Allied Telesis AT-8550 - Manual (Page 157)

AT-9108, AT-8518, AT-8525, and AT-8550 User’s Guide
9-3
Overview of RIP
RIP is an Interior Gateway Protocol (IGP) first used in computer
routing in the Advanced Research Projects Agency Network
(ARPAnet) as early as 1969. It is primarily intended for use in
homogeneous networks of moderate size.
To determine the best path to a distant network, a router using RIP
always selects the path that has the least number of hops. Each
router that data must traverse is considered to be one hop.
Routing Table
The routing table in a router using RIP contains an entry for every
known destination network. Each routing table entry contains the
following information:
❑ IP address of the destination network
❑ Metric (hop count) to the destination network
❑ IP address of the next router
❑ Timer that tracks the amount of time since the entry was last
updated
The router exchanges an update message with each neighbor every
30 seconds (default value), or if there is a change to the overall
routed topology (also called triggered updates). If a router does not
receive an update message from its neighbor within the route
timeout period (180 seconds by default), the router assumes the
connection between it and its neighbor is no longer available.
Split Horizon
Split horizon is a scheme for avoiding problems caused by including
routes in updates sent to the router from which the route was
learned. Split horizon omits routes learned from a neighbor in
updates sent to that neighbor.
Poison Reverse
Like split horizon, poison reverse is a scheme for eliminating the
possibility of loops in the routed topology. In this case, a router
advertises a route over the same interface that supplied the route,
but the route uses a hop count of 16, defining it as unreachable.
Triggered
Updates
Triggered updates occur whenever a router changes the metric for a
route, and it is required to send an update message immediately,
even if it is not yet time for a regular update message to be sent. This
will generally result in faster convergence, but may also result in
more RIP-related traffic.