16–Using Cavium Teaming Services
General Network Considerations
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Topology Change Notice (TCN)
A bridge or switch creates a forwarding table of MAC addresses and port numbers
by learning the source MAC address 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 by using a topology change notice
(TCN) BPDU. The TCN is sent from the affected bridge/switch to the root
bridge/switch. As soon as a bridge or 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.This causes every bridge to reduce the MAC table
aging time to 15 seconds for a specified amount of time. This 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 or Edge Port
To reduce the effect of TCNs on the network (for example, increasing flooding on
switch ports), end nodes that are powered on/off often should use the Port Fast or
Edge Port setting on the switch port they are attached to. 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 TCN when the port is going up or down.
Layer 3 Routing and Switching
The switch that the teamed ports are connected to must not be a Layer 3 switch or
router. The ports in the team must be in the same network.
