command no auto-summary

ronal120

what is the function of command

#no auto-summary

??

I fail to understand

thanks!

AD227529

If you are using a routing protocol that auto-summarizes (such as RIP or EIGRP) it will send the subnet mask of the classful boundary, and not the actual subnet mask. The command no auto or no auto summary will turn this feature off in RIP and EIGRP, and send the actual subnet mask. If you are using OSPF, you don't have to worry about it because OSPF does not auto-summarize.

ccnxjr

You'll use it to prevent the classful auto-summarization of discontiguous networks, especially when your given a sliver of a slice.

Lets say your given the block 192.168.7.128/25 (total of 126 hosts), and an uplink ip address of 172.16.7.1 /30
And you need to divide that up amongst 3 departments,
so you go about it like this
192.168.7.128/27
which gives you
.128
.160
.192
.224
(30 hosts each, total of 120 hosts)

Your routing protocol auto summarization function (being a lazy network admin) will just assume you have the whole class C block of 192.168.7.0/24
You can see where this will wreak all sorts of havoc!

JohnnyBiggles

AD227529 wrote: »

If you are using a routing protocol that auto-summarizes (such as RIP or EIGRP) it will send the subnet mask of the classful boundary, and not the actual subnet mask. The command no auto or no auto summary will turn this feature off in RIP and EIGRP, and send the actual subnet mask. If you are using OSPF, you don't have to worry about it because OSPF does not auto-summarize.

That being said, why is EIGRP considered classless (as is OSPF), and why is RIP considered classfull... especially if you must change both to classless manually, and both can be classless anyway... sorry just seeking clarity here. Just when I think I understand it, it confuses me again For some reason I didn't think 'no auto-summary' would apply to RIP v1 advertised network...?

capitanuionut

JohnnyBiggles wrote: »

That being said, why is EIGRP considered classless (as is OSPF), and why is RIP considered classfull... especially if you can change both to classless.. or classfull... sorry just seeking clarity here. Just when I think I understand it, it confuses me again For some reason I didn't think 'no auto-summary' would apply to RIP v1 advertised network...?

Only RIPv1 is classfull and you cannot use the no auto summary with RIPV1.... but now because discontigous networks and VLSM are used every where we just don't use RIPv1...instead we use RIPv2 wich is a classless routing protocol but with auto summarization activated by default..the same thing happens with EIGRP...

Auto summarization helps where you have a lot of routers/routes...and the routing table is to big and involves more CPU use...but this can generate some errors if it's used in a discontigous network...

For more safety just turn off auto summarization and use manual summarization where your design permits it..

JohnnyBiggles

capitanuionut wrote: »

Only RIPv1 is classfull and you cannot use the no auto summary with RIPV1.... but now because discontigous networks and VLSM are used every where we just don't use RIPv1...instead we use RIPv2 wich is a classless routing protocol but with auto summarization activated by default..the same thing happens with EIGRP...

Auto summarization helps where you have a lot of routers/routes...and the routing table is to big and involves more CPU use...but this can generate some errors if it's used in a discontigous network...

For more safety just turn off auto summarization and use manual summarization where your design permits it..

I saw an example where a RIP network was being set up and the 'no auto-summary' command WAS applied under a 'router RIP' command for the network advertisement for some reason and there was no 'version 2' to be found. It seems someone (a CCNA, as displayed in the profile) also says you can in the second post in this thread, hence my confusion:

AD227529 wrote: »

If you are using a routing protocol that auto-summarizes (such as RIP or EIGRP) it will send the subnet mask of the classful boundary, and not the actual subnet mask. The command no auto or no auto summary will turn this feature off in RIP and EIGRP, and send the actual subnet mask. If you are using OSPF, you don't have to worry about it because OSPF does not auto-summarize.

So do I have a second vote for clarification?

Chris_

Ripv1 is classful, although it may well allow the 'no auto-summary' command to exist within the config, it won't come into effect until version 2 is enabled.
EIGRP and RIPv2 are classless - ripv2 needs the 'no-auto' to behave as so. EIGRP also needs the 'no auto' but in the most recent IOS it is on by default.
Hope this helps.