Enhanced Interior Gateway Protocol (EIGRP)
EIGRP is an classless enhanced distance vector routing protocol. EIGRP is cisco proprietary so you have to have all Cisco products in order to run EIGRP. EIGRP maintains 2 separate tables, similar to OSPF, that it uses to keep information about the network. These tables are the topology and neighbor tables. We'll go more into these when we get into verifying our EIGRP configuration. For now let's go over a few key concepts for EIGRP
Protocol-Dependent Modules (PDM)
EIGRP uses a feature called PDM's to support multiple network layer routed protocols such as -
- IP
- IPX
- IPv6
- AppleTalk
Reliable Transport Protocol (RTP)
EIGRP uses RTP to manage communication between routers. RTP is a proprietary protocol that only EIGRP uses. RTP can multicast and unicast update information. EIGRP uses a multicast address of 224.0.0.10 to send routing updates to multiple routers at a time. If a router does not respond then EIGRP will switch over to unicast and attempt to update the neighbor again. By default if EIGRP doesn't get a reply after 16 unicasts the neighbor is considered dead. RTP is a layer 4 protocol, think of it similarly to TCP except its used specifically for EIGRP routing updates.
EIGRP Neighbors
EIGRP uses a processes of discovery neighbors to exchange routes with. EIGRP routers will send out hellos packets to discover neighboring routers. Once routers are discovered they must meet a few of criteria before they become neighbors -
EIGRP uses AS numbers in a similar fashion as OSPF areas. Each AS in EIGRP is a group of routers that will share routing updates.
K-values are EIGRP metric values, these are values that can be manipulated in order to adjust the metric of EIGRP routes.
- Each router must receive a hello packet from the other.
- Each router must be be running EIGRP in the same Autonomous System (AS).
- Each router must have identical K-values
EIGRP uses AS numbers in a similar fashion as OSPF areas. Each AS in EIGRP is a group of routers that will share routing updates.
K-values are EIGRP metric values, these are values that can be manipulated in order to adjust the metric of EIGRP routes.
Diffusing Update Algorithm (DUAL)
DUAL is the algorithm that EIGRP uses to calculate the best path for each network. DUAL is what makes EIGRP one of the fastest routing protocols to converge. DUAL allows for backup routes that allows EIGRP to instantly bring up alternative links should a primary path fail. DUAL also supports VLSM so we will continue working with our previous network.
EIGRP Terminology
There are some EIGRP terminology that you need to be familiar with before we get into configuring out network. Let's go over these now.
- Feasible Distance (FD) - Feasible distance is the metric of all the routes to a network, including the distance to the neighbor. The route with the lowest FD will be added to the routing table.
- Advertised Distance (AD) - Advertised distance is the metric of a network not including the distance to the neighbor. This is the metric that the neighbor is reporting to the router. The router will take the AD and add the metric for the distance between the router and the neighbor to get the FD.
- Successor Route - A successor route is the best route to a network. The successor route has the lowest FD and is added to the routing table.
- Feasible Successor - The feasible successor route is a backup to the successor route. If a route has a AD that is less than the FD of the successor route then it is considered a feasible successor. Feasible successor routes are stored in the topology table.
Let's look at the above network from the perspective of router 6 learning about router 9. When this network comes up and EIGRP begins to exchange routing information. Router 6 and 7 will exchange hellos and begin the neighbor process. Router 7 will share information about how to get to router 9. Router 7 will send router 6 an advertised distance to router 9. The advertised distance is the distance from rotuer 7 to router 9. Router 6 will take the advertised distance and add the distance from router 6 to router 7 to get a feasible distance.
We'll go over these more in the next section when we configure and verify our EIGRP network.
We'll go over these more in the next section when we configure and verify our EIGRP network.