CCIE Candidate

hello interval - the amount of time between EIGRP hellos.  5 second default for local and 60 second for NBMA interfaces.

full update - a complete EIGRP routing table update, happening when neighbor adjacencies form.

partial update - an update just with changes to the EIGRP topology.

route tag field - a field that can be used by EIGRP when routes are redistributed.

next-hop field - the field in an EIGRP advertisement where the next hop for a route is described. Typically, but not always the router itself.

MD5 - a hashing algorithm used to for digest creation and authentication

DUAL - diffusing update algorithm. Used by EIGRP to compute routes.

Hold timer - How long to keep a neighbor up without hearing from him.

K value - a multiplier used in EIGRP metric computation.

neighbor - 2 routers that have formed an adjacency by exchanging hellos and routes. Neighbors can only become so if their subnets match, k-values and other parameters match.

adjacency - when 2 routers are neighbors, they are said to be adjacent

RTP - reliable transport protocol. Used by EIGRP, IP protocol 88.

SRTT - smoothed round-trip time. A slowly changing measurement of round-trip time between neighbors, used in RTO calculations.

RTO - retransmission timeout. The time before an RTP sender will resend a packet because he didn’t receive an ACK.

Update - what a router does when his topology changes. He lets his neighbors know about the change in the form of an update. An ACK is expected.

Ack - an acknowledgement of a received packet

query - EIGRP will send a multicast query to neighbors if a route has gone active.

reply - EIGRP will respond to queries with unicast replies.

Hello - the EIGRP multicast message sent to 224.0.0.9, announcing the router’s presence.

Goodbye - notifies EIGRP neighbors of a graceful shutdown.

RD - reported distance. How far away the EIGRP neighbor thinks the route is.

FD - feasible distance. The best computed metric of all possible paths to a subnet.

feasibility condition - where the reported distance is lower than the feasible distance.

successor route - the route associated with the feasible distance, i.e. the best computed route for a subnet.

feasible successor - a route that, while not the best route, could become the best route if the successor route fails.

input event - when EIGRP receives an update from a neighbor about a topology change.

local computation - what an EIGRP router does in reaction to an input event.
active - when a successor route has failed and there is no feasible successor, EIGRP will place the route in active state and query neighbors regarding the route.

passive - a stable EIGRP route is in a passive state.

going active - EIGRP has placed a route into active status

stuck-in-active - when a query has been sent to neighbors regarding an active route, but not all replies have been received before the active timer expires, the route is said to be “stuck-in-active”

query scope - the number of EIGRP neighbors that receive a query
EIGRP stub router - a router that is a stub is not supposed to be used as a transit router, and therefore doesn’t receive queries for active routes

limiting query scope - reducing how many routers receive EIGRP queries, using route-summarization and stub routers

variance - allowing routes with small metric differences to be considered as “equal cost routes” by EIGRP

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