Understanding How Backbonefast Works - Cisco 7604 Configuration Manual

Understanding How BackboneFast Works

Figure 21-1 UplinkFast Example Before Direct Link Failure
Switch A
(Root)
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 21-2 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.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
21-4
L1
L2
Blocked port
Switch C
L1
L2
Link failure
Switch C
Switch B
L3
Switch B
L3
UplinkFast transitions port
directly to forwarding state
Chapter 21 Configuring Optional STP Features
Figure 21-2. This switchover takes
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