Cisco ME 3400 Series Ethernet Access Switches

Introduction

This document describes a known limitation on the Metro Ethernet (ME) 3400 switch that uses Class of Service (CoS) based ingress classification.

Problem

Ingress policy-map classifying traffic, based on CoS values, does not function while another based on Differentiated Services Code Point (DSCP) values functions.

Common Configuration

class-map match-any ef>
 match ip dscp ef
class-map match-any af31
 match ip dscp af31
class-map match-any cos-3 
 match cos 3
class-map match-any cos-5 
 match cos 5

policy-map qoscos
 class cos-5     police cir 5000000
  conform-action set-cos-transmit 5
  exceed-action drop
 class cos-3<
  police cir 5000000
  conform-action set-cos-transmit 3
  exceed-action drop


policy-map qosdscp
 class ef
  police cir 5000000
   conform-action set-cos-transmit 5
    exceed-action drop
 class af31
  police cir 5000000
   conform-action set-cos-transmit 3
   exceed-action drop

Working Configuration

interface GigabitEthernet0/2
  port-type nni
  switchport trunk allowed vlan 1091
  switchport mode trunk
  service-policy input qosdscp

Non-Working Configuration

interface GigabitEthernet0/2
  port-type nni>
  switchport trunk allowed vlan 1091
  switchport mode trunk
  service-policy input qoscos

Solution

For traffic that is routed on the ME-3400, the ingress policy-map matching CoS markings are not hit. It is possible to view the traffic that is routed by both the global routing table and Virtual Routing and Forwarding (VRF). However, for L2 switched traffic, this policy-map functions properly.

When a policy-map is applied on a switch platform, it must be programmed into the Ternary Content Addressable Memory (TCAM), so that Quality of Service (QoS) operations do not depend on the CPU. Programming differs between the L2 and the L3 TCAM on the ME3400. For a policy map with match CoS based class-maps only, L2 TCAM is programmed, while for match DSCP based classes, L3 TCAM is programmed.

For routed traffic, only the L3 TCAM is looked up. The entry is not found, and all traffic goes through as unpoliced/unmarked. However for L2 switched traffic, L2 TCAM lookup is performed which results in a hit against match CoS based classes. This behavior is as designed.