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Chapter 2: EIGRP Overview and Neighbor Relationships 23
Foundation Topics
EIGRP CCNA Review
All the CCNP exams consider CCNA materials as prerequisites, so the Cisco Press CCNP
Exam Certification Guide series of books also assumes the reader is already familiar with
CCNA topics. However, the CCNP exams do test on features that overlap with CCNA.
Additionally, most people forget some details along the way, so this section reviews the
CCNA level topics as a brief refresher.
To that end, this section begins with a review of EIGRP configuration using only the
router eigrp and network commands. Following that, the next section details the key
fields used to verify that EIGRP is working. Finally, the last part of this introduction summarizes
the basic EIGRP internals behind this initial simple example.
Configuration Review
Cisco IOS uses the router eigrp asn command, plus one or more network net-id wildcard-
mask subcommands, to enable EIGRP on the router and on router interfaces. The
rules for these commands are as follows:
1. Neighboring routers’ router eigrp asn commands must be configured with the same
ASN parameter to become neighbors.
2. IOS enables only EIGRP on interfaces matched by an EIGRP network command.
When enabled, the router does the following:
a. Attempts to discover EIGRP neighbors on that interface by sending multicast EIGRP
Hello messages
b. Advertises to other neighbors about the subnet connected to the interface
3. If no wildcard-mask is configured on the EIGRP network command, the command’s
single parameter should be a classful network number (in other words, a class A, B, or
C network number).
4. If no wildcard-mask is configured on the EIGRP network command, the command
enables EIGRP on all of that router’s interfaces directly connected to the configured
classful network.
5. If the network command includes a wildcard-mask, the router performs access control
list (ACL) logic when comparing the net-id configured in the network command
with each interface’s IP address, using the configured wildcard-mask as an ACL wildcard
mask.
Example 2-1 shows a sample configuration for each router in Figure 2-1, with several variations
in the network commands to make the details in the preceding list more obvious.
Key
Topic
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R3
Chapter 2: EIGRP Overview and Neighbor Relationships 25
IOS compares the 10.1.0.0 value with each interface IP address, but only for the bit positions
for which the WC mask lists a binary 0. For example, 0.0.31.255 represents 19 binary
0s, followed by 13 binary 1s, so R2 would compare the first 19 bits of 10.1.0.0 with the
first 19 bits of each interface’s IP address. (Note that Appendix B lists a binary/decimal
conversion table.)
Two features of the mechanics of the network command require a little extra attention.
First, IOS may convert the address portion of the network address wc-mask command
before putting the command into the running-config. Just as IOS does for the address/WC
mask combinations for the access-list command, IOS inverts the WC mask and then performs
a Boolean AND of the address and mask. For example, if you type the network
10.1.1.1 0.0.255.255 command, IOS inverts the WC mask (to 255.255.0.0), ANDs this
value with 10.1.1.1, resulting in 10.1.0.0. As a result, IOS stores the command network
10.1.0.0 0.0.255.255.
The second feature is that when you know for sure the values in the network command,
you can easily find the range of interface addresses that match the address/WC mask
combination in the network command. The low end of the range is the address as listed in
the network command. To find the high end of the range, just add the address and WC
mask together. For example, the network 10.1.0.0 0.0.31.255 command has a range of
10.1.0.0 through 10.1.31.255. (Note that the math suggested in this paragraph does not
work when the wildcard mask does not have a single string of consecutive binary 0s followed
by a single string of consecutive binary 1s.)
Finally, on R3, the network 10.1.0.0 0.0.255.255 command tells R3 to enable EIGRP on
all interfaces whose IP addresses begin with 10.1, which includes all three interfaces on
R3, as shown in Figure 2-1.
Taking a step back from the details, this config has enabled EIGRP, with ASN 1, on all
three routers, and on all interfaces shown in Figure 2-1—except one interface. R2’s Fa0/1
interface is not matched by any network commands on R2, so EIGRP is not enabled on
that interface. The next section reviews the commands that can be used to confirm that
EIGRP is enabled, the interfaces on which it is enabled, the neighbor relationships that
have been formed, and which EIGRP routes have been advertised and learned.
Verification Review
Even before starting to configure the routers, an engineer first considers all requirements.
Those requirements lead to a design, which in turn leads to a chosen set of configuration
commands. Then, the verification process that follows must consider the design requirements.
The goal of verification is to determine that the internetwork works as designed,
not just that some EIGRP routes have been learned.
For the purposes of this section, assume that the only design goal for the internetwork in
Figure 2-1 is that EIGRP be used so that all routers have routes to reach all subnets shown
in the figure.
To verify such a simple design, an engineer should start by confirming on which interfaces
EIGRP has been enabled on each router. The next step should be to determine if the
EIGRP neighbor relationships that should occur are indeed up and working. Then, the
EIGRP topology table should be examined to confirm that there is at least one entry for
each subnet or network in the design. Finally, the IP routes on each router should be examined,
confirming that all routes are known. To that end, Table 2-2 summarizes five key
show commands that provide the information to answer these questions:
Note: The table mentions some information that is covered later in this chapter (passive
interfaces) or in other chapters (successor/feasible successors).
Example 2-2 shows samples of each command listed in Table 2-2. Note that the output
highlights various samples of items that should be verified: the interfaces on which EIGRP
is enabled, the known neighbors, the subnets in the topology table, and the EIGRP routes.