Administrator Guide

VLANs 755
3 Configure a rate-limiting ACL to ensure that the voice VLAN does not
present a denial-of-service threat. A G.711 voice stream generates 64 Kbps,
which translates to 80 bytes of uncompressed voice every 10 ms. Overhead
adds 40 bytes, so the phone will generate 100 to 120 byte packets every
second per voice stream, or about 96 Kbps. The rate limit below will
permit a single voice stream.
console(config)#mac access-list extended dot1p-5-limit
console(config-mac-access-list)#permit any any cos 5 rate-
limit 100 64
console(config-mac-access-list)#permit any any
console(config-mac-access-list)#exit
4 Configure port 10 to be in access mode.
console(config)#interface gi1/0/10
console(config-if-Gi1/0/10)#switchport mode access
5 Configure the switch to tell the IP phone to use VLAN 25 and 802.1p
priority 5 for voice traffic.
console(config-if-Gi1/0/10)#voice vlan 25
console(config-if-Gi1/0/10)#voice vlan dot1p 5
6 Enable IEEE 802.1p trust mode on the interface. This is the switch
default.
console(config-if-Gi1/0/10)#classofservice dot1p trust
7 Configure internal CoS queue 2 as strict priority to ensure that egressing
voice traffic is transmitted first on this interface. This reduces latency for
transmitted voice traffic.
console(config-if-Gi1/0/10)#cos-queue strict 2
8 Map 802.1p priority 5 onto internal CoS queue 2. This is the switch
default mapping.
console(config-if-Gi1/0/10)#classofservice dot1p-mapping 5 2
9 Rate limit incoming IEEE 802.1p priority 5 traffic
console(config-if-Gi1/0/10)#mac access-group dot1p-5-limit in
Steps 6–8 are required to be configured on all ports that carry voice traffic
end-to-end, including the switch ports connected to other switches and the
ports on other switches that will carry voice traffic. It may be desirable to
configure steps 6–8 globally.