EIGRP(Enhanced Interior Gateway Routing Protocol)

Some facts

• It’s enhanced distance-vector protocol.


• It’s classless.


• Supports manual route summarization


• Performs autosummarization by default


• Network layer.


• The range of valid AS numbers is 1 through 65,535


• The fastest convergence


• It uses the concept of an autonomous system.


• Wildcards are used


• Supports MD5 auth


• It’s hybrid routing protocol because it has characteristics of both distance-vector and link-state protocols.


• EIGRP has a maximum hop count of 255 (the default is set to 100).


• It provides routing support for multiple Network layer protocols: IP, IPX, AppleTalk, and now IPv6


• Communication via Reliable Transport Protocol (RTP) - Cisco proprietary protocol, is used for transmitting EIGRP messages between routers. It can provide reliable packet delivery when necessary, by using sequence number, acknowledgement, and retransmission with unicast (a packet will be retransmitted up to 16 times if no acknowledgement is received, before declaring the neighbor router is dead).


• Diffusing Update Algorithm (DUAL).It is used to determine a route to network, when successor failed and there is no feasible successors. This algoritm just asks neighbor routers for route.


• Technologies: Split horizon, Poison Reverse.


• Conseptions:

1. Reported distance - distance to the remote network, reported by the neighbor router.


2. Feasible distance - This is the best metric along all paths to a remote network, including the metric to the neighbor that is advertising that remote network. This is the route that you will find in the routing table because it is considered the best path.


3. Successor - next hop router with the best metric, is placed in routing table of router, there might be several successors, if they have equal metrics.


4. Feasible successor - backup next hop router. EIGRP will keep up to six feasible successors in the topology table. If a nonsuccessor route’s RD is less than the FD, the route is a feasible successor route.

• All data is stored in three tables:

1. Neighbor table - stores data about neighboring routers.


2. Topology table – stores contains the aggregation of the routing tables gathered from all directly connected neighbors. There also may be successors and feasible successors for some routes in this table. Destination may be passive or active. Passive means that route is stable. Active means that route is updating. In Cisco routers route may be: Passive, Active, Update, Query, Reply.


3. Routing table – contains all actual routes with their successors and possible feasible successors, if load balancing is enabled.

• It uses 4 metrics(by default delay and bandwidrth are used):

1. Total Delay (the sum of all the delay values for all links in the route, with a unit of “tens of microseconds.”)


2. Minimum Bandwidth ( the lowest-bandwidth link in the route,using a unit of kilobits per second)


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


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

• The router uses formula to calculate overall metric:

metric=(107/least-bandwidth + cumulative delay)*256

Bandwidth - kilobits per second.

Delay - tens of microseconds

In this case k1=1,k2=0,k3=1,k4=0,k5=0 by default(only Bandwidth and Delay is used). You can change their value with command:

Router(config-router)#metric weights …

• Administrative distance - 90 for internal route(in one AS) an 170 for external route(in different AS or from other protocol).


• Multicast 224.0.0.10 for Hello messages, but updates can also be sent to unicast address. FF02::A for IPv6.


• EIGRP can provide equal-cost load balancing of up to four links by default, but it is possible to use 16 links with command:

(config-router)#maximum-paths

• EIGRP can provide unequal-cost load balancing with command

variance.

• First routers have to establish neighborship , to do it, three conditions must be met:

1. Hello or ACK received


2. AS numbers match


3. Identical metrics (K values)


4. Auth passed


5. The same subnet

Show commands

show ip eigrp topology all-links

Shows all routes, not only successors and feasable successors.

Debuging

debug eigrp packets

Displays the contents of EIGRP packets.

debug eigrp fsm

Displays changes to the EIGRP successor and feasible successor routes.

debug ip eigrp

Displays similar output to the debug eigrp packets command, but specifically for IP.

EIGRP packet types

• Hello packet It is used for discovering neighbors and maintaining neighbor relationship. It is sent to neighbors periodically (hello interval). The default hello interval is: Non-broadcast multi-access (NBMA) network with bandwidth Other networks (e.g. Ethernet, point-to-point serial links, high speed frame relay, etc.) - 5 seconds . A neighbor is assumed dead if no hello message from that neighbor is received before the hold timer expires (default = 3 times of the hello timer value). All routes learned from that neighbor will also be deleted. Neighbor routers can use different hello and hold-time intervals. A router informs the neighbors its hold-time interval through hello packets. It is sent as multicast. Acknowledgement is not required (i.e. unreliable delivery).


• Update packet It is used for sending routing information to neighbors as follows: Full routing updates (i.e. the whole topology table) during initialization of the routing process. Incremental routing updates (i.e. routing updates about the paths that have been changed) when there is a change in the network topology or metric. It is sent to the relevant routers as unicast or multicast. Acknowledgement is required (i.e. reliable delivery).


• Query packet It is used for querying neighbors if they have feasible successors for a destination network. It is sent during a diffusing computation (to be explained later in this Section). It is sent as multicast. Acknowledgement is required (i.e. reliable delivery).


• Reply packet It is used for replying a query with the requested routing information (e.g. information of the best path to the destination network, or destination unreachable). It is sent as unicast. Acknowledgement is required (i.e. reliable delivery).


• Acknowledgement (ACK) packet It is used for acknowledging the receipt of an update / query / reply packet. It is sent as unicast. Acknowledgement is not required (i.e. unreliable delivery).


• Redistribution is required when more than one EIGRP session or process is running and they are identified with different ASNs. Redistribution shares topology information between EIGRP sessions.