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EIGRP is a Cisco proprietary protocol, that as an administrative distance of 90 and it is an enhanced distance vector protocol, relying on the Diffused Update Algorithm (DUAL) to calculate the shortest path to a destination network.
EIGRP Basic Configuration:
To start the EIGRP process in configuration mode type:
Router(config)# router eigrp [AS number]
Configure which interfaces should participate in the EIGRP process.
Router(config-router)# network [ip address]
or
Router(config-router)# network [ip address] [wildcard mask]
The AS number must be the same on all routers within the EIGRP domain, otherwise a neighbor relationship won't be established. The ip address in the network statement can be a classfull address (doesn't need a subnet/wildcard mask) this will set all interfaces within that classfull network to be part of the EIGRP process, or a classless address with the respective subnet/wildcard mask, this can be either a subnet or a 32 bits address.
After issuing this commands EIGRP will try to establish a neighboring relationship with the neighbor router, if this relationship is successful, EIGRP will start sharing routing information with that neighbor.
EIGRP Concepts and Terminology:
EIGRP uses two different tables that are populated with information gathered from its neighbors.
Neighbor table - This is where the router keeps a record of neighbors Ip addresses, Interface where the neighbor is connected to and Hold time values.
Topology table - List all the routes that are available to EIGRP, the command "show ip eigrp topology" display only the routes that meet the feasible condition and the command "show ip eigrp topology all-links" display the entire topology table including successor, feasible successor, and non-successor routes.
Advertised or Reported Distance (AD/RD) - The Advertised Distance (AD) is the distance from a given neighbor to the destination router.
Feasible Distance - The Feasible Distance (FD) is the distance from the current router to the destination router.
Feasibility Condition - Within EIGRP there is requirement which must be met for a route to be considered feasible and loop-free. This requirement states that in order for a route to be feasible the Advertised Distance of the alternate route must be lower than that of the Feasible distance of the current route.
Successor - The Successor(s) are the current routes which are the best and are entered into the routing table. By default, these are the ones with the lowest metric and which meet the feasibility condition.
Feasible Successor - If an alternate route exists and it meets the requirements of the Feasibility Condition then it is considered a Feasible Successor.
EIGRP Verification Commands:
Note: Below is an explanation of some of the verification commands, for a full list refer to Cisco WebSite
Router# show ip eigrp interfaces
List working interfaces of which EIGRP is enabled, it omites passive interfaces.
Output overview:
Router#show ip eigrp interfaces
EIGRP-IPv4 Interfaces for AS(100)
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Et0/0 1 0/0 11 0/2 50 0
Et0/1 1 0/0 14 0/2 60 0
Se2/0 1 0/0 16 0/15 91 0
Interface: Interface over which EIGRP is configured.
Peers: Number of directly connected EIGRP neighbors.
Xmit Queue Un/Reliable: Number of packets remaining in the Unreliable and Reliable transmit queues.
Mean SRTT: Mean smooth round-trip time (SRTT) interval (in seconds).
Pacing Time Un/Reliable: Pacing time (in seconds) used to determine when EIGRP packets should be sent out the interface (unreliable and reliable packets).
Multicast Flow Timer: Maximum number of seconds for which the router will send multicast EIGRP packets.
Pending Routes: Number of routes in the packets in the transmit queue waiting to be sent.
The command "show ip eigrp interfaces detail" can also be used to show additional information such as, hello-interval and hold time values, if split horizon is enable/disable, authentication mode and key-chain, etc...
Router# show ip eigrp neighbors
Lists known neighbors; does not list neighbors for which some mismatched parameter is preventing a valid EIGRP neighbor relationship.
Output overview:
Router#show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(100)
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
2 14.0.0.4 Se2/0 13 00:00:25 46 276 0 15
1 13.0.0.3 Et0/1 11 00:00:26 15 200 0 25
0 12.0.0.2 Et0/0 11 00:00:27 9 200 0 15
H (Handle): Order of when the neighbor adjacency was established. The first neighbor will have a value of 0, the second neighbor a value of 1 and so on.
Hold (sec): This is the holddown timer per EIGRP neighbor. Once this timer expires the router will drop the neighbor adjacency. The default holddown timer is 15 seconds. Any EIGRP packet after the first hello will reset the holddown timer.
Uptime: Shows how long the neighbor has been up. This number should be up for as long as the link has been up.
SRTT (Smooth round-trip time): The number of milliseconds it takes to send an EIGRP packet to the neighbor and receive an acknowledgment packet back.
RTO (Retransmission timeout): The amount of time in milliseconds that EIGRP will wait before retransmitting a packet from the retransmission queue to his neighbor.
Q Cnt (Q count): The number of EIGRP packets (Update, Query or Reply) in the queue that are awaiting transmission. Ideally this number should be 0 otherwise it might be an indication of congestion on the network.
Seq Num (Sequence number): This will show the sequence number of the last update,query or reply packet the router received from its EIGRP neighbor.
Router# show ip eigrp topology
Lists all successor and feasible successor routes known to this router. It does not list all known topology details.
Output overview:
Router#show ip eigrp topology
EIGRP-IPv4 Topology Table for AS(100)/ID(14.0.0.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 14.0.0.0/24, 1 successors, FD is 2169856
via Connected, Serial2/0
P 34.0.0.0/24, 1 successors, FD is 307200
via 13.0.0.3 (307200/281600), Ethernet0/1
via 14.0.0.4 (2195456/281600), Serial2/0
P 13.0.0.0/24, 1 successors, FD is 281600
via Connected, Ethernet0/1
P 12.0.0.0/24, 1 successors, FD is 281600
via Connected, Ethernet0/0
P 23.0.0.0/24, 2 successors, FD is 307200
via 12.0.0.2 (307200/281600), Ethernet0/0
via 13.0.0.3 (307200/281600), Ethernet0/1
P 35.0.0.0/24, 1 successors, FD is 2195456
via 13.0.0.3 (2195456/2169856), Ethernet0/1
P - Passive: No EIGRP computations are being performed for this destination.
A - Active: EIGRP computations are being performed for this destination.
U - Update: Indicates that a pending update packet is waiting to be sent for this route.
Q - Query: Indicates that a pending query packet is waiting to be sent for this route.
R - Reply: Indicates that a pending reply packet is waiting to be sent for this route.
r - Reply status: Indicates that EIGRP has sent a query for the route and is waiting for a reply from the specified path.
Successors: Number of successors. This number corresponds to the number of next hops in the IP routing table. If "successors" is capitalized, then the route or next hop is in a transition state.
FD: Feasible distance. The feasible distance is the best metric to reach the destination or the best metric that was known when the route went active.
via: IP address of the peer that told the software about this destination.
(2195456/2169856): The first number is the EIGRP metric that represents the cost to the destination. The second number is the EIGRP metric that was advertised by the peer.
The command "show ip eigrp topology all-links" can be used to show the entire topology including non-successor routes.
