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PM3351-SW Datasheet(PDF) 101 Page - PMC-Sierra, Inc |
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PM3351-SW Datasheet(HTML) 101 Page - PMC-Sierra, Inc |
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101 / 268 page  PM3351 ELAN 1X100 DATA SHEET PMC-970113 ISSUE 3 SINGLE PORT FAST ETHERNET SWITCH PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE 95 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. On the ELAN 1x100, hash lookups are initiated at two different points in the frame switching process. Firstly, the source and destination MAC addresses of all frames received on the MAC port are looked up in the address table automatically by the DMA Controller; this results in two hash lookup operations being performed for each incoming frame. Secondly, another address table search is done if a frame was copied over the PCI expansion bus and the Learn flag bit is set in the Flags field of the data descriptor associated with the frame. This hash lookup is done on the source MAC address within the frame.1 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. 31 24 23 0 Unused Pointer to bucket/tag (NULL if none) ... Unused Pointer to bucket/tag (NULL if none) 4N Unused Pointer to bucket/tag (NULL if none) 4N+4 Unused Pointer to bucket/tag (NULL if none) 4N+8 Unused Pointer to bucket/tag (NULL if none) 4N+12 Unused Pointer to bucket/tag (NULL if none) ... The hash pointer array occupies the locations in memory described by the address map given previously. Initially, all the pointers in the array are set to zero (NULL), indicating that no MAC addresses are present in the address table. As MAC addresses are learned (or statically created), the hash buckets or forwarding tags corresponding to these addresses are inserted into the address table, and the relevant pointers in the array are updated to point to the linked-lists of hash buckets and/or forwarding tags. Note that newly learned addresses are always placed at the head of any linked-list thus created. The size of the hash array, which determines the probability of collisions (i.e., the probability that multiple MAC addresses will be associated with a single hash pointer 1 The setting of the Learn bit is an indication by the remote device that it has determined that the source MAC address of the frame under question was not seen before by the system, and needs to be entered into the address table; a hash lookup on the source MAC address is done first as a precaution, to ensure that the MAC address does not already exist in the table. |
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