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OECG – Chapter 9 Definitions

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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