vlan config

[gavin han]

Hi,

I've folowing scenario:

<<<routerA>>>-----L2 connection---<<<routerB>>>----layer3 connection----<<<RouterC>>>

router A and B are nexus 7K switches while router C is 3750Switch. and router B and C have layer 3 connection.

I've vlan 5 on A which is not defined on router B and router C. so I need to span vlan 5.

what would be best way to span it to Router C?

[Eugene Lau]

Hi Gavin,

Just wanted to check what you meant by SPAN and if it means to have VLAN 5 exist across all devices, what is the requirement for that?

From your current description, I wouldnt see a need to change unless were trying to achieve something.

A---- L2 ---- B <<--- can be a point-to-point interface linke B--- L3 --- C

A-- L2 B could be using a VLAN5 SVI to provide that connectivity where as B ---- L3 ---- C simply uses a IP address on the physical interface. The difference is that A-- B would run a L2 protocol such as STP and B--- C would not. Just a design choice as any routing/IP traffic would still be switched in hardware.

HTH.

Eugene

[gavin han]

Hi,

Yes, I meant to have vlan 5 exists across all devices.
since there is layer 2 connectivity between A and B i can do it by creating layer-2 vlan, correct?

but there is a layer-3 connectivity between B and C. so to get vlan 3 on router C, I'll have to create layer-3 inteface for that vlan? and how do I implement routing for vlan 5?

[Eugene Lau · Accepted solution]

Hi Gavin,

Just to ensure we understand each other - the current diagram - the Layer2 domain is partitioned by the L3 link between B-C. Thus, if VLAN 5 is created on the 3750, it has no relationship to the VLAN 5 on the Nexus. If the L3 link between B-C is the only physical connection to the Nexus, then this statement would stand true for all VLAN's created on the 3750.

So in A--- L2 --- B --- L3 --- C - if you created VLAN 3 on 3750, it would be isolated from A,B.

Let me use an example with IP address - keeping it simple we will assume no management domains/vpc's etc, just basic L2 and L3.

A VLAN 5 1.1.1.1/30 -----L2  ----- VLAN 5 1.1.1.2/30 B L3 2.2.2.2/30 ---------- L3 2.2.2.1/30 C ----- VLAN 3 10.x.x.x/24

In this case, you could static route or use a routing protocol to ensure that A knows about the 10.x.x.x network on the 3750. 3750 would need to know about the 1.1.1.x network on Nexus. Sames goes for any other IP subnets that exist.

Starting point:

http://www.cisco.com/en/US/docs/switches/datacenter/sw/5_x/nx-os/interfaces/configuration/guide/if_layer3int.html#wpxref79308

Some configuration examples:

http://www.cisco.com/en/US/products/ps9402/prod_configuration_examples_list.html

As you state by design VLAN 5 should be across A,B and C - basically in the same subnet, then C will need to be connected into the A-B topology by Layer 2. Essentially VLAN 5 is in the same "domain" across A,B,C.

Ultimately, it depends what the design needs to achieve.

HTH

Eugene.

[gavin han]

Thanks for your reply.

My ultimate goal is to that VLAN5 would belong to same subnet.

i.e.

A(vlan5-192.168.1.0/24)---Layer2---B(vlan5-192.168.1.0/24)---layer3---C(vlan5-192.168.1.0/24)

there has been be only layer-3 connection (no layer-2 connection) between B and C.

[Eugene Lau · Accepted solution]

OK, great!

If that's your ultimate goal, then converting B---- C into a Layer2 trunk link will achieve that. You're on your way to planning a migration!

[gavin han]

Thanks. but link between B and C has to be layer-3 only