PM3350-SW datasheet(91/224 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

The hash table consists of three types of data structures: an array referred to as the

hash pointer array of between 256 and 8,192 pointers, a set of 64-byte data structures

called hash buckets which hold information for locally reachable hosts on a per MAC

address basis, and a set of 16-byte forwarding tags which hold abbreviated information

for hosts reachable via external devices on a per-MAC address basis. (Note that the

forwarding tags are only 16 bytes in terms of formatted data, but are actually stored into

64-byte blocks of memory for simplicity.)

Each pointer in the hash array points to a linked-list of zero or more hash buckets

and/or forwarding tags. Each hash bucket or forwarding tag corresponds to a particular

MAC address; only one hash bucket or forwarding tag may be present for a given MAC

address in any device. (In addition, only one hash bucket may be present for a given

MAC address in a complete system; however, multiple forwarding tags may be present

in the various devices that point to this hash bucket.)

To access the hash bucket for a specific MAC address, the hash lookup engine in the

DMA Controller first creates a

hash key and uses it to generate an index into the hash

pointer array. The hash key is obtained by dividing the 48-bit MAC address into three

16-bit blocks:

32 31

16 15

Block 2Block 1Block 0

The three blocks are logically XORed together to create a single 16-bit value, which is

then logically ANDed with a configurable 16-bit bitmask to obtain the index into the hash

pointer array. The purpose of the bitmask is to limit the range of the indices to the

actual size of the hash table. If there are 8,192 entries in the table, the bitmask should

be defined such that the 13 LSBs are set to '1', and the 3 MSBs are set to '0'.

The hash lookup engine uses the index to locate the corresponding pointer within the

hash pointer array, and reads the pointer to obtain the first element of the linked-list of

hash buckets and/or forwarding tags that must be searched. It then scans the linked-

list, comparing the complete 48-bit MAC address to be resolved with the MAC address

fields within the hash buckets or forwarding tags, until the required hash bucket or

forwarding tag is found (or no more entries are present in the linked-list). If a match is

found, then the search is declared successful, and the memory address of the hash

bucket or forwarding tag is passed to the Switch Processor. If the complete linked-list is

traversed without finding a match, then the search is considered to have failed (i.e., the

specified source or destination MAC address is not present in the address table), and

an indication is accordingly passed to the Switch Processor by the hardware.

Hash Pointer Array

The hash pointer array is simply a linear array of 4-byte elements; each element

contains a pointer to a linked list of hash buckets and/or forwarding tags.

PM3350-SW Datasheet(PDF) 91 Page - PMC-Sierra, Inc

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