CISCO Online Training Interview Questions

• What is a Partial Sequence Number Packet (PSNP)? How is it used?

A: A Partial Sequence Number Packet contains a listing of one or more LSPs. It has two uses: On point-to-point networks, it is used to acknowledge the receipt of LSPs. On broadcast networks, it is used to request LSPs.

• What is the purpose of the Overload (OL) bit?

A: An IS-IS router uses the Overload bit to inform its neighbors that it is experiencing a memory overload and cannot store the entire link state database.

• What is the purpose of the Attached (ATT) bit?

A: The Attached bit is used by L1/L2 routers to inform L1 routers that it is attached to the L2

• What metrics are specified by the ISO for IS-IS? How many of these metrics does the Cisco IOS support?

A: The ISO specifies four metrics: Default, Expense, Delay, and Error. Cisco supports only the

Default metric.

• What is the maximum value of the IS-IS default metric?

A: The maximum value of any of the IS-IS metrics is 63.

• What is the maximum metric value of an IS-IS route?

A: The maximum metric value of an IS-IS route is 1023.

• What is the difference between a level 1 IS-IS metric and a level 2 IS-IS metric?

A: L1 IS-IS metrics apply to intra-area routes, and L2 IS-IS metrics apply to inter-area routes.

• What is the difference between an internal IS-IS metric and an external IS-IS metric?

A: Internal metrics apply to routes to destinations within the IS-IS domain. External metrics apply to routes to destinations external to the IS-IS domain.

• From what sources can a route be redistributed?

A: Routes that are learned from another routing protocol, static routes, or a direct connection to the destination network can be redistributed into a routing domain.

• What is the purpose of an administrative distance?

A: In contrast to metrics, which are used to determine the best path among multiple routes to the same destination discovered by the same routing protocol, administrative distances are used to determine the best path among multiple routes to the same destination discovered by different routing protocols.

• How can administrative distances cause problems when redistributing?

A: A route to a destination within a routing domain with a higher administrative distance can be redistributed into a routing domain with a lower administrative distance. If that route is redistributed back into the higher-distance domain, packets might be misrouted into the lower-distance domain.

• How can redistribution from a classless to a classful routing protocol cause problems?

A: Redistributing variably subnetted destination addresses from a classless domain into a classful domain can cause problems.

• Which IP IGPs can use the default redistribution metric, and which IGPs must have a metric configured in order for redistribution to work?

A: OSPF and IS-IS understand the default metric. RIP, IGRP, and EIGRP do not.

• What is the difference between using the metric keyword with the redistributecommand and using the default-metric command?

A: The metric command assigns a metric to specific redistribution statements. The default-metric command assigns a metric to all redistribution commands that do not include the metric command.

• What is the purpose of the subnets keyword when redistributing OSPF?

A: Without the subnets keyword, only major network addresses that are not directly connected to the router will be redistributed.

• How is the null interface useful when summarizing routes?

A: A router that originates a summary route should use the null interface as the next hop of the summary route. Any packets that match the summary route, but for which there is no more-specific route to the packet’s destination address, will be dropped. This prevents the router from forwarding “lost” packets.

• What is the destination address of default routes used by the open protocols?

A: The default route address is 0.0.0.0.

• How are default routes identified and advertised by IGRP and EIGRP?

A: IGRP and EIGRP advertise a default address as an external address type.

• Can a static route to 0.0.0.0 be used as the default route on a router running IGRP?

A: Yes.

• What is a stub router? What is a stub network?

A: A stub router is a router with only a single link to another router. A stub network is a network with only one attached router.

• What is an advantage of using default routes instead of a full routing table?

A: Using a default route rather than a full routing table can conserve router memory by keeping the table small and can save router processing cycles by limiting the routing information that must be processed.

• What is an advantage of using a full routing table instead of a default route?

A: Using a full routing table rather than a default route can make routing more accurate.

• What data link protocol does On-Demand Routing use to discover routes?

A: ODR uses Cisco Discovery Protocol (CDP) to discover routes.

• What IOS restrictions are placed on ODR?

A: ODR is available in IOS 11.2 and later.

• What media restrictions are placed on ODR?

A: The medium over which ODR is to run must support SNAP.

• How are route maps similar to access lists? How are they different?

A: Route maps are similar to access lists in that they define match criteria and an action to take in theevent of a match. Route maps are different from access lists in that they not only specify matchcriteria but also specify set criteria. The set action can modify a route or route a packet according tothe parameters of the packet.

• What are policy routes?

A: Policy routes are static routes that use route maps to determine which packets should be routed andwhere the packets should be routed.

• What are route tags?

A: Route tags are fields within routing information packets that allow external information to be carried through the routing domain.

• In what way do route tags affect routing protocols?

A: Route tags have no effect on the routing protocols that carry them.