VOIP QoS on Low-speed links

[ashraf.ali]

Hi,

For doing QoS over a low speed link where voice and data co-exists, I have configured Link fragmentation and interleaving to fragment the jumbograms to improve link efficiency. QoS does work with LLQ configured for Voice, but I have problems in data transfers. The MS Exchange server is not able to comunicate with the remote end servers with LFI enabled on the link.

Any help on this will be appreciated.

Thanks

regards

Ashraf

[jasyoung]

Make sure you're doing actual LFI as in link fragmentation, rather than IP-level fragmentation. If you're using the 'ip mtu' or 'mtu' commands to force a smaller IP packet MTU, this can sometimes cause communications problems. You should be using either 'ppp multilink fragment' or frame-relay fragmentation, depending on your media type.

PPP multilink LFI:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b75d2.html

Frame-relay fragmentation:

http://www.cisco.com/en/US/tech/tk652/tk698/technologies_configuration_example09186a0080094af9.shtml#topic9

[markedwards]

Have you seen any problems with PPP for LFI affecting packets larger then a given MTU size? (similar to what you see in VPN deployments)

We have enabled PPP/LFI for a VoIP on a FR-to-ATM 512K PVC. Traffic with a size greater then 1230, has a high failure rate (sometimes up to 100% failure). the DF bit is not being set.

Thanks ME