Vpc and pvst+

How are vpc and pvst reconciled? Normally, a switch sends a BPDU toward the root bridge, but with vpc, both uplinks are supposed to be active....so what then? Does the STP secondary have to use the crisslink to send to the root?

"In any STP environment the root bridge is only so a loop free  topology can be created. Once the network is stable it does not mean  that all traffic must go via the root bridge. It can take the shortest  path to the destination.
This doesn't just apply to Nexus, it is with all switches."
Without spending cycles over thinking this statement and trying to find a scenario that matches, let me concede that youre right, but this is not typical. In 99% of environemnts in a data center, you have a very typical multi-tiered architecture. You know...the usual stuff... access switches dual homed (typically, not always, I know) to a pair of distro switches that may or may not act as the L2/L3 boundary. The L3 boundary at distro is what Im thinking of now.
[EDIT - 10 minutes later] - Just thought of a scneario that describes what youre saying....an access switch dual homed to a pair of distros which are then uplinked to a pair of cores which act as root bridges for all the vlans as well as the L3 boundary. A host is hanging off of one of the distros. The frame will head in the direction of the root bridge in the core BUT get diverted to the distro switch that the host is connected to. [EDIT END]
"So lets say you had two access switches connected with vPCs to the Nexus  pair. If a device on one access switch in vlan 10 sends a packet to  another device in vlan 10 on the second switch then the access switch  would pick one of the etherchannel ports, send it to one of the Nexus  switches. Lets say this Nexus switch is not the STP root. The Nexus  switch should then send it direct to the second access switch ie. no  need to go via the root bridge. "
OK, what if the destination host is connected to the root bridge for vlan 10...you know, one of the vPC switches in the pair? The access switch may send the frame on one of its port channel ports to the non-root, BUT then the frame would have to traverse the crosslink, not because the root bridge is the root but because the final destination is connected to the root and is accessible to the non-root through the crosslink. Yes?
On the other hand, in a non-vPC setup, the frame would have gone to the root brodge immediately. The observation here is that there is an element of path inefficiency with vPC.

Similar Messages

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