Summarization of Routes
Can be called 3 different things:
1. Route Aggregation
2. Route Summarization
3. Supernetting
How to manually calculated a summarized route:
172.1.4.0 /25
172.1..4.128 /25
172.1.5.0 /25
72.1.6.0 /24
172.1.7.0 /24
Binary Chart:
1 6 3 1 8 4 2 1
2 4 2 6
8
——————-
-|-|-|-|-|-|-|-|
——————–
0 0 0 0 0 1 0 0
172.1.(last in common octets)
22 23 24 placeholders
4.0 00000 1 0 0.00000000
4.128 00000 1 0 0.10000000
5.0 00000 1 0 1.00000000
6.0 00000 1 1 0.00000000
7.0 00000 1 1 1.00000000
*Notice the 22nd place holder is the last in common. Choose this place.
To find the IP subnet and subnet mask to summarize this route, you use the 22nd placeholder for the subnet mask (/22). And you keep the number 4 value in the 3rd octet because it is the value for the 22nd placeholder.
Answer: 172.1.4.0 /22
It is that simple. Do not try to make it harder than this…
Example from http://www.routemyworld.com **** Great site!
Route Summarization Strategies
The following example illustrates a method of summarize a group of subnets.
10.3.4.0/24
10.3.5.0/24
10.3.6.0/24
10.3.7.0/24
1. Convert the addresses to binary
2. Find the all the common bits from left to right
3. Convert all the bits that are in common back to decimal, and leave all the rest of the bits to zero.
4. The number of bits that are in common will be their subnet mask
10.3.4.0: 00001010 00000011 00000100 00000000
10.3.5.0: 00001010 00000011 00000101 00000000
10.3.6.0: 00001010 00000011 00000110 00000000
10.3.7.0: 00001010 00000011 00000111 00000000
The bits in red are the common bits. When the bits in red are converted back to decimal, the end
result is 10.3.4.0. This is the summary subnet.
To calculate the mask, count all the bits in red. There are 22 bits. Therefore the summary route is
10.3.4.0/22 or subnet mask 255.255.252.0
Manual Route Summarization
Reduces the size of the routing table by aggregating a group of routes into a single route that is advertised to the other routers. As a result, there is speedier process in routing packets as well as less amount of processing power is consumed. Needs to be configured in order for it to occur to manually configure summary route with EIGRP (or RIP v2), use the interface subcommand ip
summary-address
R1(config-if)#ip summary-address eigrp 1 10.2.0.0 255.255.0.0
R2(config-if)#ip summary-address eigrp 1 10.3.0.0 255.255.0.0
