Understanding How Backbonefast Works - Cisco 7609 Configuration Manual

Understanding How BackboneFast Works

If Switch C detects a link failure on the currently active link L2 on the root port (a direct link failure),
UplinkFast unblocks the blocked port on Switch C and transitions it to the forwarding state without
going through the listening and learning states, as shown in
approximately one to five seconds.
Figure 16-2 UplinkFast Example After Direct Link Failure
Switch A
(Root)
Understanding How BackboneFast Works
BackboneFast is initiated when a root port or blocked port on a network device receives inferior BPDUs
from its designated bridge. An inferior BPDU identifies one network device as both the root bridge and
the designated bridge. When a network device receives an inferior BPDU, it indicates that a link to which
the network device is not directly connected (an indirect link) has failed (that is, the designated bridge
has lost its connection to the root bridge). Under normal STP rules, the network device ignores inferior
BPDUs for the configured maximum aging time, as specified by the STP max-age command.
The network device tries to determine if it has an alternate path to the root bridge. If the inferior BPDU
arrives on a blocked port, the root port and other blocked ports on the network device become alternate
paths to the root bridge. (Self-looped ports are not considered alternate paths to the root bridge.) If the
inferior BPDU arrives on the root port, all blocked ports become alternate paths to the root bridge. If the
inferior BPDU arrives on the root port and there are no blocked ports, the network device assumes that
it has lost connectivity to the root bridge, causes the maximum aging time on the root to expire, and
becomes the root bridge according to normal STP rules.
If the network device has alternate paths to the root bridge, it uses these alternate paths to transmit a new
kind of Protocol Data Unit (PDU) called the Root Link Query PDU. The network device sends the Root
Link Query PDU out all alternate paths to the root bridge. If the network device determines that it still
has an alternate path to the root, it causes the maximum aging time to expire on the ports on which it
received the inferior BPDU. If all the alternate paths to the root bridge indicate that the network device
has lost connectivity to the root bridge, the network device causes the maximum aging times on the ports
on which it received an inferior BPDU to expire. If one or more alternate paths can still connect to the
root bridge, the network device makes all ports on which it received an inferior BPDU its designated
ports and moves them out of the blocking state (if they were in the blocking state), through the listening
and learning states, and into the forwarding state.
Figure 16-3
directly to Switch B over link L1 and to Switch C over link L2. The Layer 2 LAN interface on Switch C
that connects directly to Switch B is in the blocking state.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide—12.1E
16-4
L1
L2
Link failure
Switch C
shows an example topology with no link failures. Switch A, the root bridge, connects
Switch B
L3
UplinkFast transitions port
directly to forwarding state
Chapter 16 Configuring Optional STP Features
Figure 16-2. This switchover takes
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