By Jeanne Perez,2014-10-14 19:39
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    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


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.


    Core Knowledge

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


    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.


    Core Knowledge

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

7. EIGRP over frame-relay



    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


     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

    referred to as the downstream router.

     The metric of the lowest-cost route, when multiplied by the variance, must be greater than the metric

    of the route to be added.


    Question: FD RD successor FS

10. EIGRP the command neighbor


    Core Knowledge

    EIGRP send hello packet to use multicast default. But the frame-relay network dont support multicast. Use the neighbor command, the route will send hello packet to the

    destination address by using unicast.

11. EIGRP ip bandwidth-percent

    Questionip bandwidth-percent eigrp 200 20是什么意思;还有eigrp的邻居需要配臵


    The 200 is the autonomous system number of the eigrp process. 20 mean that the eigrp process can use 20 percent of the bandwidth.

    In broadcast network, EIGRP dont need manual appoint neighbor, in no-broadcast network,

    EIGRP need to use neighbor command appoint peer.

12. EIGRP SIA state

    When a route goes active and queries are sent to neighbors, the route will remain active until a reply is received for every query. If a neighbor cannot reply because some reasons, the

    route would stay permanently active.

     Routes that have a valid successor are said to be in a ―passive‖ state. If, for some reason, a router loses a route through its successor and does not have a feasible successor for that route, then the route transitions to an ―active‖ state. In the active state, a router sends queries out to its neighbors requesting a path to the lost route.

    When an EIGRP neighbor receives a query for a route, it behaves as follows:

    If the EIGRP topology table does not currently contain an entry for the route, then the router immediately replies to the query with an unreachable message, stating that there is no path for this route through this neighbor.

    If the EIGRP topology table lists the querying router as the successor for this route and a feasible successor exists, then the feasible successor is installed and the router immediately replies to the query. If the EIGRP topology table lists the querying router as the successor for this route and a feasible successor does not exist, then the router queries all of its EIGRP neighbors except those sent out the same interface as its former successor. The router will not reply to the querying router until it has received a reply to all queries that it originated for this route.

    If the query was received from a neighbor that is not the successor for this destination, then the router replies with its successor information.

13. OSPF Area & LSA

    Question: 问在下图中!area 0area 2NSSA,中有什么类型的LSA和路由;


    Core Knowledge

Answer: Area 0: LSA 1 2 3 5

     Area 1: LSA 1 2 3 7

     Area 2: LSA 1 2 3 4 5

    Why: router CASBRrouter E产生4类的LAS进入area 2

14. OSPF virtual-link

Questions,在Router_B上重分布RIPOSPF!为什么area 0看不到重分布进来的路


    AnswerBecause all inter-area traffic must pass through the backbone; non-backbone areas

    cannot exchange packets directly. Area 2 don‘t connect to area 0, there should be a

    virtual-link in area 1 to connect area 2 and area 0.

15. OSPF stub area

    Question: OSPF中!ABR会阻拦那些类型的LSAstub区域;

    Answertype 45 LSA

16. OSPF load-balance

Question: OSPF会选择那条路径;

    Answer,,A-B-E and A-C-E. If multiple equal-cost, equal-path-type routes exist, OSPF


    Core Knowledge

    utilizes them. By default, the Cisco OSPF implementation load balances over a maximum of

    16 equal-cost paths.

注解,OSPF 选路,

    1. Select the route or routes with the most specific match to the destination address. The most specific

    match should always be the longest match the route with the longest address mask. The entries may

    be host, network, subnet, supernet or default addresses.

    2. Prune the set of selected entries by eliminating less-preferred path types.

    Intra-area paths, inter-area paths, E1 external paths, E2 external paths. 3. equal-cost load balance.

17. area nssadefault-information-originate(Not-So-Stubby-Area)

    The statement default-information-originate, added to the area nssa command, will cause the

    ABR to advertise a default route into the NSSA this time, with a type 7 LSA.

18. OSPF type of network and DR, BDR

    问在哪台设备上可以看到下面的显示,用这个命令,show ip ospf neighbor


OSPF show ip ospf neighbor输出的对不对;

    DRouter 之间是2way, DRBDRfull, p2p线路是full.



19 OSPF Area & LSA

    如图,R1上有个202.11.103.96 接口!做了redistribute connect动作!R3上做了RIP

    redistribute into OSPF的动作。

    要求,describe the routes/lsa in the area 0 and area 1


    Core Knowledge

    Area 0 LSA 1 2 3 5

    Area 1 LSA 1 2 3 7

20 OSPF metric


    Cisco router calculate a default OSPF cost as 10^8/BW, where BW is the configured bandwidth of the interface and 10^8 is the reference bandwidth. The reference bandwidth can be changed with the command auto-cost reference-bandwidth.

21 OSPF external route


    Stub area; totally stub area; nssa; totally nssa

22 OSPF type of network and DR, BDR

    Point-to-point, 有没有DR

    Broadcast DR&BDR; DRouter

    NBMA 手工配臵邻居!需要选举DR BDR unicast

    Point-to-multipoint NBMA网络的一个特殊配臵!可以看成是点到多点链路的集


    Point-to-point 不选举DR BDR 组播224.0.0.5

    Virtual link Unicast

23 OSPF DR rule



    If one or more of the eligible routers include their own address in the DR Field, the neighbor

    with the highest priority will be declared the DR. In a tie, the neighbor with the highest Router ID will be chosen.

24 OSPF neighbor and adjacency

    What is difference between OSPF neighbor and adjacencies?

    The OSPF neighbor relationships are established and maintained through the exchange of Hello packets. The purpose of the neighbor relationship is the formation of adjacencies over which to pass routing information. Only then OSPF neighbor finished database


    Core Knowledge

    synchronization, the OSPF neighbors are fully adjacent.

25 OSPF virtual-link

    Hello包经过virtual-link会被交换吗;Hello packet will be exchanged via virtual-link?


    The output shows that OSPF hellos are suppressed. This means that, while the virtual link is

    up, no hellos are exchanged. OSPF suppresses the hellos because it considers virtual links to be demand circuits (按需链路).

    Virtual links are interpreted by the router as unnumbered point-to-point networks. OSPF packets are unicast over virtual links. As on point-to-point networks, Valid neighbors will always become adjacent.

    Within each ABR, the virtual link will transition to the full functional point-to-point interface state when a route to the neighbor ABR is found in the route table. When the interface state becomes point-to-point, an adjacency is established across the virtual link.

26 IBGP&IGP synchronization

How BGP synchronization affected the route to

    When router B receives the route from router A, if the BGP synchronization is on, router B will neither enter this route into its IGP routing table nor advertise this route to router C unless router B known this route by IGP first.

    What is the BGP synchronization?

    Before a route learned from an IBGP neighbor is entered into the IGP IP routing table or is advertised to a BGP peer, the route must first be known via IGP.

27 BGP attribute

    Which BGP attribute will influence inbound flow?

    MED (Multi_Exit_Disc), as_path, origin.



    Core Knowledge

    What about the BGP administrative Distance?

    Internal BGP is 200, External BGP is 20, Backdoor EBGP 200.

29 BGP states

    BGP neighbors building process?

    Idle State ; Connect State ; Active State ; Open sent state ; Open confirm state ;

    Established state


    ATOMIC_AGGREGGATE is a well-known discretionary attribute that is used to alert

    downstream routers that a loss of path information has occurred.

    Any time a BGP speaker summarize more-specific routes into a less-specific aggregate, and path information is lost, the BGP speaker must attach the ATOMIC_AGGREGATE attribute to the aggregate route. Any downstream BGP speaker that receives a route with the ATOMIC_AGGREGATE attribute cannot make any NLRI information of that route more specific, and when advertising the route to other peer, the ATOMIC_AGGREGATE attribute must remain attached.

31 BGP attribute Local_pref



32. BGP attributes COMMUNITY

    The COMMUNITY attributes enable you to apply same policies to a group of routes. It is used to simplify policy enforcement.

    The Extcommunity can effect the best path select, the minimal Extcommunity cost will be selected, this step is fellow the next-hop metric and before the max-path. A BGP community is a group of routes that share common routing policy. They work by setting a common COMMUNITY attribute in the routes. Peers receiving those routes can recognize the COMMUNITY attribute and apply the appropriate policy.


    Core Knowledge

    33 BGP mechanism of prevent loops.


    IBGP: BGP does not advertise routes that have been learned from an IBGP peer to another

    IBGP peer.

    EBGP: BGP use the AS-path attribute to prevent loop. If a BGP speaker sees its own AS

    number in a received route from an external peer, it knows that a loop has occurred and

    ignores the route.

3.4 BGP attribute MED


R3 will choose R4 due to the lowest med

35 IBGP mechanism of prevent loops

R1(AS1)--------R2(AS2)------R3(AS2)-----R4(AS2), R1AS1, R2, R2EBGP

    R2R3IBGP, R3R4 IBGP 有一条路由从R1传给R2!问这条路由会不会在R4


    It will not appear in the R4 routing table. And there are several method to resolve this question.

    Establish IBGP peer between R2 and R4.

    Configure R3 as RR.

    Use BGP confederation in AS2.


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