I have only created VPCs between a N7K and a N5K. In those cases the N5K also has a VPC domain, peer link and peer keepalive link because it also had VPCs facing "down" toward N2K FEXs. However, I think Cisco TAC told me that the N5Ks would have had to have a VPC domain, peer link a peer keepalive just for the VPC facing the N7K.
Is this true? If that is the case, would I need the same for a VPC that is "controlled" by one N7K VDC and "terminated" on another. In other words, if my distribution VDC has a VPC domain that faces various devices throughout the data center. It is decided to vreate a VPC that faces a pair of downstream server VDCs. Do the server VDCs need a VPC domain, peer link and peer keepalive link so that this VPC fully functions like a VPC split between two N7Ks and two N5Ks?
I would assume that is not necessary because it is possible to VPCs that face non-Nexus devices that have no concept of a VPC domain, etc.
I can see where the confusion comes from. But consider the base that VPC is extending.
A etherchannel allows multiple physical links from the SAME switch to be combined into one logical interface. i.e. a max of TWO switches can participate.
A VPC allows multiple physical links from TWO switches to be combined into one logical interface. When the etherchannel capability span beyond one switch with multiple interfaces to two switches, a VPC is required.
The VPC config would include a VPC domain, Peer-keepalive, peer-link, and the "VPC XXX" under the relevant port-channel that is shared between two switches.
This is why it is possible to use VPC to non-NEXUS switches. Provided the two rules above is followed. i.e. the non-NEXUS switches or even a NEXUS switch can be used (NO VPC) if only local interfaces are used to make up the etherchannel . The other end of the logical port-channel, however may terminate on two N5K or two N7K switches. These devices will have to be setup with the VPC config listed above since the same logical port-channel has physical interface across more than one switch.
Now if you do one port channel with lets say four physical link between 4 switches you would need the VPC setup twice.
N5K1 has two links to N7K1 and N7K2.
N5K2 has two links to N7K1 and N7K2.
All four link can be setup as one logical port-channel. How? by configuring N5K1 and N5K2 in one VPC domain +VPC config, and N7K1 and N7K2 in another VPC domain + config. The rest of the config is the same as before.
Just note you must use unique VPC domains ID between devices pairs in the same Layer2 network, because the VPC domain-ID is used to create a unique VPC system MAC.
Let me make sure I've got what you are saying:
"This is why it is possible to use VPC to non-NEXUS switches. Provided the two rules above is followed. i.e. the non-NEXUS switches or even a NEXUS switch can be used (NO VPC) if only local interfaces are used to make up the etherchannel . The other end of the logical port-channel, however may terminate on two N5K or two N7K switches. These devices will have to be setup with the VPC config listed above since the same logical port-channel has physical interface across more than one switch."
1). The result of making either the non-Nexus switch (or Nexus without VPC config) would be that spanning tree would block a trunk configured between these two devices, correct?
2). If so, does this slow convergence significantly in a RPVST+ design?
3). The Port Channel number on the two non-Nexus switches (or Nexus without VPC config) should be unique? I know they don't have to match end-to-end, but does it matter if they are the same on all devices? Nexus and non-Nexus?
4). What are the ramifications in terms of convergence, etc. when only the Nexus side knows that a VPC is present?
Same rules to STP apply as for Etherchannel.
The physical interfaces are seen as one logical link, so STP will not block the port. no need to.
VPC should have no impact on the RSTP convergence time.
The port channel number between VPC devices must be the same, but does not need be unique within a STP domain.
The VPC domain ID must be unique, for the reason I already mentioned.
Again the VPC configuration should have no impact different on convergence times. The communication between VPC device are sub millisecond unless you spanning a VPC between distant remote locations. But that is a different.
Think basics. Its mostly a etherchannel, which spans up to two devices, with some extras.
Read the following design guides. this will answer all your questions