Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 80
Route Aggregation

Route aggregation is not a protocol. It is actually a definition of what we are accomplishing on the Internet routing tables. Using the example mentioned previously, you have been introduced to a concept known as route aggregation. It allows a router to summarize a group of routes as one advertisement. Imagine having one entry in the routing table to represent a large group of addresses. The router simply needs to know the prefix. This is completely possible with route aggregation, however, it is only useful when the routes are contiguous. Punching holes in the continuity of the routes reduces the efficiency of this concept.

To show this benefit clearly, I have chosen a Class A example. The network address is 20.0.0.0. The natural mask for this is /8. or 255.0.0.0. We first subnet the address using a /16 prefix, or 255.255.0.0. This allows for addresses in the range of 20.0.0.0 through 20.255.0.0. We take the 20.127.0.0 subnet and further subnet it with a prefix of /24 (255.255.255.0). Finally, we take the 20.127.1.0 subnet and apply a /27 prefix.

Route aggregation is based on the concept of a common prefix. What is the common prefix assigned to a group of IP addresses? For example, the 20.127.1.0 was subnetted to /27. However, all the subnets that are created by this can be advertised as one route: 20.127.1.0/24. This is detailed later in this section. All of the addresses in this range have the same prefix. This would indicate to all other routers that any network in the range of 20.127.1.0 should be forwarded to that router. The other routers do not care about any of the particular subnets beyond that address. The router that receives the datagram to be forwarded to any subnet below 20.127.1.0 will be identified by the router and it will forward it to the correct network.

Route Aggregation

The rules are simple:

1.  Write down the addresses in the range.

2.  Convert each address to binary, one below the other.

3.  Check for a contiguous, common prefix.

4.  Move the prefix to the last bit of the contiguous binary digit.

5.  Write the address starting the first address and apply the step 4 prefix.

Remember, do not make this complicated. It is confusing enough. Three variable subnet masks are enough to work with for most networks (business networks and ISPs excluded).