HP ProCurve 9304M Installation And Configuration Manual page 266

Routing switches
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Installation and Basic Configuration Guide
Figure 8.25
802.1W Draft 3 RSTP failover to alternate root port
The arrow shows the path
to the root bridge
Routing Switch 1
Root Bridge
Bridge priority = 2
Bridge priority = 6
Root port = 3/4
Routing Switch 3
In this example, port 3/3 on Routing Switch 3 has become unavailable. In standard STP (802.1D), if the root port
becomes unavailable, the Routing Switch must go through the listening and learning stages on the alternate port
to reconverge with the spanning tree. Thus, port 3/4 must go through the listening and learning states before
entering the forwarding state and thus reconverging with the spanning tree.
8021.W 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 8021.W Draft 3, Routing Switch 3 immediately fails over to port 3/4, without the delays caused by the
listening and learning states.
8021.W Draft 3 selects the port with the next-best cost to the root bridge. For example, on Routing Switch 3, port
3/3 has the best cost to the root bridge and thus is selected by STP as the root port. Port 3/4 has the next-best
cost to the root bridge, and thus is selected by 8021.W Draft 3 as the alternate path to the root bridge.
Once a failover occurs, the Routing Switch no longer has an alternate root port. If the port that was an alternate
port but became the root port fails, standard STP is used to reconverge with the network. You can minimize the
reconvergence delay in this case by setting the forwarding delay on the root bridge to a lower value. For example,
if the forwarding delay is set to 15 seconds (the default), change the forwarding delay to a value from 3 – 10
seconds.
During failover, 8021.W Draft 3 flushes the MAC addresses leaned on the unavailable root port, selects the
alternate port as the new root port, and places that port in the forwarding state. If traffic is flowing in both
directions on the new root port, addresses are flushed (moved) in the rest of the spanning tree automatically.
8 - 60
Port 1
/2
FWD
Port 1
/4
FWD
Port 1
/3
DISABLED
X
Port 3
/3
unavailab
le
Port 3
/4
FWD
Routing Switch 2
Port 2
/2
FWD
Port 2
/4
FWD
Port 2
/3
FWD
Port 4
/3
BLK
Port 4
/4
FWD
Routing Switch 4
Bridge priority = 4
Root port = 2/2
Alternate = 2/3, 2/4
Bridge priority = 8
Root port = 4/4
Alternate = 4/3

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