what is AUTO SUMMARIZATION?

william_yeo

hi guys, i quite confuse on the AUTO SUMMARIZATION portion. Can some one kind-hearted briefly describe on how AUTO SUMMARIZATION work. Thanks a lot ~

Mr.Noe

The summarizing of routing information, a method of shrinking the size of the routing tables.

Take 172.24.0.0/16 through 172.31.0.0/16, they all have the first 13 bits in common as 10101100.00011xxx

Therefore, as opposed to using the default 16-bit subnet mask, you can classify those eight networks using a 13-bit subnet mask, or 255.248.0.0, and a summarization network address of 172.24.0.0 therefore reducing the routing table from 8 network and eight subnet masks to a single address and subnet mask.

More efficient routing
Less proccessing load when recalculating routing tables
Less memory requirements

kplab

What Mr.Noe explained is the concept of route summarization. Auto-summarization refers to a routing protocol performs route summarization automatically at classful network boundary, i.e. advertises a single route for an entire Class A, B, or C network.

Auto-summarization can reduce the amount of information to be exchanged in the routing updates. However, it requires the subnets of every classful network be contiguous.

For a classless routing protocol (e.g. RIP-2, EIGRP), auto-summarization can be disabled using the command "(config-router)#no auto-summary".

SV

It means we should not auto-summarize when we have multiple networks that use VLSM. Please correct me if I am wrong

kplab

You are right if some of the networks are discontiguous. You can refer to this Cisco paper for details:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093fd6.shtml