Routing protocols are able to dynamically update and talk to each other about changes that may occur in a network, such as a link going down. The beauty of this is that, if designed properly, redundant links can quickly be found with out the network going down or users even knowing there is an issue. There are several different kinds of routing protocols such as distance vector, link-state, and a combination of distance vector and link-state. Below is a list of 10 important things you should know specifically about link-state routing protocols like OSPF and IS-IS to help you get ready for your CCNA Exam.
(1) Each router running a link-state routing protocol creates it’s own map or table of all the other routers in the same area.
(2) Each link-state router runs the SPF (Shortest Path First) Algorithm to determine the best route to each of the routers in the network.
(3) Link-state routers will send a topology update as soon as a link goes down instead of waiting for a timer to expire for regular updates.
(4) Link-state routers send small hello packets every 10 seconds or so just to say hi to its fellow routers.
(5) Link-state routers will send their topology table update every 30 minutes. It’s a lot bigger than a hello packet so it get’s sent out less frequently.
(6) Each link-state router must be configured to the exact same update timers or they will not become neighbors.
(7) Link-state routers can be sub-divided into separate areas so that only routers in the same area have to re-calculate their SPF algorithm when there is a topology change.
(8) Link-state advertisements are sent out via a multicast address instead of broadcasting them out to every node in the network.
(9) Link-state protocols don’t just use distance to determine the best route, but take into account the link speed as well.
(10) Link-state protocols are processor and memory intensive.
###
Image Credit: Thomas Hawk