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The Routing Information
Protocol (RIP) is a distance-vector protocol that uses hop count as its
metric. RIP is widely used for routing traffic in the global Internet
and is an interior gateway protocol (IGP), which means that it
performs routing within a single autonomous system. Exterior gateway
protocols, such as the Border Gateway Protocol (BGP), perform routing
between different autonomous systems. The original incarnation of RIP
was the Xerox protocol, GWINFO. A later version, known as routed
(pronounced "route dee"), shipped with Berkeley Standard
Distribution (BSD) Unix in 1982. RIP itself evolved as an Internet
routing protocol, and other protocol suites use modified versions of
RIP. The AppleTalk Routing Table Maintenance Protocol (RTMP) and the
Banyan VINES Routing Table Protocol (RTP), for example, both are based
on the Internet Protocol (IP) version of RIP. The latest enhancement to
RIP is the RIP 2 specification, which allows more information to be
included in RIP packets and provides a simple authentication mechanism.
IP RIP is formally defined in two
documents: Request For Comments (RFC)
1058 and 1723. RFC 1058 (1988) describes the first implementation
of RIP, while RFC 1723 (1994) updates RFC 1058. RFC 1058 enables RIP
messages to carry more information and security features.
This chapter summarizes the basic
capabilities and features associated with RIP. Topics include the
routing-update process, RIP routing metrics, routing stability, and
routing timers.
RIP sends routing-update
messages at regular intervals and when the network topology changes.
When a router receives a routing update that includes changes to an
entry, it updates
its routing table to reflect the new route. The metric value for the
path is increased by one, and the sender is indicated as the next hop.
RIP routers maintain only the best route (the route with the lowest
metric value) to a destination. After updating its routing table, the
router immediately begins transmitting routing updates to inform other
network routers of the change. These updates are sent independently of
the regularly scheduled updates that RIP routers send.
RIP uses a single routing metric
(hop count) to
measure the distance between the source and a destination network. Each
hop in a path from source to destination is assigned a hop-count value,
which is typically 1. When a router receives a routing update that
contains a new or changed destination-network entry, the router adds one
to the metric value indicated in the update and enters the network in
the routing table. The IP address of the sender is used as the next hop.
RIP prevents routing loops from
continuing indefinitely by implementing a limit on the number of hops
allowed in a path from the source to a destination. The maximum number
of hops in a path is 15. If a router receives a routing update that
contains a new or changed entry, and if increasing the metric value by
one causes the metric to be infinity (that is, 16), the network
destination is considered unreachable.
To adjust for rapid network-topology
changes, RIP specifies a number of stability features that are common to
many routing protocols. RIP, for example, implements the split-horizon
and hold-down mechanisms to prevent incorrect routing information from
being propagated. In addition, the RIP hop-count limit prevents routing
loops from continuing indefinitely.
RIP uses numerous timers
to regulate its performance. These include a routing-update timer,
a route timeout, and a route-flush timer. The
routing-update timer clocks the interval between periodic routing
updates. Generally, it is set to 30 seconds, with a small random number
of seconds added each time the timer is reset to prevent collisions.
Each routing-table entry has a route-timeout timer associated with it.
When the route-timeout timer expires, the route is marked invalid but is
retained in the table until the route-flush timer expires.
The following section focuses on the IP
RIP and IP RIP 2 packet formats illustrated in Figure 44-1 and 44-2.
Each illustration is followed by descriptions of the fields illustrated.
Figure 44-1 illustrates the IP RIP packet
format.
Figure 44-1: An IP RIP packet
consists of nine fields.
The following descriptions summarize the
IP RIP packet-format fields
illustrated in Figure 44-1:
Note Up to 25 occurrences of
the AFI, address, and metric fields are permitted in a single IP RIP
packet. (Up to 25 destinations can be listed in a single RIP packet.)
The RIP
2 specification (described in RFC 1723) allows more information to be
included in RIP packets and provides a simple authentication mechanism. Figure
44-2 shows the IP RIP 2 packet format.
Figure 44-2: An
IP RIP 2 packet consists of fields similar to those of an IP RIP packet.
The following descriptions summarize the
IP RIP 2 packet format fields illustrated in Figure
44-2:
Note Up to 25 occurrences of
the AFI, address, and metric fields are permitted in a single IP RIP
packet. That is, up to 25 routing table entries can be listed in a
single RIP packet. If the AFI specifies an authenticated message, only
24 routing table entries can be specified.
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