Cisco Systems 4500 About BackboneFast

21-14
Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 21 Configuring Optional STP Features
About BackboneFast
About BackboneFast
BackboneFast is a complementary technology to UplinkFast. UplinkFast is designed to quickly respond
to failures on links directly connected to leaf-node switches, but it does not help with indirect failures in
the backbone core. BackboneFast optimizes the topology based on the Max Age setting. It allows the
default convergence time for indirect failures to be reduced from 50 seconds to 30 seconds. However, it
never eliminates forward delays and offers no assistance for direct failures.
Note BackboneFast should be enabled on every switch in your network.
Sometimes a switch receives a BPDU from a designated switch that identifies the root bridge and the
designated bridge as the same switch. Because this should not happen, the BPDU is considered inferior.
BPDUs are considered inferior when a link from the designated switch has lost its link to the root bridge.
The designated switch transmits the BPDUs with the information that it is now the root bridge as well
as the designated bridge. The receiving switch ignores the inferior BPDU for the time defined by the
Max Age setting.
After receiving inferior BPDUs, the receiving switch tries to determine if there is an alternate path to the
root bridge.
• If the port that the inferior BPDUs are received on is already in blocking mode, then the root port
and other blocked ports on the switch become alternate paths to the root bridge.
• If the inferior BPDUs are received on a root port, then all presently blocking ports become the
alternate paths to the root bridge. Also, if the inferior BPDUs are received on a root port and no other
blocking ports exist on the switch, the receiving switch assumes that the link to the root bridge is
down and the time defined by the Max Age setting expires, which turns the switch into the root
switch.
If the switch finds an alternate path to the root bridge, it uses this new alternate path. This new path, and
any other alternate paths, are used to send a Root Link Query (RLQ) BPDU. When BackboneFast is
enabled, the RLQ BPDUs are sent out as soon as an inferior BPDU is received. This process can enable
faster convergence in the event of a backbone link failure.
Figure 21-4 shows an example of a topology with no link failures. Switch A, the root switch, connects
directly to Switch B over link L1 and to Switch C over link L2. In this example, because switch B has a
lower priority than A but higher than C, switch B becomes the designated bridge for L3. Consequently,
the Layer 2 interface on Switch C that connects directly to Switch B must be in the blocking state.
Figure 21-4 BackboneFast Before Indirect Link Failure
L1
L2 L3
Switch C
Switch A
(Root) Switch B
Link failure
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BackboneFast transitions port
through listening and learning
states to forwarding state