Exchanging Protocols

With the introduction of classless routing, it was not possible for classful routing protocols such as RIP and IGRP to understand entire routing tables. In some cases, routing packets to destinations within the same major network is no longer possible. Therefore, it may become necessary to exchange one protocol for another.

Take, for example, the case of the Internet making use of class A networks between multiple customers. As discussed in Chapter 6, "Routing Information Protocol," RIPV1 will not accept routes across a different major network for its own connected network, and therefore will not route packets to that destination.

Now, consider the network shown in Figure 12-1: Y.com. The Y.com network owns part of the class A network space. In this case, the section of this same major network space is given to another enterprise: X.com. ISP will either run BGP with X.com or will use a static route for the routes of X.com. Similarly, X.com will run BGP with the ISP or will run a static default route ( toward the ISP.

Figure 12-1. Parts of the Class A Network Split between Two Organizations—a Problem for

RIP with Discontiguous Networks

If Y.com runs BGP with ISP, it will learn about network from ISP. Even if Y.com wants to redistribute this BGP route into RIP, it cannot because RIPV1 will not redistribute BGP. This is because one of the redistributing router's interfaces is connected to the same major network. This behavior indicates a discontiguous network and is not supported by RIPV1, which will cause problems for Y.com when it attempts to route packets to parts of the class A that it owns.

In the second situation, Y.com is not running a BGP with ISP, and has a default route toward the ISP. Even then, with classful protocols such as RIPV1 and IGRP, the router will not route a packet toward a default router for a subnet of its own connected network. In this case, if router R1 receives a packet that must be routed to, R1 checks its routing table because this subnet is not part of the range it owns. R1 will not have the route to in its table, so it will try to find the next feasible route—in this case, it is

In a classless protocol, is considered a route to all destinations. In classful protocols, however, it is not considered a route to all destinations, except for subnets of the connected networks. This is because all the subnets of the connected network should be contained within the routing table. In this case, R1 will not route packets to using the default route, and will drop the packet. Therefore, connectivity will not take place between Y.com and X.com.

You can use the ip classlesscommand in the Cisco router to overcome this problem, but if you are receiving parts of networks from multiple points, you cannot route to one of the parts of the network. Put simply, you can avoid this problem by using Cisco IOS to some extent, but you cannot avoid the shortcomings of the protocol completely.

Another reason for migrating classful protocols such as RIP or IGRP involves support for VLSM. As the network grows, administrators begin to realize that wasting address space merely to accommodate the protocol becomes a serious issue.

RIP and IGRP do not support VLSM, so all the interfaces included in the same major network should have the same mask. When connecting a large number of hosts in broadcast media for a class B network, you would mask with 24. For point-to-point connections, you might use a 30-bit mask to use the same subnet for 64 point-to-point connections.

Another reason for migrating to another protocol is faster convergence. Again, the older classful protocols are periodic, which means that you must put the route in holddown and flush states. In the worst case, this could take minutes to converge. Because of the rapid pace of the Internet, this sluggish convergence is unacceptable.

Classful protocols also lack the capability to summarize within the network. The administrator, for example, might want to summarize some routes within a region to diminish the size of the routing table.


Essentially, migration of routing protocols is carried out to improve classfulness; and to provide support for VLSM, support for discontiguous networks, scaling, and faster convergence.

The third situation relates to the scaling protocols, but more in terms of the ISP space rather than the enterprise space. ISPs often mistakenly advertise customer routes into the IGP because ISPs usually undergo tremendous growth. When the ISP begins redistribution, its customer base is modest in size. However, the customer routes increase as the business expands. Before long, an ISP could have 5,000 to 6,000 customer routes floating in its IGP, so if a problem occurs on the customer networks, it can wreak havoc on the ISP networks. These issues are discussed in detail in the next section.

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