auto-summary command

[tigerxxxx]

hello my friends

i would like to ask what this command means ? and when i need that?

(about eigrp)thank u

[ankurbhasin]

Hi tigerxxxx,

To allow automatic summarization of subnet routes into network-level routes, use the auto-summary command in router configuration mode.

Route summarization reduces the amount of routing information in the routing tables.

By default, Border Gateway Protocol (BGP) does not accept subnets redistributed from an Interior Gateway Protocol (IGP). To allow the software to create summary subprefixes to the classful network boundary when crossing classful network boundaries, use the auto-summary command.

Again when do you need it , it depends on your network and your requirement and how your network is design and how many routes do you have in your routing table.

Check out this link

http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123cgcr/iprrp_r/ip2_a1g.htm#wp1038925

HTH

Ankur

[liviu.gheorghe]

EIGRP is a distance vector protocol with some improvements. Being a DV protocol, the default behaviour is to do automatic summarization of subnets to their classfull network when they are advertised over an interface configured with an IP from a different major network.

For example, you have a router with a number of interfaces configured with subnets from major network 10.0.0.0 (10.1.1.0/24, 10.1.2.0/24, etc) and another interface configured with 192.168.1.0/24.

Eigrp is running on all interfaces in the router.

When the router will send an update on the 192.168.1.0 interface about the 10.1.x.0/24 subnets, it will summerize the update to 10.0.0.0/8.

If you don't want this to happen, you shoud use the "no auto-summary" command under router eigrp configuration.