VPC & STP - Layer 2 extension

[sblavanya]

Hi All

We have layer 2 extension between the datacenter as given in diagram

Are the STP port roles given in the diagram correct ? Im not sure why the priority of both N7k's were given the same (4096).. both the N7k's assume that they are the root bridge for this VLAN. This is fine for a switch connecting through VPC since the port channel uses both the root bridges. What will then happen to non-VPC devices connected on the left (towards 6500) ? which N7k will the downstream devices designate as the root ?

Also is there an advantage to configure VPC in N7k end for the links going to 6500 ? 6500 doesnt understand vpc anyways.. what is the advantage of configuring vpc with a non-vpc device?

since im extending the layer 2 between datacenters , do  i need to look for any caveats ?

[Jon Marshall]

Firstly if both switches have the same priority then the lowest mac-address will be elected the root bridge in a normal setup. Are both Nexus switches claiming themselves to be root ?

Are the switches on the left (forming part of the square) 6500s ?

Yes there is an advantage to connecting devices that don't "understand" vPC. Basically you can run an etherchannel from an 6500 switch to both Nexus switches using a vPC on he nexus end. The advantage is that all links from the 6500 will be forwarding ie. STP will not have to block any of the physical links in the etherchannel.

Jon

[sblavanya]

Jon

Yes.. the switch on the left side is 6500.. just normal SUP.. not VSS...

what if the port channels between the nexus & 6500 is not in VPC.. will stp block one whole port channel, even if the links terminate on 2 different switchess ???

are the spanning tree interface status right based on my assumption given in the diagram?

[sblavanya]

Hi All

Had another query... referring back to diagram above :

L2 connectivity end-to-end would work if VLAN 5 is extended with the same IP range (172.16.5.0/24) .. we came up with another requirement here.. the new DC will be having a new subnet (instead of 172.16.5.0/24).. say 10.16.5.0/24

old subnet - 172.16.5.0/24 (VLAN 5)

new subnet - 10.16.5.0/24 (VLAN 5) --> im doing layer 2 extension here

I want to use the same VLAN number (VLAN 5)  to ease administration/management during transition.. Can i use secondary ip address concept here to extend the VLAN between the datacenters with the same VLAN # ....

config ex:

old DC:

int vlan 5

ip address 172.16.5.2 255.255.255.0

ip address 10.16.5.4 255.255.255.0 secondary (dummy IP which will not be the def gw) --> just for local ARP

hsrp configs for 172.16.5.2 network..

no hsrp config for 10.16.5.x network

New DC:

int vlan 5

ip address 172.16.5.4 255.255.255.0 (dummy IP)   --> just for local ARP

ip address 10.16.5.2 255.255.255.0 secondary (real hsrp gateway)

hsrp configs for 10.16.5.x network

no hsrp config for 172.16.x.x network

Complicated, but thought of seeing if this is a possibility