01-25-2018 03:38 AM - edited 03-08-2019 01:33 PM
Short
MST region boundary towards Rapid-PVST-MEC and Rapid-PVST Synchronization effects on that MEC
Description of topology
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
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
01-25-2018 08:52 AM
Hello Niklas,
your understanding looks like correct even if there is something new in comparison to the following thread by Cisco expert Tallet
see
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
01-25-2018 12:19 PM
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
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