Layer 2 Qos and DSCP

Unanswered Question
Apr 20th, 2009

Hi all,

We want to implement VoIP using DSCP.

Our firewall is a cisco asa 5510 in our HQ, our branch office has a 5505. Internal netwerk are 3560 switches.

During the prepartion phase, we noticed our WAN links are only supporting 802.1p (Layer 2 qos). Is this giving any problems when using VoIP between our sites?

Beste regards


I have this problem too.
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Jon Marshall Mon, 04/20/2009 - 09:48


Not a problem as such but you will have to map DSCP to CoS to go over the WAN and them map CoS back to DSCP at the other end. This is assuming that you want to prioritise the traffic on the WAN. If you don't then simply leave the DSCP value as is.

For referecnce VOIP is usually DSCP EF and CoS 5.


jorg.ramakers Mon, 04/20/2009 - 11:01

Thanks Jon,

But what is the main difference between 802.1p (l2 Qos) and L3 qos (dscp, cos)?

I thought that 802.1p doesn't prioritise, just marks (colours) the traffic?



Jon Marshall Mon, 04/20/2009 - 11:17


Neither DSCP or CoS (802.1p) prioritize automatically. It s up to you to define what level of service you want to allocate to specific markings. Note that you have said L3 QOS is dscp and CoS but it isn't. L3 is DSCP and ToS (Type of Service). CoS is L2.

So DSCP and CoS merely mark the packet. It is then up to you to define how you want that value to be treated ie. priority queued, a certain level of bandwidth allocated, shaped or policed to a certain amount.

DSCP allows for 64 values. CoS and ToS are much more limited, 8 values only, so when you map back and forth between them you can lose some granularity.

But you can define the same treatment for DSCP EF and CoS map and then map VOIP into both.


Joseph W. Doherty Mon, 04/20/2009 - 11:29

Could there be problem on the WAN link if it only supports 802.1p? It depends.

It's not so much a question of L2 or L3 markings, but whether you have any congestion that adversely impacts your traffic and whether you can manage it.

Markings, whether L2 or L3, are just a handy tag for downstream devices to quickly recognize a set of traffic that should be treated a certain way.

What's generally important for VoIP, is to insure such traffic has the bandwidth it needs and it not impeded by other traffic sharing a path.

Likely, you'll want to insure your VoIP is dequeued first, including when provided to the WAN, and if the WAN is some kind of "cloud", insure the VoIP is marked so the cloud provider can also insure VoIP goes first if there is congestion.


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