Understanding How Backbonefast Works - Cisco 7604 Configuration Manual

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Understanding How BackboneFast Works

Figure 21-1 UplinkFast Example Before Direct Link Failure

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

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

Blocked port

Link failure

UplinkFast transitions port

directly to forwarding state

Configuring Optional STP Features

21-2. This switchover takes

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