MLPPP and QoS

Unanswered Question
May 22nd, 2009

We have a 3 MB MPLS connection to a new site. Cisco Call Manager servers are at another site on the other side of the WAN. We have some questions on the QoS configuration. Our local router is a 3845, the far end is also a 3845.

The 2 T-1's are bundled with MLPPP. Once a call is established, does the established audio stream go across: a) the “bundle” or b) one of the T-1's?

If it goes across one the T-1's, could the packets go back and forth between the 2 T-1's? Our network team has recounted to me that when voice traffic was going down BOTH the ISP "A" DS3 and the AT&T MPLS DS3 at a larger site, voice quality suffered. Once it was directed down one path or the other, voice quality is fine.

Our other engineers were thinking that we should unbundle the 3MB and use PBR to direct voice down one T1 or the other.

What does this group think?

I have this problem too.
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Collin Clark Fri, 05/22/2009 - 10:07

It depends on how your load-sharing is configured.

Per-packet load balancing allows the router to send data packets over successive equal-cost paths without regard to individual destination hosts or user sessions. Path utilization is good, but packets destined for a given destination host might take different paths and might arrive out of order.

Per-destination load balancing allows the router to use multiple, equal-cost paths to achieve load sharing. Packets for a given source-destination host pair are guaranteed to take the same path, even if multiple, equal-cost paths are available. Traffic for different source-destination host pairs tend to take different paths.

Note If you want to enable per-packet load sharing to a particular destination, then all interfaces that can forward traffic to the destination must be enabled for per-packet load sharing.

To enable load balancing for Cisco Express Forwarding (CEF), use the ip load-sharing command in interface configuration mode.

Also, is this a single ended site?

http://www.cisco.com/en/US/docs/ios/12_3/switch/command/reference/swi_i1.html#wp1081905

lhoyle Fri, 05/22/2009 - 10:20

We do not have any load-sharing setup at all. I will attach a config.

This site is a large site, will have 100+ phones and is connected by the MPLS to our core. There is a smaller site, connected to the large site via single T1, with 2801's on both sides. There are 15-20 phones at that smaller site.

Paolo Bevilacqua Fri, 05/22/2009 - 10:26

You need not to worry about how MLPPP sends the packets, because it's designed so that packets are never forwarded out of sequence.

Technically, MLPPP sends to small the interface queue, then reorders any out of sequence packet using frame sequence and buffer.

Collin Clark Fri, 05/22/2009 - 10:41

Load sharing is there, but it has the defaults (which is good) so they don't show up in the config.

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