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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 60 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
11
RIP Command Codes
The field VERSION contains the protocol version number (1 in this case), and is used by the receiver to verify which
version of RIP the packet was sent.
RIP 1 Message
RIP is not limited to TCP/IP. Its address format can support up to 14 octets (when using IP, the remaining 10 octets
must be zeros). Other network protocol suites can be specified in the Family of Source Network field (IP has a value
of 2). This will determine how the address field is interpreted.
DES-3810 Series Layer 3 Managed Ethernet Switch Web UI Reference Guide
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.
Update Acknowledgement
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