Aggregated routes and addresses
Just as an aggregate interface combines multiple interfaces into one virtual interface, an aggregate route combines multiple routes into one. This reduces the amount of space those routes require in the routing tables of the routers along that route. The trade-off is a small amount of processing to aggregate and de-aggregate the routes at either end.
The benefit of this method is that you can combine many addresses into one, potentially reducing the routing table size immensely. The weakness of this method is if there are holes in the address range you are aggregating you need to decide if its better to break it into multiple ranges, or accept the possibility of failed routes to the missing addresses.
For information on aggregated routes in BGP, see
“ATOMIC_AGGREGATE”, and
“Aggregate routes and addresses”.
To manually aggregate the range of IP addresses from 192.168.1.100 to 192.168.1.103
1. Convert the addresses to binary
192.168.1.100 = 11000000 10101000 00000001 01100100
192.168.1.101 = 11000000 10101000 00000001 01100101
192.168.1.102 = 11000000 10101000 00000001 01100110
192.168.1.103 = 11000000 10101000 00000001 01100111
2. Determine the maximum number of matching bits common to the addresses.
There are 30-bits in common, with only the last 2-bits being different.
3 Record the common part of the address.
11000000 10101000 00000001 0110010X = 192.168.1.100
4 For the netmask, assume all the bits in the netmask are 1 except those that are different which are 0.
11111111 11111111 11111111 11111100 = 255.255.255.252
5 Combine the common address bits and the netmask.
192.168.1.100/255.255.255.252
Alternately the IP mask may be written as a single number:
192.168.1.100/2
6 As required, set variables and attributes to declare the routes have been aggregated, and what router did the aggregating.