back to back VPC

[haitham.jneid]

Hi Experts,

please check the attached diagram, am confused which N7K should be the STP root bridge and which one will be the secondary root.

also which ports will be in blocking mode and which will be in forwarding mode.

what is the recommended STP config for such design.

Note: same VLANs will be carried between these 2 N7Ks.

thanks,

Haitham

[ahmedshoaib]

Hi;

When you configure back to back vPC, all the links will be in forwarding state b/w vPC domain 1 & vPC domain 3. 

With reference to STP, it's depend on your configuration. If you configure N7K-A in vPC domain 1 will be STP root it's become root or else. It's always better to configure STP root and secondary root instead of automatic selection.

Thanks & Best regards;

[haitham.jneid]

Hi Ahmed,

I know that all links between VPC domains will be up. but what about the peer links in both domains?? are they gonna be both up in domain and 1 and 3?

please if you have a clear answer let me know.

thanks,

Haitham

[ahmedshoaib]

Hi;

Both peer-link (in vPC domin 1 & 3) will also be up and working fine. There will be no issue.

Be remember configure the STP properly.

Thanks & Best regards;

[haitham.jneid]

Hi Ahmed,

Both vpc domains carry the same VLANs over thr vpc links and the peer links as well.

If I make N7K-A stp root primary and N7K-B stp root secondary, am afraid that peer links between C and D will be blocked. 

Did u test this scenario before?

Appreciate ur support bro.

Thanks

[ahmedshoaib]

Hi Haitham;

Yes, I already implement the design which you are looking for. In my case in vPC domain 3, I have pair of Nexus 5548.

Note: Unlike a regular spanning-tree deployment, a vPC deployment can have two root ports.

You can also go through the Cisco document vPC best practices design guide for more detail.

http://www.cisco.com/c/dam/en/us/td/docs/switches/datacenter/sw/design/vpc_design/vpc_best_practices_design_guide.pdf

Thanks & Best regards;