PM3350-SW datasheet(131/224 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

129

Of the three components, the first has already been described in the sections dealing

with frame switching and inter-device frame transfers (see above). The other two will be

described in detail in this section.

Background Scanning Task

The background scanning task is invoked periodically (during idle periods, when frame

switching is not occurring) to search the entire address table for hash buckets that need

to be removed. The scan period is configurable, and defaults to 1 second.

The first step executed by the background scanning task is to determine the limit

imposed on the age of hash buckets. If a topology change was not detected, this limit is

simply the contents of the MaxAge variable. If, however, a topology change occurs, this

limit is set to the contents of the ForwardDelay variable for a period equal to the

ForwardDelay time, after which it reverts to the MaxAge time. Thus, for example, if the

default values of 300 seconds and 2 seconds, respectively, are used for MaxAge and

ForwardDelay, then for the 2 seconds following a topology change the age limit is set to

2 seconds, after which the age limit is set to 300 seconds. After the limit has been set,

the CLOCK register pair is read and preserved as the current time against which the

AgeControl fields in the hash buckets will be compared.

The next step is to scan the address table. To implement this, the hash pointer array is

simply scanned linearly, starting at the first pointer and terminating at the last. If any

pointer is found to be non-NULL, this indicates that a list of one or more hash buckets

and/or forwarding tags is attached to that entry; the list is then sequentially scanned

until no more elements remain in the list, after which the scanning returns to the next

hash pointer array entry. After the last entry in the hash pointer array has been

checked, the background scanning task returns.

For each hash bucket or forwarding tag found, the Flags field is checked to determine

whether it is a hash bucket, and also whether it can be aged out. If so, the AgeControl

field of the hash bucket is read and subtracted from the current time to give the age of

the hash bucket, and the result is compared to the age limit. If the comparison fails (i.e.,

the age of the hash bucket is greater than the limit), then the hash bucket is removed

from the linked list; otherwise, the hash bucket is left untouched and scanning

proceeds.

Once a hash bucket that has aged out has been located and removed from the address

table, it must be freed. This is not done immediately, however, as there may be other

devices in the system with forwarding tags that reference this hash bucket; if the hash

bucket is destroyed before deleting these forwarding tags, then incoming frames on

other devices that are directed to the corresponding MAC address may cause

inconsistencies and failures.

PM3350-SW Datasheet(PDF) 131 Page - PMC-Sierra, Inc

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