PM3350-SW datasheet(101/224 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

pool to be completely empty at any point. The Switch Processor will declare a fatal

system error and attempt to initiate a hardware reset if this occurs.

The data descriptor pool is used for supplying data descriptors. It consists of a linked-

list of 20-byte fixed-size memory blocks, and is maintained entirely by the Switch

Processor firmware. Blocks are allocated from this pool as required when a data

descriptor must be created to hold the information for received frames or messages.

The hash bucket pool consists, in a similar manner, of a linked-list of 64-byte blocks

that are intended to be used to create hash buckets and forwarding tags when MAC

addresses are learned. One minor difference between this pool and the others is that

the blocks in this pool are pre-formatted for rapid learning; all of the fields of the hash

buckets in this pool, with the exception of the MAC address and the local port descriptor

address, are initialized to their default values. This allows the learning process to simply

allocate a free hash bucket, fill in the MAC address and local port descriptor pointer,

and place the hash bucket into the address table, thereby minimizing the time spent

formatting the hash bucket. When hash buckets are de-allocated to the free hash

bucket pool (during aging, or topology changes), they are again re-formatted before

being placed on the free pool. As the aging and topology change handling are

principally non-time-critical background tasks, the additional time required to pre-format

the hash buckets is not a significant issue.

When forwarding tags are created, only the first 16 bytes of the memory block is used;

the remaining 48 bytes are left untouched (and preserve their original formatting).

Forwarding tags are required during the learning of MAC addresses reachable by

remote devices: they are allocated by the Switch Processor firmware during the learning

process carried out when specially marked frames are received over the PCI expansion

bus, and de-allocated (again by the Switch Processor firmware) when the

corresponding MAC addresses are aged out. Removal of forwarding tags takes place

only after their parent source hash buckets (on the remote devices) are removed. More

details on the creation and removal of forwarding tags and hash buckets are provided in

the sections dealing with learning and aging below.

The sizes and locations of the various free pools are completely configurable via

parameters in the boot image, and can be adjusted by the system implementer as

desired. These parameters implicitly determine the total amount of buffering in the

system, the broadcast frame reception rate, the number of MAC addresses supported,

and so on.

Spanning Tree Support Data Structures

Upon start-up, if spanning tree is enabled, the ELAN 8x10 creates a set of data

structures required for implementing the IEEE 802.1d spanning tree algorithm. (In a

multiple-ELAN 8x10 system, only the master device constructs the spanning tree data

structures and implements the spanning tree algorithm; the slave devices exchange

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