Back-Back (Double Sided) VPc Confusion

[j.a.m.e.s]

Dear All,

Having recently upgraded a Nexus 5k estate from v5 to v7, I was surprised to find some of the VPC configuration disappeared during the upgrade and spanning-tree was blocking one of the port-channels.

When I tried to recover this by restoring the VPC commands, I saw "ETHPORT-5-IF_DOWN_PORT_CHANNEL_MEMBERS_DOWN" in the logs.

After discussion with TAC, it appears our back-back design was invalid and I guess the new code enforced good behaviour.

Having read up on the back-back design, I'm confused as to how the topology should actually be configured. I've put three diagrams together:

1. What we had working in v5 (1.jpg)

2. What TAC proposed (2.jpg)

3. What I think we should do based on Cisco docs such as this. (3.jpg)

Would folks mind reviewing my diagrams and advising on whether 2.jpg or 3.jpg are supported topologies and what the advantages of either may be?

Thank you very much for any assistance you can offer.

Regards

James. 

[Reza Sharifi]

Hi,

The recommended design for this type of scenario is to put all 4 links between all 4 switches in one vPC/Portchannel.

See link and have a look at design 1, 2 and 3.  Also figure 6, 7 and table-1.

http://www.cisco.com/c/dam/en/us/products/collateral/switches/nexus-7000-series-switches/C07-572830-00_Agg_Dsgn_Config_DG.pdf

HTH

[j.a.m.e.s]

Hi Reza,

I think you're saying the design drawn out in 3.jpg (attached to my original post) is recommended?

I have read the document you reference and I think 3.jpg resembles design 2 (page 7), but Cisco don't provide enough detail for me to be certain. The aspects of Cisco's design 2 diagram that are unclear to me are:

1. Can the 'straight uplinks' and 'crossover uplinks' be port-channels?

2. Does Cisco's blue colouring of the uplinks denote use of a single VPC ID?

I'm slightly surprised that Cisco don't go into more detail about this as it seems like an easy area to go wrong. There is also quite a good config sample over at NetCraftsmen, but this corresponds to design 3 (page 8 in Cisco's doc).

Regards

James.

[Reza Sharifi · Accepted solution]

Hi,

You are correct. The documentations are not very clear and don't provide good examples with configs. I also say the Netcraftsman link, but they are only using 2 links between all 4 switches.

1. Can the 'straight uplinks' and 'crossover uplinks' be port-channels?

Yes, they can.

2. Does Cisco's blue colouring of the uplinks denote use of a single VPC ID?

That is correct. Blue coloring in figure-7 means all links are in one vpc (vpc 51) I also think there is a mistake in figure-6, as it shows both red and blue links are in one vpc (51) where it supposed to be 2 vpcs (one for each color or 51 & 52).

HTH

[j.a.m.e.s]

Thank you for your assistance Reza, I will report back on any issues we have in migrating to this design.

[j.a.m.e.s]

In case it helps others, our SE provided some useful documentation here:

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

Page 50 - 53 shows how it should be done including a configuration sample. This seems to align with my original diagram (3.jpg).