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CORE

By Jeanne Perez,2014-10-14 19:39
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CORE

    Core Knowledge

    Core Knowledge

1. RIP metric

    Question: what is the metric that from router A to router B which both running RIP? Answer: one hop

    Why: at router A, the route to router B is generated by the layer 3 switcher, so the metric is one hop. If is there is a loopback interface at router B, and in the same AS, then the route for this lo interface will be generated by router B and advertise to router A. At router A the loopback routes metric is two hop. RIPnGmetric要加1

2. 问,RIPv2 使用什么决定路由;

    : By hop.

    Why: Metric of RIPv2 is a hop count between 1 and 16. RIPv2 use the metric do route

    decision.

3. RIP metric

    Question: what maximum of the metric account in RIP. How many hops mean that the destination is unreachable?

    Answer: the maximum is 15. 16 hops means unreachable.

    Why: An unreachable network is indicated by setting the hop count to 16, which RIP interprets as an infinite distance. metric>15时都表示不可达。

4. EIGRP neighbor

    Question: There is a SW between R1 and R2, SW is running STP protocol, R1 and R2 run EIGRP, when the link between R1 and SW is down, how R2 knows R1s neighbor is down.

    Answer: EIGRP use hello packet to discovery and recovery neighbors. On the Ethernet

    network, Hellos are multicast every five seconds. When R2 receives a Hello packet from R1, the packet will include a hold time. The hold time tells R2 the maximum time it should wait to receive subsequent Hellos from R1. If the hold timer expires before a Hello is received,

    the neighbor R1 is declared unreachable and DUAL is informed of the loss of the neighbor.

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    Core Knowledge

    Then R2 know the neighbor is down. On Ethernet, the hold time is 15s (three times the Hello

    interval).

    Why: Because EIGRP updates are nonperiodic, it is especially important to have a process whereby neighbors (EIGRP-speaking routers on directly connected networks) are discovered and tracked. On most networks, Hellos are multicast every five seconds, minus a small random time to prevent synchronization. On multipoint X.25, Frame Relay, and ATM interfaces, with access link speeds of T1 or slower, Hellos are unicast every 60 seconds. This longer Hello interval is also the default for ATM SVCs and for ISDN PRI interfaces. In all cases, the Hellos are unacknowledged. The default Hello interval can be changed on a per interface basis with the command ip hello-interval eigrp.

    When a router receives a Hello packet from a neighbor, the packet will include a hold time. The hold time tells the router the maximum time it should wait to receive subsequent Hellos. If the hold timer expires before a Hello is received, the neighbor is declared unreachable and DUAL is informed of the loss of a neighbor. By default, the hold time is three times the Hello interval.

5. EIGRP unequal load-balance

    Question: if we set the variance 2, how will router A choose the path to the 172 network? Answer: A-B-E and A-C-E. The metric of the lowest-cost route is 20, the variance is 2, so the route whose metric exceeds 40 will not be considered a feasible route. Why: The variance command is used to determine which routes are feasible for unequal-cost load sharing. Variance defines a multiplier by which a metric may differ, or vary, from the metric of the lowest-cost route. Any route whose metric exceeds the metric of the lowest-cost route, multiplied by the variance, will not be considered a feasible route.

6. EIGRP over frame-relay

Question: Hub点是物理口封装FRR2使用p-t-p子接口!为什么EIGRP起不来。

    Answer: config EIGRP on the frame-relay network, we need to use the broadcast keyword

    when config the PVC or manually identify each neighbors using the ―neighbor‖ command.

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    Core Knowledge

    For this question, may not use the two ways both.

7. EIGRP over frame-relay

    问,Hub点使用点到多点!spoke点使用点到点。问为什么邻居不能建立;

    Answer:

    First: frame-relay map need to add broadcast keyword then eigrp neighbor can be establish.

    Second: If frame-relay map dose not use broadcast keyword then need to use neighbor command in the eigrp processes of all the eigrp routers.

8. EIGRP unequal load balance

    Answer: The EIGRP unequal load balance enables the router do load balance on links with unequal metrics. The router spread the traffic load inversely proportional to the metrics of the links.

    Why: The variance command is used to determine which routes are feasible for unequal-cost load sharing. Variance defines a multiplier by which a metric may differ, or vary, from the metric of the lowest-cost route. Any route whose metric exceeds the metric of the lowest-cost route, multiplied by the variance, will not be considered a feasible route.

     The maximum-paths limit must not be exceeded as a result of adding the route to a load-sharing

    "group."

     The next-hop router must be metrically closer to the destination. That is, its metric for the route must

    be smaller than the local router's metric. A next-hop router, being closer to the destination, is often