Foundry Configuration Guide for the FESX, FSX, and FWSX
7 - 54
© Foundry Networks, Inc.
December 2005
Figure 7.25
802.1W Draft 3 RSTP failover to alternate root port
In this example, port 3/3 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 3/4 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 3/4, 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 3/3 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 802.1W Draft 3 as the alternate path to the root bridge.
Once a failover occurs, the 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, 802.1W 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.
Switch 3
Switch 2
Port 2/4
FWD
Bridge priority = 6
Root port = 3/4
Port 2/2
FWD
Port 2/3
FWD
Port 1/2
FWD
Port 3/4
FWD
Port 1/4
FWD
Port 1/3
DISABLED
Port 4/4
FWD
Port 3/3
unavailable
Port 4/3
BLK
Switch 1
The arrow shows the path
to the root bridge
Bridge priority = 8
Root port = 4/4
Alternate = 4/3
Bridge priority = 4
Root port = 2/2
Alternate = 2/3, 2/4
Root Bridge
Bridge priority = 2
X
Switch 4
Содержание FastIron Edge Switch X424
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