Lab: the QoS policy that will not attach

A branch WAN link with voice, video and everything else competing for it. The configuration is three layers and it fails at the last one with a message that does not obviously say what is wrong.

Do this

• Write class maps that match the traffic you care about.
• Write a policy map that gives each class a share.
• Attach it to the WAN interface in the outbound direction.

R1(config)# class-map match-all VOICE
R1(config-cmap)# match ip dscp ef
R1(config)# policy-map WAN-OUT
R1(config-pmap)# class VOICE
R1(config-pmap-c)# priority percent 20
R1(config)# interface Serial0/0/0
R1(config-if)# service-policy output WAN-OUT

The point

Three layers, and each one can only see the one below it:

• class-map says what the traffic is;
• policy-map says what to do about it;
• service-policy says where.

The percentages across all classes cannot exceed the reservable bandwidth, and the attach fails if they do — with a message about bandwidth, on the interface, which looks like an interface problem rather than a policy one.

The other fault worth meeting here: QoS only does anything when there is congestion. A policy on an uncongested link has no visible effect at all, which makes people think it did not apply. Check show policy-map interface for the class counters rather than looking for a change in behaviour.

Try breaking it

• Make the percentages add up to 120 and read the error carefully.
• Attach it inbound instead of outbound.
• Match on an access list that matches nothing, and watch the counters stay