Been there, done that (inhereted a less than optimal network design). In fact, I'm sitting in the middle of one now, and running into some similar issues.

Using as trunk doesn't stop you from having an SVI on each end of the trunk - my biggest concern in this case would be to stop the conflicting VLAN's from being carried on the trunk.

Maybe you can implement a two-fold solution - put in your trunk and a new VLAN design using new VLAN numbers, and only carry those on the trunks - then use an SVI on anoter VLAN to carry the OSPF to link with the old setup, and slowly migrate things across.

Sorry I can't be more help. Hope you sort it out!

Cheers.

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