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RIP Version 2 (RIP-2) Message Format and Features
(Page 2 of 3)
RIP-2 Version-Specific Features
RIP-2 represents a very modest change
to the basic Routing Information Protocol. RIP-2 works in the same basic
way as RIP-1 (part of why I was able to describe the operation of both
in the same general section.) In fact, the new features introduced in
RIP-2 are described as extensions to the basic protocol,
conveying the fact that they are layered upon regular RIP-1 functionality.
The five key RIP-2 extensions are:
- Classless Addressing Support and Subnet Mask
Specification: When RIP-1 was developed, the use of subnets in IP
(as described in RFC 950) had not yet been formally defined. It was
still possible to use RIP-1 with subnets, through the use of a heuristic
to determine if the destination is a network, subnet or host. However,
there was no way to clearly specify the subnet mask for an address using
RIP-2 adds explicit support for subnets by allowing a subnet mask within
the route entry for each network address. It also provides support for
subnet masking (VLSM) and classless
- Next Hop Specification: In RIP-2, each
RIP entry includes a space where an explicit IP address can be entered
as the next hop router for datagrams intended for the network in that
entry. This feature can help improve efficiency of routing by eliminating
unnecessary extra hops for datagrams sent to certain destinations.
One common use of this field is when the most efficient route to a network
is through a router that is not running RIP. Such a router will not
exchange RIP messages and would therefore not normally be selected by
RIP routers as a next hop for any network. The explicit Next Hop
field allows the router to be selected as the next hop regardless of
- Authentication: RIP-1 included no authentication
mechanism, which is a problem because it could potentially allow a malicious
host to muck up an internetwork by sending bogus RIP messages around.
RIP-2 provides a basic authentication scheme, which allows routers to
ascertain the identity of a router before it will accept RIP messages
- Route Tag: Each RIP-2 entry includes a
Route Tag field, where additional information about a route can
be stored. This information is propagated along with other data about
the route as RIP entries are sent around the internet. A common use
of this field is when a route is learned from a different autonomous
system, to identify the autonomous system from which the route was obtained.
- Use of Multicasting: To help reduce network
load, RIP-2 allows routers to be configured to use multicasts
instead of broadcasts for sending out unsolicited RIP Response
messages. These datagrams are sent out using the special reserved multicast
address 126.96.36.199. All routers on an internetwork must obviously use
multicast if this is to work properly.
As you can see, many of these extensions
require more information to be included with each advertised route.
This is where all that extra space in the message format
of RIP-1 routing entries comes in handy, as we will see shortly.
Key Concept: RIP-2 is the most recent version of RIP used in IPv4. It includes a number of enhancements over the original RIP-1, including support for subnet masks and classless addressing, explicit next-hop specification, route tagging, authentication and multicast. For compatibility, it uses the same basic message format as RIP-1, putting the extra information required for its new features into some of the unused fields of the RIP-1 message format.
RIP-2 messages are exchanged using
the same basic mechanism as RIP-1 messages. Two different message types
exist, RIP Request and RIP Response. They are sent using
Datagram Protocol (UDP) using UDP reserved
port number 520. The semantics for the
use of this port is the same as for RIP-1. For convenience, I repeat
the description here:
- RIP Request messages are sent to UDP destination
port 520. They may have a source port of 520 or may use an ephemeral
- RIP Response messages sent in reply to
an RIP Request are sent with a source port of 520, and a destination
port equal to whatever source port the RIP Request used.
- Unsolicited RIP Response messages (sent
on a routine basis and not in response to a request) are sent with both
the source and destination ports set to 520.
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Version 3.0 - Version Date: September 20, 2005
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