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EIGRP is an advanced distance vector routing protocol developed by Cisco. EIGRP is suited for
many different topologies and media. In a well-designed network, EIGRP scales well and
provides extremely quick convergence times with minimal overhead. EIGRP is a popular choice
for a routing protocol on Cisco devices.
Introducing EIGRP
EIGRP is a Cisco-proprietary routing protocol that combines the advantages of link-state and
distance vector routing protocols. EIGRP is an advanced distance vector or hybrid routing
protocol that includes the following features:
■ Rapid convergence: EIGRP uses the Diffusing Update Algorithm (DUAL) to achieve
rapid convergence. A router that uses EIGRP stores all available backup routes for
destinations so that it can quickly adapt to alternate routes. If no appropriate route or backup
route exists in the local routing table, EIGRP queries its neighbors to discover an alternate
route.
172 Chapter 5: Implementing EIGRP
■ Reduced bandwidth usage: EIGRP does not make periodic updates. Instead, it sends partial
updates when the path or the metric changes for that route. When path information changes,
DUAL sends an update about only that link rather than about the entire table.
■ Multiple network layer support: EIGRP supports AppleTalk, IP version 4 (IPv4), IP version
6 (IPv6), and Novell Internetwork Packet Exchange (IPX), which use protocol-dependent
modules (PDM). PDMs are responsible for protocol requirements that are specific to the
network layer.
■ Classless routing: Because EIGRP is a classless routing protocol, it advertises a routing
mask for each destination network. The routing mask feature enables EIGRP to support
discontiguous subnetworks and variable-length subnet masks (VLSM).
■ Less overhead: EIGRP uses multicast and unicast rather than broadcast. As a result, end
stations are unaffected by routing updates and requests for topology information.
■ Load balancing: EIGRP supports unequal metric load balancing, which allows
administrators to better distribute traffic flow in their networks.
■ Easy summarization: EIGRP enables administrators to create summary routes anywhere
within the network rather than rely on the traditional distance vector approach of performing
classful route summarization only at major network boundaries.
Each EIGRP router maintains a neighbor table. This table includes a list of directly connected
EIGRP routers that have an adjacency with this router.
Each EIGRP router maintains a topology table for each routed protocol configuration. The
topology table includes route entries for every destination that the router learns. EIGRP chooses
the best routes to a destination from the topology table and places these routes in the routing table,
as illustrated in Figure 5-1.
Figure 5-1 EIGRP Tables
IP EIGRP Neighbor Table
Next-Hop Router Interface
The IP Routing Table
Destination 1 Interface
IP EIGRP Topology Table
Destination 1 Interface
List of All Routes Learned
from Each EIGRP Neighbor
List of All Best Routes from
EIGRP Topology Table and
the Other Routing Processes
List of Directly Connected
Routers Running EIGRP
Implementing EIGRP 173
In EIGRP, the best route is called a successor route while a backup route is called the feasible
successor. To determine the best route (successor) and the backup route (feasible successor) to a
destination, EIGRP uses the following two parameters:
■ Advertised distance: The EIGRP metric for an EIGRP neighbor to reach a particular
network
■ Feasible distance: The advertised distance for a particular network learned from an EIGRP
neighbor plus the EIGRP metric to reach that neighbor
A router compares all feasible distances to reach a specific network and then selects the lowest
feasible distance and places it in the routing table. The feasible distance for the chosen route
becomes the EIGRP routing metric to reach that network in the routing table.
The EIGRP topology database contains all the routes that are known to each EIGRP neighbor.
Routers A and B send their routing tables to Router C, whose table is displayed in Figure 5-2. Both
Routers A and B have pathways to network 10.1.1.0/24, as well as to other networks that are not
shown.
Figure 5-2 Router C EIGRP Tables
Router C has two entries to reach 10.1.1.0/24 in its topology table. The EIGRP metric for Router
C to reach both Routers A and B is 1000. Add this cost (1000) to the respective advertised distance
IP EIGRP Neighbor Table
Next-Hop Router Interface
Router A
Router B
Ethernet 0
Ethernet 1
IP EIGRP Topology Table
Network
Feasible Distance
(EIGRP Metric)
EIGRP
Neighbor
Advertised
Distance
10.1.1.0/24
10.1.1.0/24
2000
2500
1000
1500
Router A (E0)
Router B (E1)
The IP Routing Table
Network
Metric
(Feasible Distance)
Next Hop
(EIGRP Neighbor)
Outbound
Interface
10.1.1.0/24 2000 Ethernet 0 Router A
Successor
Feasible Successor
174 Chapter 5: Implementing EIGRP
for each router, and the results represent the feasible distances that Router C must travel to reach
network 10.1.1.0/24.
Router C chooses the least-cost feasible distance (2000) and installs it in the IP routing table as
the best route to reach 10.1.1.0/24. The route with the least-cost feasible distance that is installed
in the routing table is called the successor route.
Router C then chooses a backup route to the successor called a feasible successor route, if one
exists. For a route to become a feasible successor, a next-hop router must have an advertised
distance that is less than the feasible distance of the current successor route.
If the route through the successor becomes invalid, possibly because of a topology change, or if a
neighbor changes the metric, DUAL checks for feasible successors to the destination route. If one
is found, DUAL uses it, avoiding the need to recompute the route. If no feasible successor exists,
a recomputation must occur to determine the new successor.