CCNA Interview Questions and Answers -7
1. What is Graceful Shutdown and GoodBye message in EIGRP?
When an EIGRP process is shut down, router sends out “goodbye” messages to its neighbors. The neighbors can then immediately begin recalculating paths to all the destinations that went through that shutdown router without having to wait for the hold timer to expire.
2. Maximum path load balanced by EIGRP?
up to 32 equal-cost entries can be in the routing table for the same destination. The default is 4. We can also set the maximum-path to 1 disables load balancing.
Router(config)#router eigrp 100
Set the maximum number of parallel routes that EIGRP will support to 6
3. How EIGRP support unequal load balancing?
EIGRP also support unequal cost path load balancing. Use the variance n command in order to instruct the router to include routes with a metric of less than n times the minimum metric route for that destination. The variable n can take a value between 1 and 128.
4. What does the word serno mean on the end of an EIGRP topology entry when you issue the show ip eigrp topology command?
#show ip eigrp topology
P 172.22.71.208/29, 2 successors, FD is 46163456
via 172.30.1.42 (46163456/45651456), Serial0.2, serno 7539273
via 172.30.2.49 (46163456/45651456), Serial2.6, serno 7539266
Ans- Serno stands for serial number. When DRDBs are threaded to be sent, they are assigned a serial number. If you display the topology table at the time an entry is threaded, it shows you the serial number associated with the DRDB.
Threading is the technique used inside the router to queue items up for transmission to neighbors. The updates are not created until it is time for them to go out the interface. Before that, a linked list of pointers to items to send is created (for example, the thread).
These sernos are local to the router and are not passed with the routing update.
5. What percent of bandwidth and processor resources does eigrp use?
Eigrp version 1 introduced a feature that prevents any single eigrp process from using more than fifty percent of the configured bandwidth on any link during periods of network convergence. Each as or protocol (for instance, ip, ipx, or appletalk) serviced by eigrp is a separate process. You can use the ip bandwidth-percent eigrp interface configuration command in order to properly configure the bandwidth percentage on each wan interface. Refer to the eigrp white paper for more information on how this feature works.
In addition, the implementation of partial and incremental updates means that eigrp sends routing information only when a topology change occurs. This feature significantly reduces bandwidth use.
The feasible successor feature of eigrp reduces the amount of processor resources used by an autonomous system (as). It requires only the routers affected by a topology change to perform route re-computation. The route re-computation only occurs for routes that were affected, which reduces search time in complex data structures.
6. Does eigrp support aggregation and variable length subnet masks?
Yes, eigrp supports aggregation and variable length subnet masks (vlsm). Unlike open shortest path first (ospf), eigrp allows summarization and aggregation at any point in the network. Eigrp supports aggregation to any bit. This allows properly designed eigrp networks to scale exceptionally well without the use of areas. Eigrp also supports automatic summarization of network addresses at major network borders.
7. Can i configure more than one eigrp autonomous system on the same router?
Yes, you can configure more than one eigrp autonomous system on the same router. This is typically done at a redistribution point where two eigrp autonomous systems are interconnected. Individual router interfaces should only be included within a single eigrp autonomous system. Cisco does not recommend running multiple eigrp autonomous systems on the same set of interfaces on the router. If multiple eigrp autonomous systems are used with multiple points of mutual redistribution, it can cause discrepancies in the eigrp topology table if correct filtering is not performed at the redistribution points. If possible, cisco recommends you configure only one eigrp autonomous system in any single autonomous system. You can also use another protocol, such as border gateway protocol (bgp), in order to connect the two eigrp autonomous systems.
8. If there are two eigrp processes that run and two equal paths are learned, one by each eigrp process, do both routes get installed?
No, only one route is installed. The router installs the route that was learned through the eigrp process with the lower autonomous system (as) number. In cisco ios software releases earlier than 12.2(7)t, the router installed the path with the latest timestamp received from either of the eigrp processes. The change in behavior is tracked by cisco bug id cscdm47037.
9. When i configure eigrp, how can i configure a network statement with a mask?
The optional network-mask argument was first added to the network statement in cisco ios
software release 12.0(4)t. The mask argument can be configured in any format (such as in a
network mask or in wild card bits). For example, you can use network 10.10.10.0 255.255.255.252
or network 10.10.10.0 0.0.0.3.
10. What is “goodbye” message received in eigrp?
The goodbye message is a feature designed to improve eigrp network convergence. The goodbye message is broadcast when an eigrp routing process is shut down to inform adjacent peers about the impending topology change. This feature allows supporting eigrp peers to synchronize and recalculate neighbor relationships more efficiently than would occur if the peers discovered the topology change after the hold timer expired.
The following message is displayed by routers that run a supported release when a goodbye message is received: Apr 26 13:48:42.523: %dual-5-nbrchange: ip-eigrp(0) 1: neighbor 10.1.1.1 (ethernet0/0) is down: interface goodbye received
A cisco router that runs a software release that does not support the goodbye message can misinterpret the message as a k-value mismatch and display the following message:-
Apr 26 13:48:41.811: %dual-5-nbrchange: ip-eigrp(0) 1: neighbor 10.1.1.1 (ethernet0/0) is down: k-value mismatch Obviously, the signalling to a neighbor that a protocol has been gracefully shutdown means good things for protocol convergence and loop prevention in a distance vector protocol. The point that i think is important is that a network that has some ios 15.1m and more mainstream sofware might see error messages about k-value mismatch and think that something is broken. In this case, the error message is exactly correct, and can be safely ignored.
As always, it depends™ on your exact configuration, its possible that someone has actually configured k-values (but it’s unlikely these days) and the message is telling you.
11. Who does load-balancing when there are multiple links to a destination?
Load balancing is a standard functionality of the cisco ios® router software, and is available across all router platforms. It is inherent to the forwarding process in the router and is automatically activated if the routing table has multiple paths to a destination. It is based on standard routing protocols, such as routing information protocol (rip), ripv2, enhanced interior gateway routing protocol (eigrp), open shortest path first (ospf), and interior gateway routing protocol (igrp), or derived from statically configured routes and packet forwarding mechanisms. It allows a router to use multiple paths to a destination when forwarding packets.
12. How can i use only one path when a router has two equal cost paths?
Configure the bandwidth value on the interfaces to default, and increase the delay on the backup interface so that the router does not see two equal cost paths. Or you can also limit Max-path to 1 for load balancing.
13. What is the difference in metric calculation between eigrp and igrp?
1. Eigrp has totally replaced the obsolete igrp
2. Eigrp is a classless routing protocol while igrp is a classful routing protocol
3. Eigrp uses the dual while igrp does not
4. Eigrp consumes much less bandwidth compared to igrp
5. Eigrp expresses the metric as a 32 bit value while igrp uses a 24 bit value
14. What is the eigrp stub routing feature?
The enhanced interior gateway routing protocol (eigrp) stub routing feature improves network stability, reduces resource utilization, and simplifies stub router configuration. Stub routing is commonly used in a hub and spoke network topology.
15. How can i send a default route to the stub router from the hub?
Do this under the outbound interface on the hub router with the ip summary-address eigrp x 0.0.0.0 0.0.0.0command. This command suppresses all the more specific routes and only sends the summary route. In the case of the 0.0.0.0 0.0.0.0, it means it suppresses everything, and the only route that is in the outbound update is 0.0.0.0/0. One drawback to this method is that eigrp installs a 0.0.0.0/0 route to null0 is the local routing table with an admin distance of 5.
16. What are different route types in eigrp?
Internal route—routes that are originated within the autonomous system (as).
Summary route—routes that are summarized in the router (for example, internal paths that have been summarized).
External route—routes that are redistributed to eigrp.
17. What is an offset-list, and how is it useful?
The offset-list is an feature used to modify the composite metrics in eigrp. The value configured in the offset-list command is added to the delay value calculated by the router for the route matched by an access-list. An offset-list is the preferred method to influence a particular path that is advertised and/or chosen.
18. What does the neighbor statement in the eigrp configuration section do?
The neighbor command is used in eigrp in order to define a neighboring router with which to exchange routing information. Due to the current behavior of this command, eigrp exchanges routing information with the neighbors in the form of unicast packets whenever the neighbor command is configured for an interface.
19. Why does the eigrp passive-interface command remove all neighbors for an interface?
The passive-interface command disables the transmission and receipt of eigrp hello packets on an interface. Unlike igrp or rip, eigrp sends hello packets in order to form and sustain neighbor adjacencies. Without a neighbor adjacency, eigrp cannot exchange routes with a neighbor. Therefore, the passive-interface command prevents the exchange of routes on the interface. Although eigrp does not send or receive routing updates on an interface configured with the passive-interface command, it still includes the address of the interface in routing updates sent out of other non-passive interfaces.
20. Why are routes received from one neighbor on a point-to-multipoint interface that runs eigrp not propagated to another neighbor on the same point-to-multipoint interface?
The split horizon rule prohibits a router from advertising a route through an interface that the router itself uses to reach the destination. In order to disable the split horizon behavior, use the no ip split-horizon eigrp as-numberinterface command. Some important points to remember about eigrp split horizon are:
Split horizon behavior is turned on by default.
When you change the eigrp split horizon setting on an interface, it resets all adjacencies with eigrp neighbors reachable over that interface.
Split horizon should only be disabled on a hub site in a hub-and-spoke network.
Disabling split horizon on the spokes radically increases eigrp memory consumption on the hub router, as well as the amount of traffic generated on the spoke routers.
The eigrp split horizon behavior is not controlled or influenced by the ip split-horizon command.
21. What are the primary functions of the pdm?
Eigrp supports 3 protocol suites: ip, ipv6, and ipx. Each of them has its own pdm. These are the primary functions of pdm:
Maintaining the neighbor and topology tables of eigrp routers that belong to that protocol suite
Building and translating protocol specific packets for dual Interfacing dual to the protocol specific routing table Computing the metric and passing this information to dual; dual handles only the picking of the feasible successors (fss) Implement filtering and access lists.
Perform redistribution functions to/from other routing protocols.
22. What are the various load-balancing options available in eigrp?
The offset-list can be used to modify the metrics of routes that eigrp learns through a particular interface, or pbr can be used.
23. What does the %dual-5-nbrchange: ip-eigrp(0) 100: neighbor 10.254.0.3 (tunnel0) is down: holding time expired error message mean?
This message indicates that the router has not heard any eigrp packets from the neighbor within the hold-time limit. Because this is a packet-loss issue, check for a layer 2 problem.
24. From the 16:29:14.262 poison squashed: 10.x.x.x/24 reverse message, what does poison squashed mean?
The router threads a topology table entry as a poison in reply to an update received (the router sets up for poison reverse). While the router is building the packet that contains the poison reverse, the router realizes that it does not need to send it. For example, if the router receives a query for the route from the neighbor, it is currently threaded to poison. Thus, it sends the poison squashed message.
25. What is OSPF Routing Protocol?
Open shortest path first is an Open Standard Link State routing protocol which works by using Dijkastra algorithm to initially construct the shortest paths and follows that by populating the routing table with resulting best paths.
26. What are the steps required to change Neighborship into adjacency?
1.Two-way communication (using Hello Protocol).
2.Database Synchronization which means exchange of Database Description (DD) packets, Link State Request (LSR) packets, Link State Update (LSU) packets. After Database synchronization is complete, the two routers are considered adjacent.
27. Explain LSA (Link-State Advertisement), LSU (Link State Update) and LSR (Link State Request)?
The LSAs (Link-State Advertisements) are used by OSPF routers to exchange routing and topology information. When two neighbors decide to exchange routes, they send each other a list of all LSAs in their respective topology database. Each router then checks its topology database and sends Link State Request (LSR) message requesting all LSAs that was not found in its topology table. Other router responds with the Link State Update (LSU) that contains all LSAs requested by the neighbor.
28. Explain OSPF Router ID?
Router Id is used to identify the Router. Highest IP address of the router’s loopback interfaces is chosen as the Router ID, If no loopback is present than highest IP address of the router’s physical interfaces will be chosen as Router ID. OSPF prevents neighborships between routers with duplicate RIDs. All OSPF RIDs in a domain should be unique. OSPF Router ID should not be changed after the OSPF process is started and the OSPF neighborships are established. If you change the OSPF router ID, we need to either reload the IOS or use “clear ip ospf process” command (restart the OSPF process) for changed RID to take effect.
To manually configure the router ID
R1(config)# router ospf 5
R1(config-router)# router-id 126.96.36.199
29. Can we use OSPF without backbone area?
Yes, but than only intra-area communication is possible. Inter-area communication is not possible without backbone area.
30. What is the difference between an OPPF neighbor and an adjacent neighbor?
LSAs are exchanged only among adjacent routers not among neighbor routers.