User's Manual
275
Figure 1.4 Backbone Fast
Suppose the bridge priority of switch C is higher than that of switch B. When L1 is disconnected, device B is
selected to send BPDU to device C because the bridge priority is used as root priority. To device C, the
information contained by BPDU is not prior to information contained by its own. When Backbone Fast is not
enabled, the port between device C and device B ages when awaiting the bridge information and then turns
to be the designated port. The aging normally takes a few seconds. After the function is configured in global
configuration mode by running the command spanning-tree backbonefast, when the Alternate port of device C
receives a BPDU with lower priority, device C thinks that an indirect-link and root-device-reachable connection
on the port is disconnected. Device C then promptly update the port as the designated port without waiting the
aging information.
After the Backbone Fast function is enabled, if BPDU with low priority is received at different ports, the switch
will perform different actions. If the Alternate port receives the message, the port is updated to the designated
port. If the root port receives the low-priority message and there is no other standby port, the switch turns to
be the root switch.
Note that the Backbone Fast feature just omits the time of information aging. New designated port still needs
to follow the state change order: the listening state, then the learning state and finally the forwarding state.
Note:
Similar to Uplink Fast, the Backbone Fast characteristic is effective in SSTP and PVST modes.
37.1.6 Root Guard
The Root Guard attribute can prevent a port from serving as a root port after it receives a higher-priority BPDU.