D-Link xStack Reference Manual page 244

®
xStack DGS-3620 Series Layer 3 Managed Stackable Gigabit Switch Web UI Reference Guide
network). So, the vector is the network address and the distance is measured by the number of routers between
the local router and the remote network.
RIP measures distance by an integer count of the number of hops from one network to another. A router is one hop
from a directly connected network, two hops from a network that can be reached through a router, etc. The more
routers between a source and a destination, the greater the RIP distance (or hop count).
There are a few rules to the routing table update process that help to improve performance and stability. A router
will not replace a route with a newly learned one if the new route has the same hop count (sometimes referred to
as 'cost'). So learned routes are retained until a new route with a lower hop count is learned.
When learned routes are entered into the routing table, a timer is started. This timer is restarted every time this
route is advertised. If the route is not advertised for a period of time (usually 180 seconds), the route is removed
from the routing table.
RIP does not have an explicit method to detect routing loops. Many RIP implementations include an authorization
mechanism (a password) to prevent a router from learning erroneous routes from unauthorized routers.
To maximize stability, the hop count RIP uses to measure distance must have a low maximum value. Infinity (that is,
the network is unreachable) is defined as 16 hops. In other words, if a network is more than 16 routers from the
source, the local router will consider the network unreachable.
RIP can also be slow to converge (to remove inconsistent, unreachable or looped routes from the routing table)
because RIP messages propagate relatively slowly through a network.
Slow convergence can be solved by using split horizon update, where a router does not propagate information
about a route back to the interface on which it was received. This reduces the probability of forming transient
routing loops.
Hold down can be used to force a router to ignore new route updates for a period of time (usually 5 seconds) after
a new route update has been received. This allows all routers on the network to receive the message.
A router can 'poison reverse' a route by adding an infinite (16) hop count to a route's advertisement. This is usually
used in conjunction with triggered updates, which force a router to send an immediate broadcast when an update
of an unreachable network is received.
RIP Version 1 Message Format
There are two types of RIP messages: routing information messages and information requests. Both types use the
same format.
The Command field specifies an operation according the following table:
Command
1
2
3
4
5
9
10
Description
Request for partial or full routing information.
Response containing network-distance pairs from sender's routing table.
Turn on trace mode.
Turn off trace mode.
Reserved for Sun Microsystems internal use.
Update Request.
Update Response.
243