QTECH
Software Configuration Manual
7-86
RIP uses the following mechanisms to prevent routing loops.
·
Counting to infinity. The metric value of 16 is defined as unreachable. When a routing loop occurs, the
metric value of the route will increment to 16.
·
Split horizon. A router does not send the routing information learned from a neighbor to the neighbor to
prevent routing loops and save the bandwidth.
·
Poison reverse. A router sets the metric of routes received from a neighbor to 16 and sends back these routes
to the neighbor to help delete useless information from the neighbor’s routing table.
·
Triggered updates. A router advertises updates once the metric of a route is changed rather than after the
update period expires to speed up the network convergence.
7.2.2
RIP Version
RIP has two versions, RIPv1 and RIPv2.
RIPv1, a Classful Routing Protocol, supports message advertisement via broadcast only. RIPv1 protocol messages do
not carry mask information, which means it can only recognize routing information of natural networks such as Class
A, B, and C. That is why RIPv1 does not support discontiguous subnet.
RIPv2 is a Classless Routing Protocol. Compared with RIPv1, RIPv2 has the following advantages.
·
Supporting route tags. The route tag is used in routing policies to flexibly control routes.
·
Supporting masks, route summarization and classless inter-domain routing (CIDR).
·
Supporting designated next hop to select the best next hop on broadcast networks.
·
Supporting multicast routing update to reduce resource consumption.
·
Supporting Plain text authentication and MD5 authentication to enhance security.
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Note : RIPv2 has two types of message transmission : broadcast and multicast.
Multicast is the default type using 224.0.0.9 as the multicast address. The interface
working in the RIPv2 broadcast mode can also receive RIPv1 messages.
7.2.3
RIP Message Format
7.2.3.1 RIPv1 message format
A RIP message consists of the Header and up to 25 route entries.
RIPv1 Message Format
·
Command : The type of message. 1 indicates Request, 2 indicates Response.
·
Version : The version of RIP, 0x01 for RIPv1.
·
AFI : Address Family Identifier, 2 for IP.
·
IP Address : Destination IP address of the route; can be a natural network, subnet or a host address.
·
Metric : Cost of the route.