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  • Adjust MTU of Phones/Switches

    We are having some issues with some of our remote users and our network admin feels it has to do with the MTU size.

    Please let me know if this can be done - and how... If it requires rebooting; etc, I will do it friday (updating to 9.1 then anyway so already have scheduled down time).

    His comment:
    When voice packets are 1418 bytes or larger, theyre broken up to fit within the MTU limit after encapsulations (IPsec). If, for example, voice packet is 1500b, it could be broken up into two pieces, 1418b and 82b. This is not optimal. If packets are send out of order and not reassemble in time, it could overflow the buffer and be dropped. Since this is an UDP packet, its not retried, thus, would produce the broken-up sound XXXX experienced. Im not even sure UDP packets are resequenced, that is, played in order it was sent instead of received.

    If ShoreTel has a DHCP parameter to control MTU, Id like to have ALL phones set to 1400b, to eliminate the fragmentations.

  • #2
    I don't see anything in the site planning or admin guides about setting the MTU. I don't think you can.

    Looking at voice UDP packets, they are typically 200 bytes (g711) per IP packet. I don't use Linear broadband but at 256kb of bandwidth per call that should be about 640 bytes per IP packet. These are not even close to 1400 bytes.

    UDP RTP packets are sequenced and packets received out of order are dropped.

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