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2010-06-30 23:28:21

EIGRP routing
Show Answers:

  1. What are the four components of EIGRP?
    • The four components of EIGRP are as follows:
    • - Protocol-independent modules
    • - Reliable Transport Protocol (RTP)
    • - Neighbor discovery/recovery
    • - Diffusing Update Algorithm (DUAL)
  2. By default, what does EIGRP use for calculating routes?
    Bandwidth and delay.By default, bandwidth and delay are used by EIGRP to calculate its metric. EIGRP can also be configured to use reliability, load, and maximum transmission unit (MTU). The metric of EIGRP is the metric of IGRP multiplied by 256 for improved granularity.
  3. What are three general steps that EIGRP uses to add routes to the router's routing table?
    The three general steps that EIGRP uses to add routes to the router's routing table are as follows:Step 1. Discover other EIGRP routers attached to the same subnet and form a neighbor relationship with the discovered routers. All discovered routers are kept in the router's EIGRP neighbor table.Step 2. Exchange network topology information with all discovered neighbors. This information is stored in the EIGRP topology table.Step 3. Run DUAL on all topology information and put the lowest-metric routes in the routing table.
  4. How does EIGRP discover neighbors?
    EIGRP neighbors are discovered through Hello messages. On most networks, Hello messages are multicast every 5 seconds to address On Frame Relay and link speeds of T1 (1.544 Mbps) or slower, Hellos are unicast every 60 seconds.
  5. When routing information changes in the routing table, how does EIGRP send updates?
    When routing information changes, EIGRP sends update messages to all neighbors, informing them of the change. If EIGRP has to send to multiple neighbors on the same subnet, the update messages are multicast to IP address If sending updates to one router, the messages are unicast to the neighbor.
  6. What is the EIGRP neighbor table?
    The EIGRP neighbor table lists all adjacent routers. Each EIGRP router maintains a neighbor table.
  7. What is the EIGRP topology table?
    The EIGRP topology table contains all learned routes to a destination. In other words, the topology table holds all feasible routes in its table.
  8. In EIGRP, what is a successor?
    A successor is a route selected as the primary route used to reach a destination. It is the route kept in the routing table.
  9. In EIGRP, what is the feasible successor?
    The feasible successor is the backup route. These routes are selected at the same time the successors are identified, but they are only kept in the topology table, not the routing table. They are used for fast convergence. If the successor fails, the router can immediately route through the feasible successor. Multiple feasible successors can exist for a destination.
  10. In EIGRP, what is the advertised distance (AD)?
    The AD is the cost between the next-hop router and the destination.
  11. In EIGRP, what is the feasible distance (FD)?
    The FD is the metric from the local router, through the next-hop router, and to the destination.
  12. What IOS commands enable EIGRP on a Cisco router and advertise and as its directly connected networks?
    • The router eigrp process-id command, followed by the network command, enables EIGRP on the router. The following commands enable EIGRP using AS 100 and then advertise networks and
    • RouterA(config)#router eigrp 100 (100 is the AS)
    • RouterA(config-router)#network
    • RouterA(config-router)#network
  13. What command would you use to see EIGRP adjacencies?
    • The show ip eigrp neighbors command displays EIGRP adjacencies and directly connected neighbors, as follows:RouterA# show ip eigrp neighbors
    • IP-EIGRP Neighbors for process 100
    • Address Interface Holdtime Uptime Q
    • Seq SRTT RTO (secs) (h:m:s) Count Num (ms) (ms)
    • Ethernet1 13 0:02:00 0
    • 11 4 20
    • Ethernet0 14 0:02:01 0
    • 10 12 24
  14. What IOS command allows you to view all EIGRP routes in the routing table?
    The show ip route eigrp command allows you to view all EIGRP-learned routes in the routing table.
  15. How do you view the EIGRP neighbor table?
    The show ip eigrp neighbors command shows the EIGRP neighbor table.
  16. How do you view the EIGRP topology table?
    • The show ip eigrp topology command shows the EIGRP topology table, including successors and feasible successors, as follows:
    • RouterB# show ip eigrp topology
    • IP-EIGRP Topology Table for process 100
    • Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
    • r - Reply status
    • P, 1 successors, FD is 2172416
    • via (2172416/28160), Serial0
    • via (2684416/1794560), Serial1
  17. What IOS command would you use to view the EIGRP neighbor states?
    You would use the debug eigrp neighbors command to check the EIGRP neighbor states. This command displays the contents of the Hello packet used in EIGRP as well as the neighbors discovered by EIGRP.
  18. Router A is running EIGRP and has four paths to network All four paths have the same cost. Which path will Router A choose to route to network
    All four paths. By default, EIGRP can load-balance up to four equal-cost routes. This is called equal-cost load balancing. Because EIGRP has four equal-cost paths to network, all paths are included in Router A's routing table.
  19. Router A is connected to Router B through a point-to-point T1 link. Router B is connected to network on its Fast Ethernet interface. EIGRP is running on both routers. You install a second point-to-point link between the two routers for redundancy. The new link has a bandwidth of 256 kbps. Because the new link has a higher cost than the T1 link, the new link is not installed in the routing table and is idle. EIGRP only uses the T1 link to route to network You want to load-balance between the two links. How do you enable EIGRP to load-balance between the two links?
    By default, EIGRP can only load-balance equal-cost links and not load-balance between unequal-cost links. EIGRP needs to be configured to load-balance between unequal-cost links. The goal is to configure EIGRP to spread the traffic load inversely proportionally to the metrics on the two links. EIGRP uses the variance command to perform unequal-cost load balancing. The variance command defines a multiplier by which a metric can vary from the lowest-cost route. A variance of 1 means that the metrics of multiple routes must be equal.In this question, the metric of the T1 link is 1,657,856. The composite metric to network (the total of the cost of the T1 link and the Fast Ethernet interface) is 2,172,416. The composite metric of the 256-kbps link is 10,514,432. To find the variance between the two paths to perform unequal-cost load balancing, divide the metric of the 256-kbps link by the T1 link: 10,514,432/2,172,416 = 4.8. Thus to configure unequal-cost load balancing, the variance on Router A needs to be set to 5, as follows:RouterA(config)#router eigrp 100RouterA(config-router)#variance 5
  20. What is the default bandwidth of a serial interface on a Cisco router?
    1544 kbps.
  21. What type of route authentication does EIGRP support?
    EIGRP supports message digest algorithm 5 (MD5) route authentication.
  22. Why would you want to enable EIGRP route authentication on EIGRP routers?
    EIGRP route authentication causes EIGRP routers to authenticate with each other using an MD5 key digest. This prevents the introduction of unauthorized or false routing messages from unauthorized or unapproved routers.
  23. How do you enable EIGRP MD5 authentication on Cisco routers?
    • The steps to configure EIGRP authentication are as follows:
    • Step 1. Enter the interface that you want to configure authentication on.
    • Step 2. Enable MD5 authentication using the ip authentication mode eigrp process-id md5 interface command.
    • Step 3. Create an authentication key using the ip authentication key-chain eigrp process-id key-chain command. The key-chain parameter is the name of the key you want to create.
    • Step 4. Exit interface configuration mode.
    • Step 5. Identify the key chain that you configured in Step 3 using the key chain name-of-key-chain command.
    • Step 6. Create a key number with the key number command.Step 7. Identify the key string using the key-string text command.
  24. As a network administrator, you have EIGRP enabled on all network routers. Your company has two locations. The corporate network is The branch office network is Your company acquires another company, and you connect the newly acquired company through a WAN link at the branch office. The network range of the newly acquired company is You enable EIGRP on the router that connects to the new office, but users at the corporate network cannot access devices on the newly acquired company's network. Why?
    Users from the corporate network cannot access the newly acquired company's network because the network is discontiguous, and by default, EIGRP summarizes routes across classful boundaries. As a result, the router on network advertises to both the corporate network and the newly acquired network. Thus when a user at the corporate network tries to connect to a device at the newly acquired network, the router drops the packet because it thinks it is local. Additionally, the branch office router thinks it has two equal-cost paths to the network.To fix the issue, you need to disable auto-summary on the routers. This is done with the no auto-summary EIGRP router configuration mode command.
  25. What command allows you to troubleshoot EIGRP authentication?
    The debug eigrp packets command allows you to view the neighbor adjacency process. When authentication is enabled on two routers, it is part of the adjacency process, and you can view whether authentication is the cause of failed neighbor adjacencies.If the failed neighbor adjacency is due to a misconfiguration in EIGRP authentication, you will see the following debugging output from the router:*Mar 26 12:48:15.749: EIGRP: pkt key id = 2, authentication mismatch*Mar 26 12:48:15.749: EIGRP: Serial 0: ignored packet from, opc ode = 5 (invalid authentication)*Mar 26 12:48:15.749: EIGRP: Dropping peer, invalid authentication