BackboneFast: RLQ PDU

Root Link Query Protocol Data Unit:

There are two forms of RLQ PDU, RLQ Requests and RLQ Responses

RLQ Requests are sent out on ports where BPDUs are usually received, so that it can check that the switch is still connected to the Root on that port.   The outbound RLQ Requests defines what Root bridge that switch currently has in its saved BPDU and once it’s sent out a RLQ response comes back with a root bridge that can be accessed through that port.  If the two roots are the same connectivity is still good, if it isn’t that means a failure has occurred.

A switch receiving an RLQ Request immediately answers if it is aware it has lost connection to the root that’s queried because it will have a root bridge different than the one found in the RLQ PDU.

RLQ Responses are flooded on designated ports, the sender of the RLQ Request also include its own bridge ID in the PDU, this allows the sender to confirm that it’s received it’s own response to its request.

The RLQ PDU uses the same packet structure as a normal STP BPDU however there are two different cisco specific SNAP addresses used, one for the request and one for the reply.

rlqbpdu2.PNG

BackboneFast must be configured on all switches in the network in order to process these kinds of PDUs.

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