BackboneFast
BackboneFast is a Maxage optimizer. (See Figure 10-6.) In other words, BackboneFast helps get rid
of the 20 seconds that are associated with Maxage timer, which is used for
indirect failures as mentioned in the previous section. BackboneFast does this
by first detecting the indirect failure. The trigger for the indirect failure is
when the switch receives inferior BPDUs on its blocking port. The second
component to BackboneFast is verifying the failure. It does this through Root
Link Query (RLQ) protocol. The switch sends RLQ requests to the upstream switch
to find the location of the Root. Upon finding the location of the Root, the
switch expires the Maxage timer and transitions the blocking port to listening
state.
BackboneFast process is outlined in the following steps:
- Step 1. The connection between Switch1 and Switch3 fails.
- Step 2. Switch3 will start generating inferior configuration BPDUs
toward Switch2.
- Step 3. Upon receiving the inferior BPDUs, Switch2 sends a RLQ toward
the upstream switch.
- Step 4. Switch1 sends back a RLQ response that it is the
Root.
- Step 5. Switch2 now knows that it still has a path to the Root. It
can now safely transition the blocked port.
- Step 6. Switch2 transitions the blocking port into listening state.
It forwards configuration BPDUs to Switch3.
- Step 7. After 30 seconds, Switch3 has converged. This is the same
amount of time for direct failure convergence.
BackboneFast is a global command, and it should be enabled on
all switches, as shown in Example 10-10.
BackboneFast does not affect direct failure convergence times.
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