PM3350-SW datasheet(130/224 Pages) PMC

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ELAN 8X10

PM3350 ELAN 8 X10

DATA SHEET

PMC-970109

ISSUE 3

8 PORT ETHERNET SWITCH

PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND PMC-SIERRA, INC. CUSTOMERS ONLY

128

The normal aging time used by the ELAN 8x10 is set by the MaxAge variable, and may

vary between 2 and 4000 seconds, with a default of 300 seconds. This aging time is

used when there are no topology changes, and hash buckets are removed after their

corresponding source entities have been silent for the time specified by MaxAge. If a

topology change occurs, however, the aging time is set by the ForwardDelay variable,

which may also range between 2 and 4000 seconds, but defaults to 2 seconds. The

purpose of the ForwardDelay time is to quickly remove all inactive hash buckets without

forcing the entire table to be purged, allowing the outdated address table entries

resulting from the topology change to be replaced by the new ones. This mechanism

has the benefit that MAC addresses from which heavy traffic is being received (but

have not shifted physical ports) will remain untouched in the address table, while at the

same time the altered state of the network will be rapidly re-learned. The net result is

intended to minimize the impact of topology changes in a busy network. (Note that this

process is also aimed at permitting simple implementation of the IEEE 802.1D

standard.)

In some situations, it may be necessary to 'lock' certain address table entries

permanently into the address table (i.e., to partially defeat aging without completely

turning it off). Each hash bucket contains a flag (referred to as the NoAge flag) that may

be set; if set, the aging process will be prevented from removing the hash bucket or its

associated forwarding tags, if any. (Note that the AgeControl field within the hash

bucket will still be updated whenever a frame is received with the corresponding source

address.)

Three main components make up the aging process:

the work interrupt service routine that performs the primary frame dispatching

tasks is responsible for updating the AgeControl field in the source hash bucket

whenever a frame is received from the corresponding source. The work interrupt

service routine also notices when a mismatch occurs between the physical port

indicated by the source hash bucket and the actual physical port on which the

frame arrived, and flags a topology change notification to cause the address

table to be updated.

a background scanning task is implemented to periodically check the

AgeControl fields in all the hash buckets and purge any that have been aged out

(i.e., whose source entities have not been active for some time). The

background scanning task is also responsible for notifying the remainder of the

chips in a multi-device system when a hash bucket must be removed, to allow

the other chips to delete the corresponding forwarding tags. The notification is

performed via a special message (called an aging message) sent to the other

chips.

a special routine, invoked whenever an aging message is received from another

device in the system, that actually removes the forwarding tag referenced by the

message.

PM3350-SW Datasheet(PDF) 130 Page - PMC-Sierra, Inc

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