OECG – Chapter 8 Definitions


Holddown timer – when a new metric is received for a route already in the routing table, the holddown timer starts counting down, 180 seconds by default. The new metric won’t get installed until the holddown timer expires.

Invalid timer – Starts counting down every time RIP hears a route advertised. If the route is not again advertised before the invalid timer expires, the route is eligible for deletion (but not actually deleted). 180 seconds by default.

Flush timer – starts counting down at the same time the invalid timer does. If the route is no again advertised before the flush timer expires, then the route is removed from the routing table. 240 seconds be default. Once flushed, the router will accept new advertisements for this route.

Garbage timer – same thing as flush timer.

authentication – 2 routers using a password or MD5 hash as a part of their routing updates to one another. Passwords/hashes must match or advertisements are ignored.

Update timer – the interval RIP uses to send out updates, 30 second by default.

triggered updates – RIP sending out an update based on a trigger, i.e. a received update, send independently of the update timer.

flash updates – same as a triggered update

split horizon – a router will not advertise a route via the same interface the route was learned.

route poisoning – a RIP router will advertise an unreachable route with a weight of 16, effectively poisoning it to other routers.

poison reverse – a RIP router advertising a poison route right back to the router he received the route from (loop prevention mechanism)

counting to infinity – a type of routing protocol convergence event in which the metric for a route increases a little bit over time because of the advertisement of an invalid route. (I don’t remember this from the RIP chapter. Huh.)

hello interval – the time between hellos in some routing protocols.

full update – receiving the entire update in a routing advertisement.

partial update – receiving a portion of the routing table in an advertisement, only what’s changed.

route tag field – a field to hold generic information about a route often an identifying number. The tag can be used by other routers to filter with.

next hop field – tells the router where to send traffic for a given route. This isn’t necessarily the advertising router.

triggered extensions to RIP for on-demand circuits – RFC2091. RIP doesn’t have to send updates every 30 seconds in an on-demand circuit environment. Rather, RIP will send one full update, then updates only when routes change or an update is requested or when a RIP interface goes up or down.

MD5 – message digest 5. Performing a computation on a packet payload with a possibly a password, private key, and the message itself. Both sender and receiver perform the same computation and in this was know that the payload was not tampered with.

offset list – a means to add extra weight to a route metric.

prefix list – a list of IP prefixes that can be used be a router to match against for received or sent route updates. You can use a range of masks, which is way cool.

distribution list – a list of routes that will be sent in advertisements or accepted in advertisements

distance vector – RIP computes how far away a network is in hop count.

metric – the weight of a route. If 2 routers advertise the same route to a 3rd router, the metric will determine which route is more favorable.

About the author

Ethan Banks

Most people know me because I write & podcast about IT on the Packet Pushers network. I also co-authored "Computer Networks Problems & Solutions" with Russ White.

Find out more on my about page.


Most people know me because I write & podcast about IT on the Packet Pushers network. I also co-authored "Computer Networks Problems & Solutions" with Russ White.

Find out more on my about page.

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