RIPv2 Message Format

The RIPv2 message format takes advantage of the unused fields in the RIPv1 message format by adding subnet masks and other information. Figure 7-3 shows the RIPv2 message format.

Figure 7-3 RIPv2 Message Format

0 1 2 3 4 5 6 7

1 2 3 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

Command

Version

Unused (must be zero)

Address Family Identifier

Route Tag

IP address (1st route entry)

Subnet Mask

Next Hop

Metric

Address Family Identified

Route Tag

IP address (2nd route entry - up to 25)

Subnet Mask j

Next Hop

Metric

The following is a description of each field:

The following is a description of each field:

• Command—Indicates whether the packet is a request or a response message. The request message asks that a router send all or part of its routing table. Response messages contain route entries. The response message is set periodically, or as a reply to a request.

• Version—Specifies the RIP version. It is set to 2 for RIPv2 and 1 for RIPvl.

• Address Family Identifier—Specifies the address family. 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. The AFI is set to OxFFF for the first entry to indicate that the remainder of the entry contains authentication information.

• Route Tag—Provides a method for distinguishing between internal routes (learned by RIP) and external routes (learned from other protocols). Route tags are covered in Chapter 10.

• IP address—Specifies the IP address (network) of the destination.

• Subnet Mask—Contains the subnet mask for the destination. If this field is zero, no subnet mask has been specified for the entry.

• Next Hop—Indicates the IP address of the next hop where packets are sent to reach the destination.

• Metric—Indicates how many router hops to reach the destination. The metric is between 1 and 15 for a valid route, or 16 for an unreachable or infinite route.

Again, as in version 1, up to 25 occurrences of the last five 32-bit words (20 bytes) are permitted for up to 25 routes per RIP message. If the AFI specifies an authenticated message, only 24 routing table entries can be specified.

RIPv2 timers are the same as in version 1. Periodic updates are sent every 30 seconds. The default invalid timer is 180 seconds, the holddown timer is 180 seconds, and the flush timer is 240 seconds.

The RIPv2 protocol is configured just as in RIPv1 by using the global router rip command. You use a version command to enable use of the RIPv2 protocol. Then, the networks that are routed with RIP are configured with the network command. Looking again at Figure 7-2, the routers in this example are configured differently than the example configurations shown for RIPv1. First, you use the version command. Second, subnetworks can now have different subnet masks. The serial link is configured with a /30 mask. The Token Ring network is configured with a /28 mask.

The configuration examples in this section are for Router8, Router9, and Router10, as shown in Figure 7-4. In these examples, you use VLSMs and authentication is configured.

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