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Cisco QoS Suggestions

Updated May 2015

Introduction

For a more general article on QoS, see our article here. This article seeks to provide some more specific examples of QoS implemented on Cisco routers.

Cisco with Polycom Phones

By default, Polycom implements QoS by marking the DSCP value as EF in the IP header in packets from their phone. In this example, we wanted to preference VoIP packets to have priority over the other traffic leaving the LAN for the Internet. This was particularly important in this case because the Internet connection was ADSL, which had 3 MB of downstream, but only 1 MB of upstream bandwidth.

Step 1: Set up a class map to identify the packets that should be matched. This could be done in other ways, but we chose to match the DSCP value in the IP header on packets sent from the phones. On this network, presuming these packets to come strictly from the phones was a safe assumption.

  class-map match-all VOICE
   description "Prioritize VoIP packets"
   match ip dscp ef

Step 2: Set up a policy map to do something with the packets identified in the class map.

  policy-map VOICE-OUT
   class VOICE
   set precedence 1

Step 3: Apply it to the outbound interface in the outbound direction.

  interface FastEthernet0
   ip address 6.7.8.9 255.255.255.0
   ip nat outside
   duplex auto
   speed auto
   service-policy output VOICE-OUT

Step 4: Verify that the match is working:

  Router# show policy-map interface FastEthernet0
   FastEthernet0

   Service-policy output: VOICE-OUT

   Class-map: VOICE (match-all)
   29448101 packets, 6325546691 bytes
   5 minute offered rate 77000 bps, drop rate 0 bps
   Match: ip dscp ef (46)
   QoS Set
   precedence 1
   Packets marked 29448101

   Class-map: class-default (match-any)
   79873679 packets, 9623152335 bytes
   5 minute offered rate 17000 bps, drop rate 0 bps
   Match: any

 


 

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