Routing Protocols: EIGRP Metric

EIGRP Metric

Bandwidth

Minimum Bandwidth (in kilobits per second) along the path from router to destination network

Load

Load (number in range 1 to 255; 255 being saturated)

Delay

Total Delay (in 10s of microseconds) along the path from router to destination network

Reliability

Reliability (number in range 1 to 255; 255 being the most reliable)

MTU

Minimum path Maximum Transmission Unit (MTU) (never used in the metric calculation)

Formula

Metric = 256*([K1*Bw + K2*Bw/(256-Load) + K3*Delay]*[K5/(Reliability + K4)])

The default is for K1 and K3 to be set to 1, and the rest to zero, effectively reducing the above formula to (Bandwidth + Delay) * 256.

Routing Protocol: EIGRP TABLES

EIGRP Tables

1-Neighbor Table: 

Stores data about the neighboring routers, i.e. those directly accessible through directly connected interfaces.

Topology Table: 

Confusingly named, this table does not store an overview of the complete network topology; rather, it effectively contains only the aggregation of the routing tables gathered from all directly connected neighbors. This table contains a list of destination networks in the EIGRP-routed network together with their respective metrics. Also for every destination, a successor and a feasible successor are identified and stored in the table if they exist. Every destination in the topology table can be marked either as "Passive", which is the state when the routing has stabilized and the router knows the route to the destination, or "Active" when the topology has changed and the router is in the process of (actively) updating its route to that destination.

Routing table: 

Stores the actual routes to all destinations; the routing table is populated from the topology table with every destination network that has its successor and optionally feasible successor identified (if unequal-cost load-balancing is enabled using the variance command). The successors and feasible successors serve as the next hop routers for these destinations.

Routing Protocol: Enhanced Interior Gateway Routing Protocol

EIGRP

EIGRP is an enhanced version of IGRP. The same distance vector technology found in IGRP is also used in EIGRP, and the underlying distance information remains unchanged. The convergence properties and the operating efficiency of this protocol have improved significantly. This allows for an improved architecture while retaining existing investment in IGRP.

The Diffusing Update Algorithm (DUAL) is the algorithm used to obtain loop-freedom at every instant throughout a route computation. This allows all routers involved in a topology change to synchronize at the same time. Routers that are not affected by topology changes are not involved in the recomputation. The convergence time with DUAL rivals that of any other existing routing protocol.



EIGRP has been extended to be network-layer-protocol independent, thereby allowing DUAL to support other protocol suites.

EIGRP Characteristics

• Fast convergence.

• Support for VLSM.

• Partial updates conserve network bandwidth.

• Support for IP, AppleTalk, and IPX.

• Runs directly over IP, using protocol number 88.

• Support for all Layer 2 (data link layer) protocols and topologies.

• Sophisticated metric that supports load-balancing across unequal-cost paths .

• Use of multicast (and unicast where appropriate) instead of broadcasts.

• Support for authentication.

• Manual summarization at any interface.

• Uses multicast 224.0.0.10.