RIP (Cont.)

RIP Timers: - RIP uses numerous timers to regulate its performance. These include a routing-update timer, a route-timeout timer, 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 amount of time added whenever the timer is reset. This is done to help prevent congestion, which could result from all routers simultaneously attempting to update their neighbors. 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.

RIP Packet Format

Figure: An IP RIP Packet Consists of Nine Fields 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: An IP RIP Packet Consists of Nine Fields:

• 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 - 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) - 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.

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 that is not supported by RIP.

Figure: An IP RIP 2 Packet Consists of Fields Similar to Those of an IP RIP Packet shows the IP RIP 2 packet format.

Figure: 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: An IP RIP 2 Packet Consists of Fields Similar to Those of an IP RIP Packet:

• 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 - Has a value set to zero.
• Address-family identifier (AFI) - Specifies the address family used. RIPv2's AFI field functions identically to RFC 1058 RIP's AFI field, with one exception: 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.

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. Given that individual table entries aren't fragmented into multiple packets, RIP does not need a mechanism to resequence datagrams bearing routing table updates from neighboring routers.