Catalyst 3550 Access Switch

1P3Q2T (recommended) or 4Q2T WRR default - Each queue is given a relative weight with one priority queue Default values are as follows: CoS Value CoS Priority Queues 0, 1 1 2, 3 2

Move CoS 5 traffic to queue 4 Trust Cisco IP Phone on access links Trust DSCP or CoS on uplinks CoS-to-DSCP mappings

Classify and mark mission-critical traffic using ACLs

Distribution

Access

Distribution

Access

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For the Catalyst 3550 switches, the recommended configuration for the transmit interface is one priority queue and three queues, each with a single drop threshold.

Scheduling is performed using WRR, in which each queue is given a relative weight while the priority queue is serviced exhaustively. The default CoS to queue mapping is as follows:

■ Frames marked with a CoS 6 and 7 will go into queue 4 (priority queue for the Catalyst 3550).

■ Frames marked with a CoS 4 and 5 will go into queue 3.

■ Frames marked with a CoS 2 and 3 will go into queue 2.

■ Frames marked with a CoS 0 and 1 will go into queue 1.

When servicing voice traffic, CoS 5 frames should be configured to go to the priority (expedite) queue. By default, Cisco IP Phones forward voice traffic with an 802.1Q priority of 5 (CoS 5).

At the access layer switch, configure the trust boundary to trust only the CoS marking on the Cisco IP Phone, not the CoS marking on the PC. Also configure the trust boundary to trust the DSCP or CoS marking from the distribution layer switches, depending on the capabilities of the distribution layer switch that is attached.

For Layer 2-to-Layer 3 QoS mappings, the Catalyst 3550 has default CoS-to-DSCP mappings. However, you can manually configure these default mappings to override the default mappings. The default CoS-to-DSCP map is as follows:

CoS-to-DSCP Default Mapping

Marker

Value

CoS Values

0

1

2

3

4

5

6

7

DSCP Values

0

8

16

24

32

40

48

56

As a general rule, it is best to classify and mark the traffic as close to the source as possible. Therefore, at the access layer switches, you can configure class-based markings to classify and mark the traffic.

mls qos interface GigabitEthernet0/12 description Uplink to Distribution no ip address mls qos trust dscp priority-queue out wrr-queue co s-map 4 5

mls qos mls qos map cos-dscp 0 8 16 26 34 46 48 56

class-map match-all voice-bearer match access-group 101 class-map match-all voice-control match access-group 102 class-map match-all mission-critical match access-group 103

interface GigabitEthernet0/12 description Uplink to Distribution no ip address mls qos trust dscp priority-queue out wrr-queue co s-map 4 5

policy-map mark class voice-bearer set ip dscp ef class voice-control set ip dscp 26 class mission-critical set ip dscp af31

access-list 101 permit udp any any range 16384 32767 access-list 102 permit tcp any any range 2000 2002 access-list 102 permit tcp any any eq 1720 access-list 102 permit tcp any any range 11000 11999 access-list 102 permit udp any any eq 2427 access-list 103 permit <Mission-Critical Traffic>

interface FastEthernet0/1 description to IP Phone cdp enable no ip address service-policy input mark mls qos trust cos mls qos trust device cisco-phone switchport priority extend cos 0 switchport voice vlan 111 switchport access vlan 11 priority-queue out wrr-queue cos-map 4 5

<output omitted>

The figure shows a sample QoS configuration on a Catalyst 3550 access switch.

When classification of traffic by the access layer switch is required, the Catalyst 3550 provides a powerful set of features for classifying traffic as the traffic enters the network. This configuration example uses three class maps to classify traffic into three classes (voice-bearer, voice-control, and mission-critical) using three extended IP access control lists (ACLs). The access ports are configured to trust only the CoS markings from the Cisco IP Phone; all other traffic will be classified and marked using the traffic policy called "mark." This traffic policy is used to mark the voice-bearer class with a PHB of expedited forwarding (EF), the voice-control (call-signaling) class with a DSCP of 26, and the mission-critical class with a DSCP of AF 31. This traffic policy is then applied to the switch access ports.

Note If you are using the Catalyst 2950, the ACLs are limited to match a single TCP or User

The uplink switch ports are configured to trust the DSCP markings from the distribution layer switches. Layer 3-capable distribution layer switches are being used in this topic.

The Catalyst 3550 supports multiple VLANs on the access port to which the Cisco IP Phone is attached. The switchport voice vlan interface configuration command is used to configure voice VLAN on the port. In this example, all the voice traffic from the access port will belong to VLAN 111 while the data traffic will belong to VLAN 11.

To ensure EF for the voice-bearer traffic, all CoS 5 traffic is configured to go into queue 4 (the priority queue [PQ]) along with the CoS 6 and CoS 7 traffic.

The default CoS-to-DSCP mapping is also changed so that CoS 3 (voice control) maps to AF31 (26), CoS 4 (videoconferencing) maps to AF41 (34), and CoS 5 (voice bearer) maps to DSCP EF (46).

Datagram Protocol (UDP) port. For example, the Catalyst 2950 ACLs cannot be used to match against a UDP or TCP port range for matching VoIP RTP bearer and control traffic.

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