Understanding Linux Network Internals

Timers

The STP uses both per-bridge and per-port timers . In Tables 15-4 and 15-5, you can see the default timeouts, and what the allowed values are, for per-bridge and per-port timers, respectively.^[*]

Table 15-4. Bridge timers

Timer	Default value (in seconds)	Allowed range
a See the section "Topology Changes."
Hello	2	1-10
Topology Change	Forward Delay + Max Age	Not configurable
TCN	Hello time	Not configurable
Addresses Aging	300 or Forward Delaya	Not configurable

Table 15-5. Port timers

Timer	Default value (in seconds)	Allowed range
Message Age	20	6-40
Forward Delay	15	4-30
Hold	1	Not configurable

Note that not all timers are user configurable. Also note that some timers share the same configuration (the TCN and Hello timers, for example) so that a configuration change for a timer may affect others as well.

These are the bridge timers:

Hello

Used to regularly generate configuration BPDUs. Only the root bridge uses it.

TCN

Used by a bridge that has detected a topology change and must notify the root bridge about it. See the section "Topology Changes."

Topology change

Used by the root bridge to remember to set a specific flag in its configuration BPDUs. This flag is used to notify the other bridges about a topology change. See the section "Topology Changes."

Aging timer

Used to clean up stale addresses from the forwarding database. This timer is used by the bridge regardless of whether the STP is used. See the section "Short Aging Timer."

Each ...