Pop Quiz - EIGRP and DUAL experts

Discussion in 'Cisco' started by jimbo, Dec 17, 2004.

  1. jimbo

    jimbo Guest

    Question #1 EIGRP

    Exactly when is the RTP protocol used? Is it used only when traffic
    crosses WAN links or is it also used on a local LAN? Is it used only
    for certain packet types or all packet types? Is it always used on
    routing updates?


    Question #2 DUAL
    DUAL will reject a route update if the REPORTED DISTANCE is less then
    the FEASIBLE DISTANCE of the successor - to prevent a possible routing
    loop. But, if the successor is removed and DUAL performs a diffused
    computation, and gets a reply advertising THE SAME EXACT NETWORK IT
    REJECTED BEFORE, DUAL will accept the route. Right?

    So how does DUAL know for sure the route is not looped when it accepts
    it. How is the guaranteed 100% loop-free promise kept in this

    jimbo, Dec 17, 2004
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  2. jimbo

    Scooby Guest


    From Cisco's site...

    Reliable Transport Protocol (RTP) is responsible for guaranteed, ordered
    delivery of Enhanced IGRP packets to all neighbors. It supports intermixed
    transmission of multicast or unicast packets. For efficiency, only certain
    Enhanced IGRP packets are transmitted reliably. On a multiaccess network
    that has multicast capabilities, such as Ethernet, it is not necessary to
    send hello packets reliably to all neighbors individually. For that reason,
    Enhanced IGRP sends a single multicast hello packet containing an indicator
    that informs the receivers that the packet need not be acknowledged. Other
    types of packets, such as updates, indicate in the packet that
    acknowledgment is required. RTP contains a provision for sending multicast
    packets quickly when unacknowledged packets are pending, which helps ensure
    that convergence time remains low in the presence of varying speed links.
    The DUAL finite-state machine embodies the decision process for all route
    computations by tracking all routes advertised by all neighbors. DUAL uses
    distance information to select efficient, loop-free paths and selects routes
    for insertion in a routing table based on feasible successors. A feasible
    successor is a neighboring router used for packet forwarding that is a
    least-cost path to a destination that is guaranteed not to be part of a
    routing loop. When a neighbor changes a metric, or when a topology change
    occurs, DUAL tests for feasible successors. If one is found, DUAL uses it to
    avoid recomputing the route unnecessarily. When no feasible successors exist
    but neighbors still advertise the destination, a recomputation (also known
    as a diffusing computation) must occur to determine a new successor.
    Although recomputation is not processor-intensive, it does affect
    convergence time, so it is advantageous to avoid unnecessary recomputations.

    Hope that helps,

    Scooby, Dec 17, 2004
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  3. jimbo

    jimbo Guest

    Hey, thanx for the reply. The stuff on RTP is helpful. I pulled down a
    couple of white papers from the site, but it did not include this

    As far as EIGRP goes, I did not see anything in what you posted that
    goes to my question. Did it make sense to you? I understand the
    mechanics of what EIGRP does for route selection, but I have yet to
    hear clear reasoning on why EIGRP will accept a route it previously
    rejected for failing to meet the ferasibility condition. How does DUAL
    know the route is loop-free the second time around?

    Thank you
    jimbo, Dec 19, 2004
  4. jimbo


    Sep 27, 2011
    Likes Received:
    DUAL accepts previously rejected route

    Hi ,

    DUAL will accept the route it rejected previously( call it routeA).
    The route (route A) was rejected earlier
    since there was a possibility of forming a loop . Note that another successor
    route was present at that time and there was a chance for loop if routeA gets
    installed in RT.

    Now the successor route gets removed.

    DUAL happens and it finds that the best route is routeA

    Now DUAL can safely install routeA , since the earlier successor route is
    not present and there is no chance of forming a loop including routeA and the
    earlier successor route.

    anish1234, Sep 27, 2011
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