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    MLPPP Link Selection Overview

    By default, E Series routers use a round-robin algorithm to select the link on which to transmit data on an MLPPP interface. The round-robin link selection method applies to both best-effort packets, such as data, and non-best-effort (high-priority) packets, such as voice and video. Best-effort packets are encapsulated with an MLPPP header that contains a sequence number, whereas non-best-effort packets are encapsulated with a PPP header that does not contain a sequence number.

    The member links in an MLPPP bundle can experience different queuing delays due to the volume of traffic transmitted on the MLPPP interface. These delays can cause packets to arrive out of order at the remote router. The effect of such delays differs for best-effort packets and non-best effort packets, as follows:

    • For best-effort packets that arrive out of order from the E Series router, the remote router can use the sequence number to reorder and forward the packets in the correct order, regardless of the order in which the packets were received.
    • For non-best-effort packets that arrive out of order from the E Series router, the lack of a sequence number prevents the remote router from being able to determine the correct order in which to forward the packets. This can cause problems with applications that require high-priority voice and video traffic transmitted on MLPPP interfaces to be received in the same order transmitted by the peer applications.

    To ensure that the E Series router maintains the proper packet order when transmitting non-best-effort traffic, you can use the ppp hash-link-selection command to enable use of a hash-based algorithm to select the link on which the router transmits high-priority packets on an MLPPP interface.

    When you use hash-based link selection instead of the default round-robin link selection for non-best-effort traffic, the router uses the IP source address (SA) and IP destination address (DA) of the packet as a hash to select the MLPPP member link on which to transmit the packet. Specifically, the router uses the hash algorithm to bind the transmission of all traffic between this IP SA and IP DA to the same member link in the MLPPP bundle.

    If the member link selected to transmit high-priority packets becomes inoperable or is removed from the MLPPP bundle, the router must select a different link on which to transmit the packets. As a result, packets transmitted on this new link might sometimes arrive at the remote destination before the traffic sent on the previously selected member link.

     

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    Published: 2014-08-14