The COMMAND field specifies an operation according the following table:
Command | Meaning |
1Request for partial or full routing information
2Response containing
3Turn on trace mode (obsolete)
4Turn off trace mode (obsolete)
5Reserved for Sun Microsystem’s internal use
9Update Request
10Update Response
11Update Acknowledgement
Table 5-12. 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 from.
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.
RIP specifies that the IP address 0.0.0.0 denotes a default route.
The distances, measured in router hops are entered in the Distance to Source Network, and Distance to Destination Network fields.
RIP 1 Route Interpretation
RIP was designed to be used with classed address schemes, and does not include an explicit subnet mask. An extension to version 1 does allow routers to exchange subnetted addresses, but only if the subnet mask used by the network is the same as the subnet mask used by the address. This means the RIP version 1 cannot be used to propagate classless addresses.
Routers running RIP version 1 must send different update messages for each IP interface to which it is connected. Interfaces that use the same subnet mask as the router’s network can contain subnetted routes, other interfaces cannot. The router will then advertise only a single route to the network.
RIP Version 2 Extensions
RIP version 2 includes an explicit subnet mask entry, so RIP version 2 can be used to propagate variable length subnet addresses or CIDR classless addresses. RIP version 2 also adds an explicit next hop entry, which speeds convergence and helps prevent the formation of routing loops.
RIP2 Message Format
The message format used with RIP2 is an extension of the RIP1 format:
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