Specifications

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6.2 Buffers

Each queue is assigned a certain amount of buffer space to store transit data. Resident on the port ASIC is buffer

memory, which is split up and allocated on a per port basis. Per port buffering for each of the linecards is detailed in

Appendix One.

6.3 Thresholds

One aspect of normal data transmission is that if a packet is dropped, it will (if we are talking TCP flows) result in that

packet being retransmitted by the TCP endpoint. At times of congestion, this can add to the load on the network and

potentially cause buffers to overload even more. As a means of ensuring the buffers do not overflow, the Catalyst

6500 switch employs a number of techniques to avoid this happening.

Thresholds are arbitrary internal levels assigned by the switch port ASIC (and configurable by the administrator) that

define utilization points in the queue at which the congestion management algorithm can start dropping data. On the

Catalyst 6500 ports, each queue structure defines a number of thresholds that are associated with input queues. The

same applies to the number of thresholds associated with output queues.

What data is eligible to be dropped when a threshold is reached is determined by the priority of the packet. Different

CoS priorities are assigned to each threshold. In this way, when the threshold is exceeded, the congestion

management algorithm immediately knows which packets with which priority value are eligible to be dropped.

6.4 Mappings

In the queues and threshold section above, it was mentioned that the CoS value in the Ethernet frame is used to

determine which queue to place the frame into and at what point of the buffer filling up is a frame eligible to be

dropped. A map is the vehicle used to assign a priority value to a threshold in a queue.

When QoS is configured on the Catalyst 6500, default mappings are enabled that define the following:

●

At what thresholds frames with specific CoS values are eligible to be dropped

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Which queue a frame is placed into (based on its CoS value)

While the default mappings exist, these default mappings can be overridden by the administrator. Mapping exist for

the following:

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Map a CoS value on an incoming frame to a DSCP value

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Map an IP Precedence value on an incoming frame to a DSCP value

●

Map a DSCP value to a CoS value for an outgoing frame

●

Map CoS values to drop thresholds on receive queues

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Map CoS values to drop thresholds on transmit queues

●

Mapping DSCP markdown values for frames that exceed policing statements

●

Mapping a CoS value to a frame with a specific destination MAC address

An example of a map can be shown in the following diagram: