3Com corebuilder 3500 Implementation Manual page 270

270 C 11: I
HAPTER NTERNET
P (IP)
ROTOCOL
Also, with route aggregation, you can forward RIP-2 updates to RIP-1
routers by "rolling up" or aggregating the subnet addresses into a single
advertisement.
See the RIP-1 versus RIP-2 discussion later in this chapter.
Go to
http://www.3com.com/technology/tech_net/white_papers
a thorough discussion of IP addressing, VLSMs, and route aggregation.
Guidelines for Using VLSMs
Consider the following guidelines when you implement VLSMs:
When you design the subnet scheme for your network, do not
estimate the number of subnets and hosts that you need. Work from
the top down until you are sure that you have accounted for all the
hosts, present and future, that you need. When you design the
network, set up the address numbers so that you can take advantage
of route aggregation.
Use RIP-2 or Open Shortest Path First (OSPF) to carry the extended
network prefix information with each route advertisement.
Make sure that the routers forward routes based on what is known as
the longest match.
For example, assume that the destination IP address of a packet is
158.101.26.48 and that the following four routes are in the routing
table:
158.101.26.0/24
158.101.3.10/16
158.101.26.32/16
158.95.80.0/8
The router selects the route to 158.101.26.0/24 because its extended
network prefix has the greatest number of bits that correspond to the
destination IP address of the packet.
To take advantage of route aggregation, use addresses that reflect the
actual network topology, as illustrated in Figure 46. This is one of the
best things you can do to increase routing efficiency.
For more information about understanding and using VLSMs, see
RFC 1219 and RFC 1878.
for