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.
Packet types
Hello – Identify neighbors, sent as periodic multicasts
Update – Advertises routes, only sent when there is a change, multicast to 224.0.0.10
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 calculation
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
DUAL
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.
Queries
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.
EIGRP Tables
Neighbor table
Stores information about neighboring EIGRP routers:
Network address (IP)
Connected interface
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
Sequence numbers
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
Topology table
Holds all routes received from neighbors, is built from updates, calculated by DUAL, and contains all the information required by the routing table
Routing table
Route types:
Internal – Paths directly within EIGRP
Summary – Internal paths which have been summarized
External – Routes redistributes into EIGRP
