Texas Instruments TNETX3270 Spanning-tree support, Aging algorithms, Frame-routing determination

TNETX3270

ThunderSWITCH 24/3 ETHERNET SWITCH

WITH 24 10-MBIT/S PORTS AND 3 10-/100-MBIT/S PORTS

SPWS043B ± NOVEMBER 1997 ± REVISED APRIL 1999

spanning-tree support

Each port provides independent controls to block reception or transmission of frames, for learning of addresses, or to disable the port on a per-port basis. Blocking can be overridden to allow reception or transmission of spanning-tree frames.

aging algorithms

time-threshold aging

When processing (learning) addresses, the IALE adds the source address to the table and tags it with a time stamp. If another frame is received with this source address, then the time stamp is refreshed. If the aging counter expires before another frame is received from this source address, then the address is deleted from the table. If the table is full, then the oldest address is deleted to make room for a new address, even if the age for this address has not expired.

table-full aging

In table-full aging, the oldest address (or one of the oldest addresses if there is more than one) is automatically deleted from the IALE records only if the table is full and a new address needs to be added to the table. In this mode, the age stamp for the addresses is not refreshed.

frame-routing determination

When a frame is received, its 48-bit destination and source addresses are extracted and the VLAN index is determined. The destination address and VLAN index are then looked up in the IALE records to determine if a match exists. If a match is found, the information associated with the record is passed on to the frame-routing algorithm. After removing disabled ports and checking for mirrored and trunking ports, the frame is sent to the correct ports and the source address time stamp is reported.

The source address and VLAN index combination also are looked up in the IALE records to determine if a match exists. If a match is found, additional information is provided to the routing process. Figure 9 provides a flow diagram of the routing algorithm.

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