Cisco 4500M Software Manual page 225

Chapter 15 Configuring STP Features
Figure 15-4 BackboneFast Before Indirect Link Failure
Switch A
(Root)
Next, assume that L1 fails. Switch A and Switch B, the switches directly connected to this segment,
instantly know that the link is down. The blocking interface on Switch C must enter the forwarding state
for the network to recover by itself. However, because L1 is not directly connected to Switch C, Switch C
does not start sending any BPDUs on L3 under the normal rules of STP until the time defined by the Max
Age setting has expired.
In an STP environment without BackboneFast, if L1 should fail, Switch C cannot detect this failure
because it is not connected directly to link L1. However, because Switch B is directly connected to the
root switch over L1, Switch B detects the failure and elects itself the root. Then Switch B begins sending
configuration BDPUs to Switch C, listing itself as the root.
Here is what happens additionally when you use BackboneFast to eliminate the time defined by the Max
Age setting (20-second) delay:
1. When Switch C receives the inferior configuration BPDUs from Switch B, Switch C infers that an
indirect failure has occurred.
Switch C then sends out an RLQ.
2.
Switch A receives the RLQ. Because Switch A is the root bridge, it replies with an RLQ response,
3.
listing itself as the root bridge.
When Switch C receives the RLQ response on its existing root port, it knows that it still has a stable
4.
connection to the root bridge. Because Switch C originated the RLQ request, it does not need to
forward the RLQ response on to other switches.
5. BackboneFast allows the blocked port on Switch C to move immediately to the listening state
without waiting for the time defined by the Max Age setting for the port to expire.
6. BackboneFast transitions the Layer 2 interface on Switch C to the forwarding state, providing a path
from Switch B to Switch A.
This switchover takes approximately 30 seconds, twice the Forward Delay time if the default forward
delay time of 15 seconds is set.
Figure 15-5
OL-6696-01
L1
L2
Blocked port
Switch C
shows how BackboneFast reconfigures the topology to account for the failure of link L1.
Switch B
L3
Software Configuration Guide—Release 12.2(25)EW
Overview of BackboneFast
15-7