When a switch receives an inferior BPDU, it indicates that a link to which
the switch is not directly connected (an indirect link) has failed (that is,
the designated bridge has lost its connection to the root bridge).
Consider the example involving three switches in a fully meshed topology.
Assume that Switch A is the root bridge. Switch C is blocking on Port P and
Switch B is the designated bridge for link L3.
- If link L1 goes down, Switch B detects the failure and sends BPDUs to
Switch C claiming itself as the new root.
- When C receives this new BPDU from B, it realizes it is inferior to the one
stored for port P and ignores it for the duration of the max_age timer (20
seconds by default).
- After max_age timer has expired, the BPDU stored on Switch C for port P
ages out. The port goes into listening state and Switch C starts sending its
BPDU to Switch B with Switch A as the Root ID.
- As soon as Switch B receives the BPDU from Switch C with the Root ID of
Switch A, then Switch B stops sending its own BID as the Root ID in its
BPDUs.
- After the expiration of the listening and learning states (twice the
fw_delay value), Port P moves to the forwarding and connectivity is restored
for Switch B.