Supports routed protocols like IP and IPv6 via protocol-dependent modules
Uses Reliable Transport Protocol (RTP, Cisco proprietary) for some traffic (updates, queries, and replies)
Uses hellos to identify/monitor neighbors
Uses the Diffusing Update Algorithm (DUAL) to select routes
EIGRP is IP protocol 88.
EIGRP supports proportional unequal-cost load-balancing among feasible routes.
Hello – Identify neighbors, sent as periodic multicasts
Update – Advertises routes, only sent when there is a change, multicast to 188.8.131.52
Ack – Acknowledges receipt of an update
Query – Used to query routes from neighbors (multicast; unicast attempted up to 16 times if multicast gets no response)
Reply – Used to answer a query (unicast)
Metric = 256 * (K1 * bandwidth + ( (K2 * bandwidth) / (256 – load) ) + K3 * delay) * ( K5 / (reliability + K4))
K values are used to distribute weight to different path aspects:
bandwidth – Defined as 107 divided by the speed of the slowest link in the path, in Kbps
load – 8-bit value, not considered by default
reliability – 8-bit value, not considered by default
delay – constant value associated with interface type; EIGRP uses the sum of all delays in the path
K defaults: K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0
K values can be manipulated by an admin, but routers must have matching K values to become neighbors
Advertised distance – Cost advertised by a neighbor to get to a destination
Feasible distance – Advertised distance + cost get to that neighbor
The feasibility requirement states "if my neighbor’s advertised distance is less than my feasible distance, the path will be loop free."
Successor – The neighbor with the best path
Feasible successor – All other neighbors which meet the feasibility requirement
Split-horizon – A network is not advertised on the link from which is learned.
When a router loses its successor and has no feasible successors, it will query all remaining neighbors for a new route. Queries are recursive and will be forwarded to other neighbors until either a route is found, or a summarization boundary is reached.
Stuck in Active (SIA) – Queries which do not return a route before the active timer expires (usually 3 minutes), the router is considered stuck in active mode.
Stores information about neighboring EIGRP routers:
Network address (IP)
Holdtime – how long the router will wait to receive another HELLO before dropping the neighbor; default = 3 * hello timer
Uptime – how long the neighborship has been established
Retransmission Timeout (RTO) – how long the router will wait for an ack before retransmitting the packet; calculated by SRTT
Smooth Round Trip Time (SRTT) – time it takes for an ack to be received once a packet has been transmitted
Queue count – number of packets waiting in queue; a high count indicates line congestion
Holds all routes received from neighbors, is built from updates, calculated by DUAL, and contains all the information required by the routing table
Internal – Paths directly within EIGRP
Summary – Internal paths which have been summarized
External – Routes redistributes into EIGRP