PM3350-SW datasheet(128/224 Pages) PMC

128 / 224 page

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

126

Remote address learning has been determined to be possible. The Switch

Processor allocates an empty 16-byte memory block from the free data

descriptor pool and formats it as a forwarding tag. It fills in the new source MAC

address (read from the IEEE 802.3 source address field of the Ethernet frame),

the pointer to the master hash bucket in the remote device (read from the

HashBkt field of the data descriptor), and the chip number (obtained from the

SrcChip field of the data descriptor) into the MACAddr, SrcHashBkt, and

ChipNum fields, respectively, of the blank forwarding tag. In addition, it fills in

the MulticastMask and FTFlags fields of the forwarding tag with their default

values.

The forwarding tag must now be inserted into the hash table at the correct point.

To do this, the Switch Processor hashes the MAC address and computes the

index into the hash pointer array (in the same manner as implemented by the

hash lookup mechanism). It then converts the index into the actual address of

the appropriate entry within the hash pointer array, reads the 24 LSBs of this

entry, and writes the 24 LSBs into the NextFT field of the newly created

forwarding tag. Finally, it writes the memory address of the forwarding tag into

the hash pointer array, which completes the insertion of the forwarding tag into

the address table.

The creation and insertion of the forwarding tag, carrying with it the necessary

information required to direct frames targeted at the given MAC address to the

remote device, completes the remote learning process. The Switch Processor

now continues with the rest of the frame switching process.

Note that the number of forwarding tags created within any of the ELAN 8x10 devices in

a system must equal the sum of the numbers of hash buckets created in all of the other

devices in the system (each new hash bucket created within any remote device results

in a forwarding tag being created within this device). In addition, the sum of the

forwarding tags and the hash buckets created by any one device cannot exceed the

pre-set maximum number of addresses that can be learned by the system.

Default Hash Buckets

A special mechanism is implemented to deal with the case when a new source MAC

address cannot be learned due to some reason (e.g., when the pool of available hash

buckets is exhausted). This mechanism is necessary as the source hash bucket pointer

that is used at various points during the switching process must always point to a valid

hash bucket data structure, as this pointer is utilized by the switching firmware to

update statistics and so on. In addition, the local port descriptor pointer within the hash

bucket must also be valid, and pointing to the correct local port descriptor

corresponding to the physical port upon which the frame arrived.

To deal with the above case, the initialization process creates eight dummy (default)

hash buckets within the Switch Processor operating environment at system start-up.

PM3350-SW Datasheet(PDF) 128 Page - PMC-Sierra, Inc

PM3350-SW Datasheet(HTML) 128 Page - PMC-Sierra, Inc

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