Allied Telesis AT-A16 Manual do Utilizador descarregar pdf (Página 31)

Switching 3-31

Software Release 2.6.4

C613-03081-00 REV A

The Spanning Tree Protocol uses three configurable parameters for the time

intervals that control the flow of STP information on which the dynamic STP

topology depends: the HELLOTIME, FORWARDDELAY, and MAXAGE

parameters. All switches in the same spanning tree topology must use the same

values for these parameters, but can themselves be configured with different,

and potentially incompatible time intervals. The parameter values actually

used by each switch are those sent by the root bridge, and forwarded to all

other switches by the designated bridges.

The HELLOTIME parameter, with a default of 2 seconds, determines how

often the switch sends Hello messages containing spanning tree configuration

information if it is the root bridge, or is trying to become the root bridge in the

network. Setting a shorter value for HELLOTIME than the default of 2 seconds

makes the network more robust; setting a longer time uses less processing

overhead.

The MAXAGE parameter, with a default of 20 seconds, determines the

maximum time that dynamic STP configuration information is stored in the

switch, before it is considered too old, and discarded. The value can be set at

approximately two seconds for every hop across the network. If this value is

too small, the STP may sometimes configure unnecessarily. If it is too long,

there can be delays in adapting to a change in the topology, for instance when a

fault occurs.

The FORWARDDELAY parameter prevents temporary loops in the network

occurring in the briefly unstable topology while a topology change is

propagated through the network. When STP is running in standard mode and

a port that has been in the Blocking state is to move into the Forwarding state,

it must first pass through the Listening and Learning states. The

FORWARDDELAY parameter determines how long the port remains in each of

these intermediate states before moving on to the Forwarding state in the

active topology; that is, half the time between when it is decided that the port

will become part of the spanning tree and when it is allowed to forward traffic.

When STP is running in rapid mode, a port only has to pass from the

Discarding state through the Learning state to reach the Forwarding State. In

this case, the FORWARDDELAY parameter should be at least half the time it

takes for a topology change message to reach the whole network. A value that

is too short risks the temporary creation of loops, which can seriously degrade

switch performance. A longer value can result in delays in the network after

topology changes. The default FORWARDDELAY value is 15 seconds.

The FORWARDDELAY, MAXAGE and HELLOTIME parameters should be set

according to the following formulae, as specified in IEEE 802.1d:

2 x (FORWARDDELAY - 1.0 seconds) >= MAXAGE

MAXAGE >= 2 x (HELLOTIME + 1.0 seconds)

To modify the parameters controlling these time intervals, use the command:

SET STP={stp-name|ALL} [FORWARDDELAY=4..30] [HELLOTIME=1..10]

[MAXAGE=6..40]

The value of the PRIORITY parameter sets the writable portion of the bridge

ID, i.e. the first two octets of the (8-octet long) Bridge Identifier. The remaining

6 octets of the bridge ID are given by the MAC address of the switches. The

Bridge Identifier parameter is used in all configuration Spanning Tree Protocol

packets transmitted by the switch. The first two octets, specified by the

PRIORITY parameter, determine the switch’s priority for becoming the root

bridge or a designated bridge in the network, with a lower number indicating a

higher priority. In a fairly simple network with a small number of switches in a

meshed topology, it may make little difference which switch is selected as the

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