Re: Rapid-PVST boundary to MST - P2p Peer(STP)

codha · ‎01-25-2018

Short

MST region boundary towards Rapid-PVST-MEC and Rapid-PVST Synchronization effects on that MEC

Description of topology

VSS01 running rapid-pvst
VSS01 connects to Nexus-dist01-vPC-pair (D01) through one MEC consisting of 4x10Gbps ports
D01 running mst
D01 connects to VSS01 through vPC-MEC as described above
D01 are Spine-switches in a FabricPath Leaf-and-Spine (Spine-and-Leaf egg-chicken-what-comes-first) topology
ACC01 are vPC Access-switches and Leaf's in the FabricPath topology.
Host N is connected to FEXNNN with spanning-tree port type edge [trunk] (Portfast a'la NX-OS)

ASCII topology

 VSS01-1 -- VSS01-2 (L2/L3)
   ||           ||  (MEC)
  D01-1  ----  D01-2 (Nexus 5000 - L2)
   | \            /  |
   |   \        /    |
   |     \     /     |
   |       \ /       | (FabricPath)
   |       / \       |
   |     /     \     |
   |   /         \   |
   | /             \ |
 ACC01-1 -------- ACC01-2 (Nexus 5000 - L2)
   | |            | |  (FabricPath)
   FEX101         FEX201 (Nexus 2000) etc...
      |
  Host N ... etc

Show commands

D01
D01-member-1# sh spanning-tree interface po99

Mst Instance Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
MST0000 Desg FWD 200 128.4194 (vPC) P2p Bound(PVST)
MST0001 Desg FWD 200 128.4194 (vPC) P2p Bound(PVST)

VSS01
VSS01#sh spanning-tree interface po99
Vlan Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0001 Root FWD 500 128.5785 P2p Peer(STP)
VLAN0002 Root FWD 500 128.5785 P2p Peer(STP)
VLAN0003 Root FWD 500 128.5785 P2p Peer(STP)
VLAN0004 Root FWD 500 128.5785 P2p Peer(STP)
VLAN0010 Root FWD 500 128.5785 P2p Peer(STP)
VLAN0011 Root FWD 500 128.5785 P2p Peer(STP)
etc...

Summary

This tells me that the IST in the MST-region is talking PVST towards VSS01 and not Rapid-PVST.

Questions

Does this mean that during a topology change in the Rapid-PVST part of the network Rapid-PVST will transit the MEC towards D01 through CST port-states (Blocking, Listening, Learning, Forwarding) for all VLANs for up to 50s as part of RSTP Synchronization?
Does this mean that during a topology change in the MST-region (On a non-edge port ofcourse) MST will transit the MEC towards VSS01 through CST port-states as part of RSTP Synchronization within MST?

And yes the long-term solution is ofcourse to migrate to one MST-region for the entire datacenter. But that is another project...

Regards!

/Niklas

Giuseppe Larosa · ‎01-25-2018

Hello Niklas,

your understanding looks like correct even if there is something new in comparison to the following thread by Cisco expert Tallet

see

https://supportforums.cisco.com/t5/other-network-infrastructure/rapid-pvst-interacting-with-mstp/m-p/653258#M179656

As you noted once a network starts to use MST interaction between MST and Rapid PVST is only a temporary solution.

And yes you should accept all the slow features of legacy old STP on the link between the VSS and the Nexus.

At the boundary the MST IST interacts with the Rapid PVST instance in vlan 1 (the untagged)

To avoid issues the root bridge has to be on the Nexus for all the Vlans because actually there is no chance to exchange information on the other Vlans.

It would be heavy from a computational point of view to replicate the IST BPDU in each Vlan permitted over the trunk in order to perform the handshake.

What is important with MST is that you can decide the Vlans to MST instance mapping even before the Vlan is created (either manually or in VTP).

VTPv3 can be of help as it provide the capability to carry additional databases in particular the one of MST.

Hope to help

Giuseppe

codha · ‎01-25-2018

Thanks for the reply!

Reading some additional threads regarding this I can conclude:

1. The previous technician who designed this topology were lucky that the Root for all VLANs was the IST within the MST-region and not in the Rapid-PVST-region (Due to RSTP Simulation between the two protocol regions) Peters excellent post in this thread: Migrating from Rapid-PVST+ to MST

2. The answer to question 1. is: "Yes, even though the interface is the Root-port for all VLANs"

3. The answer to question 2. is "Yes!"

/Niklas