Qos & MPLS

[louis0001]

Another basic question which hopefully somebody will be able to answer.

We have catalyst 2960s switches with VoIP phones connected.

Qos is setup on the switches and the uplink is via a 1941 directly into the MPLS network which has Qos on it.

Does the router have to have Qos enabled on it?

[daniel.dib]

You are buying a MPLS circuit from a provider? Does the ISP own the router or does it belong to you?

Are you setting CoS or DSCP in the switches? Remember that CoS may be removed when the layer 2 frame is rewritten so it's better to set DSCP if possible. As long as markings make it to the router then it should be fine but it's more of a question for your provider.

Daniel Dib
CCIE #37149

[louis0001]

It's DSCP on the switches and it's been set using  auto qos.

We own the routers which connect directly to their PE router on site although we do have a number of sites that connect via ADSL to their PE router in the nearest exchange.

I'm thinking that the traffic in the switch is prioritised so the outgoing traffic on the uplink to the router will be fine where it then enters the MPLS network which has QOS enabled.

Traffic coming inwards to the switch from the MPLS network will be tagged so I'm thinking that this should be OK.

[Adam Vitkovsky · Accepted solution]

Louis,

Make sure the VoIP phones are setting COS=5 and DSCP=EF (should be set like that by default) when sending frames to switch over the trunk port.

Make sure your switch trusts those markings.

If you’d like to make use of QOS on the link between switch and router it has to be a trunk port as only tagged frames carry the COS field.

On the router as well as the switch you can just honor the existing markings and schedule the traffic on egress accordingly.

As far as the transport of VoIP traffic from ISP’s PE to your router and vice versa (traffic ingress to PE from your router) make sure they have enabled the “interactive QOS profile” or basically VoIP QOS profile on the PE-CE link/VPN (it may come with an additional cost).

That is on the PE they will mark incoming (from your router) VoIP packets (DSCP EF) with MPLS EXP 5 and treat them as real-time traffic within their backbone. Also in the opposite direction i.e. from PE to your router the packets marked as DSCP EF will be departed first in case of congestion on the PE-CE link.

adam

[louis0001]

Thank you very much for the detailed reply above.

Exactly what I was after.

[Joseph W. Doherty]

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Posting

By default, Cisco routers tend to pass along L3 ToS without QoS  (extra) features enabled.  With (extra) QoS features enabled, it will usually pass along L3 ToS, unless configured to do otherwise, but it also might have different defaults for prioritization and drop preferences for different traffic markings.  The latter can be often be explicitly configured.

Many MPLS vendors offer some kind of QoS support; some charge extra for it.

If you have just a single (logical) p2p, or a (logical) hub-and-spoke, you often don't need to rely on MPLS vendor QoS.  However, if your topology routinely allows any-to-any traffic, MPLS vendor QoS is really a good thing to use it for traffic egressing from the MPLS cloud.  Also, if there is MPLS vendor QoS, you may need QoS to remark to/from your ToS markings to MPLS vendor ToS markings and/or you may need to manage congestion from your CE to PE.

If your MPLS vendor hand-off is Ethernet, and there's a logical bandwidth cap less than the physical port bandwidth, it's also usually a very good idea to shape for the logical bandwidth cap.

For VoIP, ideally you want to treat the bearer traffic as real-time (no drop, no delay) and also treat concurrent signally traffic such that it's unlikely to be dropped (guaranteed bandwidth).