Content Overview Enhanced Interior
Gateway Routing Protocol (EIGRP) is a Cisco-proprietary routing
protocol based on Interior Gateway Routing Protocol (IGRP).
Unlike IGRP, which is a classful routing protocol, EIGRP
supports classless interdomain routing (CIDR), allowing network
designers to maximize address space by using CIDR and
variable-length subnet mask (VLSM). Compared to IGRP, EIGRP
boasts faster convergence times, improved scalability, and
superior handling of routing loops. Furthermore, EIGRP can
replace Novell Routing Information Protocol (RIP) and AppleTalk
Routing Table Maintenance Protocol (RTMP), serving both IPX and
AppleTalk networks with powerful efficiency. EIGRP is often
described as a hybrid routing protocol, offering the best of
distance vector and link-state algorithms. EIGRP is an advanced
routing protocol that relies on features commonly associated
with link-state protocols. Some of the best features of OSPF,
such as partial updates and neighbor discovery, are similarly
put to use by EIGRP. However, EIGRP is easier to configure than
OSPF. EIGRP is an ideal choice for large, multi-protocol
networks built primarily on Cisco routers. This module covers
common EIGRP configuration tasks. Particular attention is paid
to the ways in which EIGRP establishes relationships with
adjacent routers, calculates primary and backup routes, and
when necessary, responds to failures in known routes to a
particular destination. A network is made up of many devices,
protocols, and media that allow data communication to happen.
When one piece of the network does not work properly, one or
two users may be unable to communicate, or the entire network
may fail. In either case, the network administrator must
quickly identify and troubleshoot problems when they arise.
Network problems commonly result from the following:
- Mistyped commands
- Incorrectly constructed or
incorrectly placed access lists
- Misconfigured routers,
switches, or other network devices
- Bad physical
connections
A network administrator should approach
troubleshooting in a methodical manner, using a general
problem-solving model. It is often useful to check for physical
layer problems first and then move up the layers in an
organized manner. Although this module will focus on
troubleshooting the operation of routing protocols, which work
at Layer 3, it is important to eliminate any problems that may
exist at lower layers. Students completing this module should
be able to: - Describe the differences between EIGRP and
IGRP
- Describe the key concepts, technologies, and data
structures of EIGRP
- Understand EIGRP convergence and
the basic operation of the Diffusing Update Algorithm
(DUAL)
- Perform a basic EIGRP configuration
- Configure EIGRP route summarization
- Describe the
processes used by EIGRP to build and maintain routing
tables
- Verify EIGRP operations
- Describe the
eight-step process for general troubleshooting
- Apply a
logical process to routing troubleshooting
- Troubleshoot a RIP routing process using show and
debug commands
- Troubleshoot an IGRP routing
process using show and debug commands
- Troubleshoot an EIGRP routing process using show and
debug commands
- Troubleshoot an OSPF routing
process using show and debug commands
Content 3.1 EIGRP Concepts 3.1.1
Comparing EIGRP with IGRP Cisco released EIGRP in 1994 as a
scalable, improved version of its proprietary distance vector
routing protocol, IGRP. The same distance vector technology
found in IGRP is used in EIGRP, and the underlying distance
information remains the same. EIGRP improves the convergence
properties and the operating efficiency significantly over
IGRP. This allows for an improved architecture while retaining
the existing investment in IGRP. Comparisons between EIGRP and
IGRP fall into the following major categories:
- Compatibility mode
- Metric calculation
- Hop
count
- Automatic protocol redistribution
- Route
tagging
IGRP and EIGRP are compatible with each
other. This compatibility provides seamless interoperability
with IGRP routers. This is important so users can take
advantage of the benefits of both protocols. EIGRP offers
multiprotocol support, but IGRP does not. EIGRP and IGRP use
different metric calculations. EIGRP scales the metric of IGRP
by a factor of 256. That is because EIGRP uses a metric that is
32 bits long, and IGRP uses a 24-bit metric. By multiplying or
dividing by 256, EIGRP can easily exchange information with
IGRP. IGRP has a maximum hop count of 255. EIGRP has a maximum
hop count limit of 224. This is more than adequate to support
the largest, properly designed internetworks. Enabling
dissimilar routing protocols such as OSPF and RIP to share
information requires advanced configuration. Redistribution,
the sharing of routes, is automatic between IGRP and EIGRP as
long as both processes use the same autonomous system (AS)
number. In Figure , RTB automatically redistributes
EIGRP-learned routes to the IGRP AS, and vice versa. EIGRP will
tag routes learned from IGRP or any outside source as external
because they did not originate from EIGRP routers. IGRP cannot
differentiate between internal and external routes. Notice that
in the show ip route command output for the routers in
Figure , EIGRP routes are flagged with D, and external routes
are denoted by EX. RTA identifies the difference between the
network learned via EIGRP (172.16.0.0) and the network that was
redistributed from IGRP (192.168.1.0). In the RTC table, the
IGRP protocol makes no such distinction. RTC, which is running
IGRP only, just sees IGRP routes, despite the fact that both
10.1.1.0 and 172.16.0.0 were redistributed from EIGRP.
Interactive Media Activity Checkbox: IGRP and EIGRP
Comparison When the student has completed this activity, the
student will be able to identify the difference between IGRP
and EIGRP.
Content 3.1 EIGRP Concepts
3.1.2 EIGRP concepts and terminology EIGRP routers keep
route and topology information readily available in RAM, so
they can react quickly to changes. Like OSPF, EIGRP saves this
information in several tables and databases. EIGRP saves routes
that are learned in specific ways. Routes are given a
particular status and can be tagged to provide additional
useful information. EIGRP maintains three tables: -
Neighbor table
- Topology table
- Routing
table
The neighbor table is the most important table
in EIGRP. Each EIGRP router maintains a neighbor table that
lists adjacent routers. This table is comparable to the
adjacency database used by OSPF. There is a neighbor table for
each protocol that EIGRP supports. When newly discovered
neighbors are learned, the address and interface of the
neighbor is recorded. This information is stored in the
neighbor data structure. When a neighbor sends a hello packet,
it advertises a hold time. The hold time is the amount of time
a router treats a neighbor as reachable and operational. In
other words, if a hello packet is not heard within the hold
time, then the hold time expires. When the hold time expires,
the Diffusing Update Algorithm (DUAL), which is the EIGRP
distance vector algorithm, is informed of the topology change
and must recalculate the new topology. The topology table is
made up of all the EIGRP routing tables in the autonomous
system. DUAL takes the information supplied in the neighbor
table and the topology table and calculates the lowest cost
routes to each destination. By tracking this information, EIGRP
routers can identify and switch to alternate routes quickly.
The information that the router learns from the DUAL is used to
determine the successor route, which is the term used to
identify the primary or best route. A copy is also placed in
the topology table. Every EIGRP router maintains a topology
table for each configured network protocol. All learned routes