Chapter 4

"Do I Know This Already?"

1.	B
2.	A
3.	B
4.	D
5.	B
6.	C
7.	D
8.	B
9.	A
10.	A, B, C, D
11.	C
12.	B

Q&A

The passive-interface interface command prevents EIGRP from speaking on an interface. If it cannot speak, it cannot form neighbors. In the example, passive-interface serial0/0 would prevent the serial interface from being involved in EIGRP.

A second way to limit EIGRP interfaces is to use the network network wildcard-mask to specify the interface that should run. In the example, network 10.1.1.1. 0.0.0.0 would limit EIGRP to fastethernet 0/0.

The preferred configuration for a hybrid multipoint NBMA network, in which one circuit is lower than the other circuits, is to create a point-to-point subinterface for the lower circuit and then to configure the bandwidth to reflect the CIR of the link. Another subinterface should be created as a multipoint interface with a configured bandwidth that equals the aggregate CIR of all the circuits—in this instance, 5 * 256 kbps, or 1280 kbps. An alternative solution is to configure each PVC as a point-to-point link.

EIGRP will assume each PVC is 56 kbps because it divides bandwidth evenly between PVCs on a multipoint interface. By default, EIGRP will limit advertisements to half that (28 kbps).

The command no auto-summary is used to turn off automatic summarization, which EIGRP applies at classful network boundaries. With automatic summarization off, manual summarization can still be used to summarize appropriately.

The ip bandwidth-percent eigrp 63 100 command overrides the default bandwidth of 50 percent that is allocated to EIGRP for network overhead. This command sets the bandwidth allocation to be 100 percent of the link for the EIGRP autonomous system of 63 on the interface upon which it is configured. This command would be used if the bandwidth command had set the logical bandwidth of the link to be artificially low.

The hold time is three times the hello timer. The hold time is 15 seconds or 180 seconds, depending on the medium.

The variance command is used to determine additional paths to be included in load-balancing traffic to remote networks. The command is used in conjunction with the multiplier number. This number multiplies the path with the best (lowest) metric by the number stated as the multiplier. Any paths that the router knows of that have a metric value equal to or less than this value are included in the paths for load balancing. The amount of traffic sent across each path will be proportional to the metric value of the path.

The command show ip eigrp topology shows the passive and active state of the routes that are contained in the table. The route is passive if the route is operational; it is set in an active state if the router must query its neighbors for alternative paths to a network. This command also shows the number of successors and the neighbors and distance information.

The interface command ip summary-address eigrp autonomous-system-number address mask is used to define the summary address to be used.

10.

The command debug eigrp packet displays the types of EIGRP packets that are both sent and received by the router.

11.

Summarization reduces the amount of resources needed by both the network and the routers within the network. Summarization limits advertisements, minimizes the size of advertisements, and optimizes routing table lookups and recalculations. It also reduces the scope of the queries sent out by a router.

12.

The stub configuration is typically used on small-capacity routers in a hub-and-spoke WAN environment. The router has a limited set of neighbors and does not need to be involved in convergence. It is not necessary, therefore, for this remote router to have a complete routing table that might overwhelm its limited resources. The remote router needs only a default route to the distribution router that can serve all its needs.

Another reason to configure the remote router as a stub is to optimize queries in the network. When a query is sent to a stub router, the router will immediately respond, thus accelerating the process.

13.

Descriptions of the parameters are listed in the following table.

Parameter	Description
connected	(Optional) Advertises connected routes
summary	(Optional) Advertises summary routes

14.

Any feasible successor in the topology table can be used in the variance command.

15.

The command ip eigrp hello-interval timer is typically used to decrease the time between hellos to ensure that the network is more stable and converges more quickly. Although this increases the amount of bandwidth consumed, it is a minimal cost. It becomes very useful in WANs, particularly when NBMA clouds are used. NBMA technologies (such as Frame Relay) assume a low bandwidth medium (less than T1 speeds) and set the hello timer to 60 seconds by default.

16.

EIGRP will assume that bandwidth is evenly distributed between PVCs on a multipoint interface. The bandwidth command serves two purposes in this situation—it is used as part of the EIGRP metric and it is used to determine how much traffic EIGRP will send over an interface. Generally, the best way to configure this interface is to take the slowest PVC and multiply it by the number of PVCs. In this case 5 * 56 = 280. A better solution would be to divide the PVCs into separate subinterfaces.

Another solution is to configure many subinterfaces, each with a point-to-point link.

17.

If the multipoint network has differing speeds allocated to the VCs, it is much easier to configure and manage an environment that has used subinterfaces, where a VC is logically treated as if it were a separate interface or point-to-point. The bandwidth command can be configured on each subinterface, which allows different speeds on each VC. In this second solution, subinterfaces are configured for the links with the differing CIRs. The links that have the same configured CIR are presented as a single subinterface with a bandwidth, which reflects the aggregate CIR of all the circuits. Cisco recommends this as the preferred solution.