Cabletron Systems STS16-20D, STS16-20R manual Filtering, Congestion Control

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Filtering

Filtering is important for a LAN switch. Filters can be used to reduce broadcast traffic, block certain protocols and provide security functions.

The SmartStack STS16-20D/STS16-20R provides filters for:

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Destination or source MAC addresses

Destination service access point (DSAP)

Subnetwork Access Protocol (SNAP) type

Each protocol filter can be applied on a per-port basis for both input and output traffic. This feature allows certain protocols to be blocked from certain ports. For example, filters can be established to allow only Systems Network Architecture (SNA) traffic to flow to ports with SNA gateways.

Source and destination MAC address filtering can be applied to all incoming frames. The MAC address filters act in one of three ways:

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Block destination address at a specific port—this prevents the specified port from sending frames to a specified destination.

Allow destination address at specific ports—this indicates that the specified port must send frames to the specified destinations only.

Force destination address to a specific port—this allows forwarding to a unicast address that has not been learned. It can also be used to limit the forwarding of Multicast addresses to a subset of ports. This last filter applies to non-source-routing frames only.

Congestion Control

At regular intervals, the switch CPU inspects the queues on all output ports. If a queue size is above a certain threshold, the port is instructed to:

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Set the transmit priority for low priority frames to a specified high level

Delete old frames from the queue until it reaches a specified size

When the queue size again comes below a normal threshold size the port is instructed to set the transmit priority back to the normal level.

SmartStack STS16-20D/STS16-20R Token Ring Switches Installation and User Guide Switch Overview