EIGRP is an enhanced version of the IGRP developed by Cisco. EIGRP uses the same distance vector algorithm and distance information as IGRP. However, the convergence properties and the operating efficiency of EIGRP have improved substantially over IGRP.

The convergence technology is based on research conducted at SRI International and employs an algorithm called the diffusing update algorithm (DUAL). This algorithm guarantees loop-free operation at every instant throughout a route computation and allows all devices involved in a topology change to synchronize at the same time. Routers that are not affected by topology changes are not involved in recomputations. The convergence time with DUAL rivals that of any other existing routing protocol.

EIGRP provides the following features:

• Increased network width--With Routing Information Protocol (RIP), the largest possible width of your network is 15 hops. When EIGRP is enabled, the largest possible width is 224 hops. Because the EIGRP metric is large enough to support thousands of hops, the only barrier to expanding the network is the transport layer hop counter. Cisco works around this limitation by incrementing the transport control field only when an IPv4 or an IPv6 packet has traversed 15 routers and the next hop to the destination was learned by way of EIGRP. When a RIP route is being used as the next hop to the destination, the transport control field is incremented as usual.
• Fast convergence--The DUAL algorithm allows routing information to converge as quickly as any other routing protocol.
• Partial updates--EIGRP sends incremental updates when the state of a destination changes, instead of sending the entire contents of the routing table. This feature minimizes the bandwidth required for EIGRP packets.
• Neighbor discovery mechanism--This is a simple hello mechanism used to learn about neighboring routers. It is protocol-independent.
• Arbitrary route summarization.
• Scaling--EIGRP scales to large networks.
• Route filtering--EIGRP for IPv6 provides route filtering using the distribute-list prefix-listcommand. Use of the route-map command is not supported for route filtering with a distribute list.

EIGRP has the following four basic components:

• Neighbor discovery--Neighbor discovery is the process that routers use to dynamically learn of other routers on their directly attached networks. Routers must also discover when their neighbors become unreachable or inoperative. EIGRP neighbor discovery is achieved with low overhead by periodically sending small hello packets. EIGRP neighbors can also discover a neighbor that has recovered after an outage because the recovered neighbor will send out a hello packet. As long as hello packets are received, the Cisco IOS software can determine that a neighbor is alive and functioning. Once this status is determined, the neighboring routers can exchange routing information.
• Reliable transport protocol--The reliable transport protocol is responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors. It supports intermixed transmission of multicast and unicast packets. Some EIGRP packets must be sent reliably and others need not be. For efficiency, reliability is provided only when necessary. For example, on a multiaccess network that has multicast capabilities (such as GigabitEthernet) it is not necessary to send hello packets reliably to all neighbors individually. Therefore, EIGRP sends a single multicast hello with an indication in the packet informing the receivers that the packet need not be acknowledged. Other types of packets (such as updates) require acknowledgment, which is indicated in the packet. The reliable transport has a provision to send multicast packets quickly when unacknowledged packets are pending. This provision helps to ensure that convergence time remains low in the presence of varying speed links.
• DUAL finite state machine--The DUAL finite state machine embodies the decision process for all route computations. It tracks all routes advertised by all neighbors. DUAL uses several metrics including distance and cost information to select efficient, loop-free paths. When multiple routes to a neighbor exist, DUAL determines which route has the lowest metric (named the feasible distance), and enters this route into the routing table. Other possible routes to this neighbor with larger metrics are received, and DUAL determines the reported distance to this network. The reported distance is defined as the total metric advertised by an upstream neighbor for a path to a destination. DUAL compares the reported distance with the feasible distance, and if the reported distance is less than the feasible distance, DUAL considers the route to be a feasible successor and enters the route into the topology table. The feasible successor route that is reported with the lowest metric becomes the successor route to the current route if the current route fails. To avoid routing loops, DUAL ensures that the reported distance is always less than the feasible distance for a neighbor router to reach the destination network; otherwise, the route to the neighbor may loop back through the local router.
• Protocol-dependent modules--When there are no feasible successors to a route that has failed, but there are neighbors advertising the route, a recomputation must occur. This is the process where DUAL determines a new successor. The amount of time required to recompute the route affects the convergence time. Recomputation is processor-intensive; it is advantageous to avoid unneeded recomputation. When a topology change occurs, DUAL will test for feasible successors. If there are feasible successors, DUAL will use them in order to avoid unnecessary recomputation.

The protocol-dependent modules are responsible for network layer protocol-specific tasks. An example is the EIGRP module, which is responsible for sending and receiving EIGRP packets that are encapsulated in IPv4 or IPv6. It is also responsible for parsing EIGRP packets and informing DUAL of the new information received. EIGRP asks DUAL to make routing decisions, but the results are stored in the IPv4 or IPv6 routing table. Also, EIGRP is responsible for redistributing routes learned by other IPv4 or IPv6 routing protocols.

1.    enable

2.    configure terminal

3.    ipv6 unicast-routing

4.    interface type number

5.    no shut

6.    ipv6 enable

7.    ipv6 eigrp as-number

8.    ipv6 router eigrp as-number

9.    eigrp router-id ip-address

10.    exit

11.    show ipv6 eigrp [as-number] interfaces [type number] [detail

1.    enable

2.    configure terminal

3.    interface type number

4.    no shut

5.    ipv6 bandwidth-percent eigrp as-number percent

1.    enable

2.    configure terminal

3.    interface type number

4.    no shut

5.    ipv6 summary-address eigrp as-number ipv6-address [admin-distance

1.    enable

2.    configure terminal

3.    interface type number

4.    no shut

5.    ipv6 authentication mode eigrp as-number md5

6.    ipv6 authentication key-chain eigrp as-number key-chain

7.    exit

8.    key chain name-of-chain

9.    key key-id

10.    key-string text

11.    accept-lifetime start-time infinite | end-time| duration seconds

12.    send-lifetime start-time infinite | end-time | duration seconds

1.    enable

2.    configure terminal

3.    interface type number

4.    no shut

5.    no ipv6 next-hop-self eigrp as-number

1.    enable

2.    configure terminal

3.    interface type number

4.    no shut

5.    ipv6 hello-interval eigrp as-number seconds

1.    enable

2.    configure terminal

3.    interface type number

4.    no shut

5.    ipv6 hold-time eigrp as-number seconds

1.    enable

2.    configure terminal

3.    interface type number

4.    no shut

5.    no ipv6 split-horizon eigrp as-number

The EIGRP stub routing feature can help to provide greater network stability. In the event of network instability, this feature prevents EIGRP queries from being sent over limited bandwidth links to nontransit routers. Instead, distribution routers to which the stub router is connected answer the query on behalf of the stub router. This feature greatly reduces the chance of further network instability due to congested or problematic WAN links. The EIGRP stub routing feature also simplifies the configuration and maintenance of hub-and-spoke networks. When stub routing is enabled in dual-homed remote configurations, it is no longer necessary to configure filtering on remote routers to prevent those remote routers from appearing as transit paths to the hub routers.

Caution

EIGRP stub routing should be used only on stub routers. A stub router is defined as a router connected to the network core or distribution layer through which core transit traffic should not flow. A stub router should not have any EIGRP neighbors other than distribution routers.

• Configuring a Router for EIGRP Stub Routing
  
• Verifying EIGRP Stub Routing
  

After you have enabled EIGRP for IPv6 on a specific interface, you can configure an EIGRP for IPv6 routing process.

• Logging EIGRP Neighbor Adjacency Changes
  
• Configuring Intervals Between Neighbor Warnings
  

EIGRP for IPv6 uses the minimum bandwidth on the path to a destination network and the total delay to compute routing metrics. You can use the metric weights command to adjust the default behavior of EIGRP for IPv6 routing and metric computations. EIGRP for IPv6 metric defaults have been carefully selected to provide optimal performance in most networks.

Note	Adjusting EIGRP metric weights can dramatically affect network performance. Because of the complexity of this task, we recommend that you do not change the default values without guidance from an experienced network designer.

By default, the EIGRP composite metric is a 32-bit quantity that is a sum of the segment delays and the lowest segment bandwidth (scaled and inverted) for a given route. For a network of homogeneous media, this metric reduces to a hop count. For a network of mixed media (e.g., GigabitEthernet, FastEthernet, Ethernet), the route with the lowest metric reflects the most desirable path to a destination.

1.    enable

2.    configure terminal

3.    ipv6 router eigrp as-number

4.    metric weights tos k1 k2 k3 k4 k5

1.    enable

2.    clear ipv6 eigrp as-number ] [neighbor [ipv6-address | interface-type interface-number]]