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core gateway
The innermost gateways of the ARPAnet. These gateways have a total picture of the reachability to all networks known to the ARPAnet with EGP. They then redistribute reachability information to all those gateways speaking EGP. It is from them your EGP agent (there is one acting for you somewhere if you can reach the ARPAnet) finds out it can reach all the nets on the ARPAnet. Which is then passed to you via Hello, gated, RIP....

count to infinity
The symptom of a routing problem where routing information is passed in a circular manner through multiple gateways. Each gateway increments the metric appropriately and passes it on. As the metric is passed around the loop, it increments to ever increasing values til it reaches the maximum for the routing protocol being used, which typically denotes a link outage.

hold down
When a router discovers a path in the network has gone down announcing that that path is down for a minimum amount of time (usually at least two minutes). This allows for the propagation of the routing information across the network and prevents the formation of routing loops.

split horizon
When a router (or group of routers working in consort) accept routing information from multiple external networks, but do not pass on information learned from one external network to any others. This is an attempt to prevent bogus routes to a network from being propagated because of gossip or counting to infinity.

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