Users Guide

11–QLogic Teaming Services
General Network Considerations
160 BC0054508-00 J
Topology Change Notice (TCN)
A bridge or switch creates a forwarding table of MAC addresses and port numbers
by learning the source MAC address that received on a specific port. The table is
used to forward frames to a specific port rather than flooding the frame to all ports.
The typical maximum aging time of entries in the table is 5 minutes. Only when a
host has been silent for 5 minutes would its entry be removed from the table. It is
sometimes beneficial to reduce the aging time. One example is when a forwarding
link goes to blocking and a different link goes from blocking to forwarding. This
change could take up to 50 seconds. At the end of the STP re-calculation a new
path would be available for communications between end stations. However,
because the forwarding table would still have entries based on the old topology,
communications may not be reestablished until after 5 minutes when the affected
ports entries are removed from the table. Traffic would then be flooded to all ports
and re-learned. In this case it is beneficial to reduce the aging time. This reduction
is the purpose of a topology change notice (TCN) BPDU. The TCN is sent from
the affected bridge or switch to the root bridge/switch. As soon as a bridge/switch
detects a topology change (a link going down or a port going to forwarding) it
sends a TCN to the root bridge through its root port. The root bridge then
advertises a BPDU with a topology change to the entire network.The
advertisement causes every bridge to reduce the MAC table aging time to
15 seconds for a specified amount of time. The time reduction allows the switch to
re-learn the MAC addresses as soon as STP re-converges.
Topology change notice BPDUs are sent when a port that was forwarding
changes to blocking or transitions to forwarding. A TCN BPDU does not initiate an
STP recalculation. It only affects the aging time of the forwarding table entries in
the switch.It will not change the topology of the network or create loops. End
nodes such as servers or clients trigger a topology change when they power off
and then power back on.
Port Fast and Edge Port
To reduce the effect of TCNs on the network (for example, increasing flooding on
switch ports), end nodes that are powered on and off frequently should use the
Port Fast or Edge Port setting on the switch port to which they are attached. Port
Fast or Edge Port is a command that is applied to specific ports and has the
following effects:
 Ports coming from link down to link up will be put in the forwarding STP
mode, instead of going from listening to learning and then to forwarding.
STP is still running on these ports.
 The switch does not generate a topology change notice when the port is
going up or down.