Spanning Tree Protocol: BackboneFast

BackboneFast is a Cisco proprietary feature that allows all switches on a network to save up to 20 seconds max age time when recovering from indirect link failure.

BackboneFast attempts to get rid of the STP standard of waiting max age seconds for timing out saved BPDUs.  Instead BackboneFast has the ability to detect indirect link failures by tracking inferior BPDUs being sent on designated port when a failure occurs.  It also has a mechanism that allows to it to immediately check if the BPDU information stored on a port is still valid via the Root Link Query PDU (RLQ PDU).

Detecting Link Failures

First the switch detects the inferior BPDU by receiving the new one being sent after a failure occurs and comparing it to the BPDU it already has stored previously.  Instead of ignoring these inferior BPDUs per the STP standard, BackboneFast begins to immediately send RLQ PDUs on all non-designated ports except the port in which the inferior BPDU was received.  Once an RLQ PDU is sent out (request) the port then waits for a RLQ Response.

Once an RLQ response comes back, if the answer is negative the port has lost its connection to the root and its BPDU can be immediately aged out, if all other non-designated ports received a negative answer the whole bridge loses the root and can restart STP calculations.

If the RLQ PDU comes back positive you can immediately age out the port that received the inferior BPDU

rlqpdu.PNG

 

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