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The Catalyst 8500 campus switch router provides extensive core Quality of Service (QoS) mechanisms that are built into the switch router architecture. These functions ensure policy enforcement and queuing of the ingress port, as well as weighted round-robin (WRR) scheduling at the egress port.
Implementation of QoS on the Catalyst 8500 is based on IP precedence. The system gathers IP precedence information from the service type field of the IP header. For an incoming IP packet, the first two (most significant) bits of the service type field determine the delay priority. The Catalyst 8500 recognizes four QoS classes, as summarized in Table 5-1.
| IP Precedence Bits | Delay Priority | Queue Selected |
|---|---|---|
0 0 0 | 0 0 | Q-0 |
0 0 1 | 0 0 | Q-0 |
0 1 0 | 01 | Q-1 |
0 1 1 | 01 | Q-1 |
1 0 0 | 1 0 | Q-2 |
1 0 1 | 10 | Q-2 |
1 1 0 | 1 1 | Q-3 |
1 1 1 | 1 1 | Q-3 |
The Catalyst 8500 can read the precedence field and switch the packet accordingly. However, the switch router cannot reclassify traffic. The edge router or switch is expected to set the precedence field according to its local policy.
The Catalyst 8500 queues packets based on the delay priority and the target next-hop interface.
You can find the effective bandwidth (in Mbps) for a particular VC with the following formula:
(W/S) x B = n Mbps
where
W | is the WRR weight of the specified queue. |
S | is the sum of the weight of all active queues on the outgoing interface |
B | is the available bandwidth in Mbps |
For example, if W is 4, S is 15, and B is 100, the equation is as follows:
(4/15) x 100 = 26 Mbps
Thus, the effective bandwidth for the specified queue in this example is 26 Mbps.
The Catalyst 8500 enables QoS-based forwarding by default. If disabled, issue the following command to enable QoS switching:
# [no] qos switching
The [no] version disables QoS switching on the entire system.
To map QoS scheduling at the system level, follow these steps:
| Step | Command | Description |
| 1 | | From global configuration mode, set the mapping between IP precedence and the WRR weight. See the syntax description below. |
| 2 | | Return to privileged EXEC mode. |
value | The precedence value (0 to 3) is derived from the IP precedence field. The higher 2-bits of the IP precedence field is used. When a precedence value x is specified, it also implicitly assigns the same WRR-weight to precedence x + 1. |
weight | The WRR-scheduling weight (1 to 15). This parameter specifies the weight assigned to traffic with the given precedence. |
To set the precedence back to the default setting for the Catalyst 8500, use the [no] version of the qos mapping precedence command.
The following defaults map the IP precedence to the WRR weights:
| IP Precedence | WRR Weight |
|---|---|
0 | 1 |
1 | 2 |
2 | 4 |
3 | 8 |
To map QoS scheduling at the interface level, follow these steps:
| Step | Command | Description |
| 1 | | From global configuration mode, assign different WRR-scheduling weights for a particular precedence traffic between a pair of interfaces. |
| 2 | | Return to privileged EXEC mode. |
To set the precedence back to the system-level default setting for the Catalyst 8500, use the [no] version of the qos mapping precedence wrr-weight command.
Both the source and destination interface parameters are optional. When both are not specified, the system-level QoS mapping is configured. Otherwise, you can specify the source interface, or the destination interface, or both to configure the WRR weight for the following:
1. Traffic streams with a certain precedence, from a particular source interface to a particular destination interface.
2. Traffic streams with a certain precedence to a particular destination interface.
3. Traffic streams with a certain precedence from a particular source interface.
The configuration takes precedence in the above order.
The Cisco IOS show commands for QoS are as follows:
# show qos switching
# show qos mapping [source fa x/0/z destination fa a/0/c]
Posted: Thu Dec 30 11:43:14 PST 1999
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