[cisco-voip] QoS on Multilink T1's

[Joseph Mays]

We have the following service policy on a router that priorities VOIP traffic according to the ef tag.

class-map match-all dscp-ef
  match ip dscp ef
!
!
policy-map queue-on-dscp
 description Prioritizes voice traffic first, signalling next.
  class dscp-ef
   priority percent 75
  class class-default
   fair-queue
   random-detect dscp-based

The router primarily contains traffic for T1's routed to several destinations.

I can demonstrate that for individual T1's the service policy does as it should. Throw normal pings at the remote end, things are low latency and no packet loss. Ping flood the remote end with 1500 byte packets and latency for normal pings and packet loss go sky high. While still pingflooding, pings tagged with DSCP ef still have low latency and no packet loss. This is all the way it should be.

However, it generally doesn't work for the multilink client on the box. In this case, while ping flooding, packets with and without the EF tag set all suffer the same high latency and packet loss during ping flood. Not surprisingly, this one client is also having VOIP call quality problems. All the clients are using the same service policy. I have been assuming that it's something about the fact that this client has two multilink T1's bonded together with multilink PPP and other clients just have a single T1.

Is there somethings special that has to done for QoS over multilink PPP? Or is there possibly some other thing affecting this one client? There are no specific access lists relating to their connection, nor to the ones that work. Really, the only thing overt that sets them different from the others is that they have bonded T1's, as shown below.

interface Multilink117870
 description Bonded Pair to Edge Outreach
 bandwidth 3072
 ip address 216.24.2.145 255.255.255.252
 no cdp enable
 ppp authorization PermT1
 ppp multilink
 ppp multilink group 117870
 service-policy output queue-on-dscp

interface Serial6/0/1:0
 description Edge Outreach (K1.HCFU.511024..SC)
 bandwidth 1536
 no ip address
 no ip redirects
 no ip proxy-arp
 encapsulation ppp
 ppp authorization PermT1
 ppp multilink
 ppp multilink group 117870
!
interface Serial6/0/2:0
 description Edge Outreach (K1.HCFU.511025..SC)
 bandwidth 1536
 no ip address
 no ip redirects
 no ip proxy-arp
 encapsulation ppp
 ppp authorization PermT1
 ppp multilink
 ppp multilink group 117870

Here is an example of a plain single T1 client config, in which case the QoS service policy works exactly as it should.

interface Serial6/0/3:0
 description Leonard Brush (K1.HCFU.511093..SC)
 bandwidth 1536
 ip address 216.24.0.53 255.255.255.252
 no ip redirects
 no ip proxy-arp
 encapsulation ppp
 ppp authorization PermT1
 service-policy output queue-on-dscp
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