QOS for Routing updates, LDP, etc

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Jan 28th, 2009
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We are needing to provide QOS to our voice traffic and we were thinking in configure Precedence 7 to this traffic. The traffic goes through an MPLS Network of 7600 as PE and 12000 as P. Can we configure the routing traffic, LDP and other signaling generated on the routers to use a service class as if the traffic were marked as precedence 6? how can we mark this traffic as precedence 6 changing the default? Is 7 the default value?


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Giuseppe Larosa Thu, 01/29/2009 - 11:48
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Hello Pedro,

I did some tests years ago

all routing protocols including OSPF, LDP and BGP uses precedence 6 with the execption of some RSVP-TE messages that are sent out with precedence 0.

Precedence 7 is not used in protocol messages.

However, the recommended value for voip bearer packets is precedence 5 that fits with DSCP EF Expedite Forwarding.

Using precedence 7 is not recommended but you can and you don't need to remark signalling packets.

I would use precedence 5 and I did in the past and also now we use it for voip.

Hope to help


Mohamed Sobair Thu, 01/29/2009 - 12:09
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The recommended marking for Voice as per RFC is IP precedence 5..

However, u cant mark with (Ip precedence 6 & 7) respectively because its reserved for (Internet & Network).

Also the recommended RFC marking for Data traffic is (IP precedence 3).

LDP has nothing to do with marking, LDP , TDP and RSVP are all Label Distribution protocols.

The default marking for the traffic if not explicitly marked is IP Precedence 1.



Giuseppe Larosa Fri, 01/30/2009 - 05:49
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Hello Mohamed,

LDP uses TCP port 646 and UDP port 646 for neighbor discovery.

So the question of how LDP messages (not LSPs) are marked can be placed and the answer is that they ip precedence 6 or DSCP CS6.

To demonstrate this is just enough to perform a packet capture on a lan segment where two routers are exchanging LDP messages

Hope to help



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