Protocol Extension to RIP

RIP Version 2 (RIP-2) made some improvements and enhancements to RIP-1. RIP-2 supports VLSM and discontiguous networks, and it offers the following enhancements:

• Route tag

• Subnet mask

• Next -hop metric

• Multicast capability

• Authentication

Figure 2-10 shows the RIP-2 packet format. The sections that follow discuss each of the enhancements and new packet fields in greater detail.

Figure 2-10. RIP-2 Packet Format

Route Tag

The Route Tag field is a 2-byte field that allows RIP routes to be assigned with a unique integer value. The routing table display shows the route tag for each RIP route, if assigned. This route tag plays an important role during redistribution with RIP. Any route that is redistributed into RIP gets tagged, to distinguish between internal RIP information and external RIP information.

When redistributed routes in RIP are assigned with route tags, it becomes easier to control redistribution of tagged routes into other protocols. Instead of matching against each route when redistributing into other protocols, RIP routes can simply be matched against the tag that they were assigned.

For example, consider that 10 static routes in a router are redistributed in RIP and are assigned a tag of 20. These static routes will be advertised in RIP as external routes with a tag of 20. If in some other router RIP is being redistributed into OSPF and OSPF wants only those 10 static routes to be redistributed, OSPF can simply match the tag information instead of listing each static route in its redistribution commands. In addition, if OSPF is being redistributed back into RIP at some other router, RIP should deny any routes that are tagged with 20. Matching against tags thus avoids IP routing loops as well.

Subnet Mask

Unlike RIP-1, RIP-2 carries subnet mask information along with the IP network number. If an IP network is variably subnetted , RIP-2 picks the subnet mask of each subnet and advertises to RIP-2 neighbors. RIP-2 routers in the network install routes with their respective subnets though a variable length of, say, /8, /15/, /24, and so on.

Support of VLSM also enables RIP-2 to understand discontiguous networks. In a discontiguous network, the IP supernet is divided by another IP block. Because RIP-2 can carry subnet mask information, each RIP-2 router has a route with the actual mask and routers can forward traffic properly.

Next Hop

The Next Hop field was added to avoid an extra hop during packet forwarding. For those familiar with OSPF, the Next Hop field holds nearly the same role as the forwarding address for OSPF external routes.

In Figure 2-11, OSPF is enabled between Router 2 and Router 5. RIP is enabled on Router 2, Router 3, and all the other routers behind Router 2 and Router 3. Router 2 is doing redistribution between OSPF and RIP. Now when a packet from Router 1 is destined for OSPF networks and arrives at Router 2, it is forwarded to Router 5.

Figure 2-11. RIP-2 Packet Format

When a packet from Router 4 destined to the OSPF network arrives at Router 3, if there is no next-hop information (in case of RIP-1), Router 3 forwards the packet to the originator, Router 2. Then Router 2 forwards it to Router 5. This is an extra hop that Router 3 must take to get to the OSPF network. With the Next Hop field in the RIP packet, when a packet destined to the OSPF network arrives at Router 3, the RIP route for the destination network has its next hop set to Router 5 instead of Router 2. As a result, Router 3 does not forward the packet to Router 2 ‚ instead, it forwards the packet straight to Router 5.

Multicast Capability

RIP-2 uses multicast when sending an update to all its neighbors. This reduces unnecessary broadcast flooding on the wire. The multicast address that RIP-2 uses is 224.0.0.9.

All devices on the wire running RIP-2 listen for RIP-2 multicast packets on 224.0.0.9 at a multicast MAC address (01-00-5E-00-00-09). Devices not running RIP-2 simply discard RIP-2 messages on the wire, reducing unnecessary load.

Authentication

RIP-2 supports simple password authentication, to validate trusted RIP-2 neighbors. RIP-2 speakers determine whether authentication is used by looking at the address family identifier (AFI) in RIP-2 packet. AFI in RIP-2 header indicates what kind of addresses are present in the rest of the packet.

If the AFI value is 0xFFFF, this means that the remainder of the entire RIP packet contains authentication information.

Figure 2-12 shows the packet format when authentication is used.

Figure 2-12. RIP-2 Packet Format for Authentication