We have a configuration with two Catalyst switches (3550 and 3560) connected with a IEEE 802.1Q trunk on port Fa0/24. Port Fa0/10 and Fa0/11 on each switch are connected to a WAN acceleration device (normal access ports but assigned to different VLANs: Fa0/10 to VLAN 30 and Fa0/11 to VLAN 31). Because the WAN acceleration device is completely transparent for the STP, it?s the same topology as directly connecting these two ports using a crossover cable ? kind of Inter-VLAN bridging. (see also the attached picture).
First I thought that port Fa0/11 on each switch will go into backup because both ports (Fa0/10 and Fa0/11) are connected to the same Ethernet segment but with PVST+ (or RPVST+) the behaviour is a quite different:
- PVST+ only selects one root bridge for both VLAN 30 and 31 and not a separate one for each VLAN (although a per-VLAN spanning tree is used).
- As you can see from the ?show spanning-tree" commands, VLAN 31 does not use the local bridge as the root bridge but it selects the root bridge of VLAN 30 and uses Fa0/11 as the root port.
- Due to this behaviour, port Fa0/11 on the root bridge (LEFT-BRIDGE-3550) is in forwarding state and only the port Fa0/11 from VLAN 31 on the other switch (RIGHT-BRIDGE-3560) is in blocking state (this port has a manual adjusted port cost of 250000, otherwise the trunk port Fa0/24 will go into blocking for VLAN 31).
So, it seems to be that PVST+ ?merges? the different VLAN instances to create a common spanning tree instance for all VLANs. Unfortunately I couldn`t find any further information about this very special setup and if this is really the expected behaviour of PVST+.
Does anybody has a more detailed explanation why and how PVST+ works in such a configuration.
The root bridge election per VLAN is won by the switch with the lowest root Bridge Identifier (BID). The BID is the bridge priority combined with the switch MAC address. Initially, BPDUs are sent from all switches and contain the BID of each switch and the path cost to reach that switch. This enables the determination of the root bridge and the lowest-cost path to the root. Additional configuration parameters that are carried in BPDUs from the root override those parameters that are locally configured so that the whole network uses consistent timers. For every BPDU that a switch receives from the root, the Catalyst central NMP processes a new BPDU and sends it out with the root information.
The topology then converges through these steps:
A single root bridge is elected for the entire spanning tree domain.
One root port (that faces the root bridge) is elected on every nonroot bridge.
A designated port is elected for BPDU forwarding on every segment.
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