RIP

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Background
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.
Routing Updates
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 Routing Metric
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.
RIP Stability Features
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 Timers
RIP uses numerous timers to regulate its performance. These include a routing-update timer,aroute
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.
Packet Formats
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.
RIP Packet Format
Figure illustrates the IP RIP packet format.
Figure An IP RIP packet consists of nine fields.
The following descriptions summarize the IP RIP packet-format fields illustrated in Figure :
Field Length,in Bytes

A = Command
B = Version Number
C = Zero
D = Address Family Identifier
E = Address
F = Metric
The following descriptions summarize the IP RIP packet-format fields illustrated in Figure
Command indicates whether the packet is a request or a response. The request asks that a router
send all or part of its routing table. The response can be an unsolicited regular routing update or
a reply to a request. Responses contain routing table entries. Multiple RIP packets are used to
convey information from large routing tables.
Version Number Specifies the RIP version used. This field can signal different potentially
incompatible versions.
Zero Not used.
Address-Family Identifier (AFI) Specifies the address family used. RIP is designed to carry
routing information for several different protocols. Each entry has an address-family identifier to
indicate the type of address being specified. The AFI for IP is 2.
Address specifies the IP address for the entry.
Metric Indicates how many internetwork hops (routers) have been traversed in the trip to the
destination. This value is between 1 and 15 for a valid route, or 16 for an unreachable route.
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.)
RIP 2 Packet Format
The RIP 2 specification (described in RFC 1723) allows more information to be included in RIP
packets and provides a simple authentication mechanism. Figure shows the IP RIP 2 packet
format.
An IP RIP 2 packet consists of fields similar to those of an IP RIP packet
Length of Field
in Octets

The following descriptions summarize the IP RIP 2 packet format fields illustrated in Figure
Command Indicates whether the packet is a request or a response. The request asks that a router
send all or a part of its routing table. The response can be an unsolicited regular routing update
or a reply to a request. Responses contain routing-table entries. Multiple RIP packets are used to
convey information from large routing tables.
Version Specifies the RIP version used. In a RIP packet implementing any of the RIP 2 fields
or using authentication, this value is set to 2.
Unused Value set to zero.

Address-Family Identifier (AFI) Specifies the address family used. RIP is designed to carry
routing information for several different protocols. Each entry has an address-family identifier to
indicate the type of address specified.
The address-family identifier for IP is 2. If the AFI for the first entry in the message is 0xFFFF, the
remainder of the entry contains authentication information. Currently, the only authentication type is
simple password.
Route Tag Provides a method for distinguishing between internal routes (learned by RIP) and
external routes (learned from other protocols).
IP Address Specifies the IP address for the entry.
Subnet Mask Contains the subnet mask for the entry. If this field is zero, no subnet mask has
been specified for the entry.
Next Hop Indicates the IP address of the next hop to which packets for the entry should be
forwarded.
Metric Indicates how many internetwork hops (routers) have been traversed in the trip to the
destination. This value is between 1 and 15 for a valid route, or 16 for an unreachable route.
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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