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Query on QOS


Team, I have made QOS on the edge router to serve the bandwidth for Voice and set it EF bit. My question is, if incoming packet is already EF bit marked and when it will reach to edge router , then what router will do. Will it remove the ef marking and again put EF marking or it will simply ignore the remarking.

WIll it create any jitter? Here is the configuration


class-map match-any VOICE

 match access-group 101
 match protocol rtp audio
 match ip dscp ef

policy-map SW-TRAFFIC
 class VOICE

 priority percent 45
  set ip dscp ef

 class class-default
  set ip dscp default

3 Replies 3

John Blakley

Are you receiving the traffic from the Internet or over something that you control like mpls or ipsec tunnel? If it's the internet, you probably won't receive dscp46 from anything on the outside because ISPs will strip it before it gets to you.

Let's say that it's MPLS. If you control both ends, and you're marking both ends outbound, you would need to have an agreement with the ISP (if you're using them as the carrier for your mpls cloud) to support that tag or they will mark it to default.

If you have an agreement and they can guarantee the marking through their network, your router will receive it marked and you can do whatever you want to it with an inbound service policy.



HTH, John *** Please rate all useful posts ***

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And just ádding what John said. If a packet comes in already marked with ef, the router will first trust it, and then it will overwrite the value with ef, so You can see it as replacing ef with ef.

And it will not introduce any extra jitter.


Joseph W. Doherty
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Routers, usually by default, just pass along a packet's ToS and also usually just ignore it.

If a router has been configured to change and/or "use" ToS, it may or may not significantly impact forwarding of the packet, including jitter.

With your posted configuration, matching for rtp audio and/or DSCP EF will add a little processing overhead.  Additionally, setting ToS in your two classes will add a little processing overhead.  Both of these should be minimal.  However, the configured policy might have a huge impact.  For example, once your VOICE class exceeds 45% utilization, voice packets might be dropped (to keep VOICE from exceeding 45% bandwidth utilization).

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