Brocade TurboIron 24X Series Configuration Guide - Manual (Page 598)

564 Brocade TurboIron 24X Series Configuration Guide
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Configuring STP related features
FIGURE 88 802.1W Draft 3 RSTP failover to alternate root port
In this example, port 3 on Switch 3 has become unavailable. In standard STP (802.1D), if the root
port becomes unavailable, the Switch must go through the listening and learning stages on the
alternate port to reconverge with the spanning tree. Thus, port 4 must go through the listening and
learning states before entering the forwarding state and thus reconverging with the spanning tree.
802.1W Draft 3 avoids the reconvergence delay by calculating an alternate root port, and
immediately failing over to the alternate port if the root port becomes unavailable. The alternate
port is in the blocking state as long as the root port is in the forwarding state, but moves
immediately to the active state if the root port becomes unavailable. Thus, using 802.1W Draft 3,
Switch 3 immediately fails over to port 4, without the delays caused by the listening and learning
states.
802.1W Draft 3 selects the port with the next-best cost to the root bridge. For example, on Switch
3, port 3 has the best cost to the root bridge and thus is selected by STP as the root port. Port 4
has the next-best cost to the root bridge, and thus is selected by 802.1W Draft 3 as the alternate
path to the root bridge.
X
The arrow shows the path
to the root bridge
Switch 1
Switch 2
Switch 4 Switch 3
Port 2
FWD Port 2
FWD
Port 4
FWD Port 4
FWD
Port 3
DISABLED
Root Bridge
Bridge priority = 2
Bridge priority = 6
Root port = 4
Bridge priority = 8
Root port = 4
Alternate = 3
Bridge priority = 4
Root port = 2
Alternate = 3, 4
Port 3
FWD
Port 3
BLK
Port 4
FWD
Port 4
FWD
Port 3
unavailable