classless routing :

Routing that send subnet mask information in the routing updates. Classless routing allows Variable-Length Subnet Mask (VLSM) and supernetting. Routing protocols that support classless routing are RIP version2, EIGRP, and OSPF. [TOC]

composite metric :

Used with routing protocols, such as IGRP and EIGRP, that use more tan one metric to find the best path to a remote network. IGRP and EIGRP both use bandwidth and delay of the line by default. However, Maximum Transmission Unit (MTU), load, and reliability of a link can be used as well. [TOC]

holddown :

The state a route is placed in so that routers can neither advertise the route nor accept advertisements about it for a defined time period. Holddown is used to surface bad information about a route from all routers in the network. A route is generally placed in holddown when one of its links fails. [TOC]

hop count :

A routing metric that calculates the distance between a source and a destination. RIP employs hop count as its sole metric. [TOC]

poison reverse updates :

The update messages are transmitted by a router back to the originator (thus ignoring the slpit-horizon rule) after route poisoning has occurred. Typically used with DV routing protocols in order to overcome large routing loops and offers explicit information when a subnet or network is not accessible (instead of merely suggesting that the network is unreachable by not including it in updates.). [TOC]

route poisoning :

Used by various DV routing protocols in order to overcome large routing loops and offer explicit information about when a subnet or network is not accessible (instead of merely suggesting that the network is unreachable by not including it in updates.). Typically, this is accomplished by setting the hop count to one more than maximum. [TOC]

split horizon :

Useful for preventing routing loops, a type of distance-vector routing rule where information about routes is prevented from leaving the router interface through which that information was received. [TOC]