Routing Information Protocol (RIP) Cont...

RIP Message Format

RIP updates are placed as UDP payload inside an IP datagram. The format is as follows:
- Command : It 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 : It specifies the RIP version used. This field can signal different potentially incompatible versions.
- Zero : This field is not actually used by RFC 1058 RIP; it was added solely to provide backward compatibility with prestandard varieties of RIP. Its name comes from its defaulted value: zero.
- Address family identifier (AFI) : It 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 : It specifies the IP address for the entry.
- Metric : It 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.

RIPv2 header explanation

- Command : Indicates whether the packet is a request or a response.
- Version : Version of RIP.
- Unused : It has a value set to zero.
- Address family identifier (AFI) : It specifies the address family used.
- Route tag : It provides a method for distinguishing between internal routes (learned by RIP) and external routes (learned from other protocols).
- IP address : It specifies the IP address for the entry.
- Subnet mask : It contains the subnet mask for the entry. If this field is zero, no subnet mask has been specified for the entry.
- Next hop : It indicates the IP address of the next hop to which packets for the entry should be forwarded.
- Metric : It 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.