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Disable Auto-Summary on Router

If I add:
router eigrp 109
no auto-summary

What exactly does it do?

I think that it advertises the mask along with the prefix.

Not assume Class A 8 bit mask, Class B 16 bit mask and Class C a 24 bit mask.

If your network is non-contiguous and not completely hierarchical then you have to disable auto-summary.

I read something about EIGRP having problems turning off auto-summary if you are using both private address space 10 and 192.168?

Also the documentation for EIGRP says that it "automatically summarizes at classful network boundaries". What exactly does that statement mean?

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Instructor
Top Expert 2015
Commented:
By default, EIGRP will summarize routes using the classful boundary when the route is advertised over a different network. For example, advertising the 172.xx.x.0/24 network out an interface with an address of 172.xx.x.1/24 will be advertised as the 172.xx.x.0/16 network.

However, if auto-summary is disabled then the route will be advertised as the 172.xx.x.0/24 network.
Commented:
> If your network is non-contiguous and not completely hierarchical then you have to disable auto-summary.
Yep, i agree with this statement. Auto-summary has no significance if an entire network is based one address range say 10.0.0.0, 172.xx.x.0 or 192.168.0.0


> I read something about EIGRP having problems turning off auto-summary if you are using both private address space 10 and 192.168?


This would be because if auto-summary is ON, route advertisements would get summarized classfully when they are advertised across a major boundry.
Meaning: If my interface address is 192.xxx.1.254/24 and i want to send the update 10.1.1.0/24 out this interface with auto-summary ON. It will get advertised as 10.0.0.0/8 (class-A = /8) out this interface. Note that if the interface address was 10.1.2.254/24 this same update would go out as 10.1.1.0/24 NOT 10.0.0.0/24. Becasue its NOT a different network boundry.

> Also the documentation for EIGRP says that it "automatically summarizes at classful network boundaries". What exactly does that statement mean?
See above explanation


Author

Commented:
thanks donjohnston and nazsky,

Where I get confused is I was thinking the auto-summary meant:

Class A routes use /8
Class B routes use /16
Class C routes use /24

Now I am thinking that auto-summary may allow all Class A routes to use a single say /10

All Class B to use say a /19

All Class C to use say a /24

If an interface has a Class B address and is a /19 but sends an update for a Class A route it will not use the aforementioned /10 but change it to a classfull /8?

If auto-summary is turned off then every route gets advertised with whatever /x it has configured on the interface that matches the network statement.

Or maybe I have that all completely wrong!

and auto-summary has to do with setting up your network hierarchically which does not make sense to me because that would mean that you would need an entire classful network for each segment of your lan
Commented:
Where I get confused is I was thinking the auto-summary meant:

Class A routes use /8
Class B routes use /16
Class C routes use /24

This is correct to an extent, this happens when the conditions are met i.e.
- auto-summary is on
and
- advertisement is across network boundaries


If an interface has a Class B address and is a /19 but sends an update for a Class A route it will not use the aforementioned /10 but change it to a classfull /8?
Yes this is correct.

Author

Commented:
>- advertisement is across network boundaries

Network boundaries are the classes A, B and C?
Commented:
Not exactly...

Same:
7.1.2.0/24
7.1.1.0/24

99.1.1.0/24
99.2.2.0/24

130.1.1.0/24
130.1.45.0/24


Different:
7.1.1.0/24
9.1.1.0/24

98.1.1.0/24
99.2.2.0/24

130.1.1.0/24
130.2.45.0/24



Author

Commented:
So a network boundary is if the first octet?

7.x.x.x is the same as any other 7.x..x.x?

7.x.x.x is different from 8.x.x.x or 9.x.x.x?
Don JohnstonInstructor
Top Expert 2015
Commented:
No. A network boundary is a different (classful) network.

Example A
172.xx.1.0/24 and 172.xx.2.0/24 are part of the same classful network.

Example B
172.xx.1.0/24 and 172.xx.1.0/24 are on different classful networks.

So in example A, the route would NOT be summarized.

In example B, the route would be summarized.

Author

Commented:
So if route 172.xx.1.0/24 was being advertised out and interface with an ip address of 172.xx.2.1/24 the original /24 would stay and not be summarized. Because both are in classful network 172.xx.0.0/16

If route 172.xx.1.0/24 was being advertised out and interface with an ip address of 172.xx.1.1/24 the original /24 would be changed to a classful /8? Because one network is in classful network 17.xx.0.0/16 and the other is in 172.xx.0.0/16?

And in both above examples auto-summary is turned ON.

If auto-summary was turned off the original /24 would stay on the route?
Don JohnstonInstructor
Top Expert 2015
Commented:
>And in both above examples auto-summary is turned ON.

Correct!

>If auto-summary was turned off the original /24 would stay on the route?

Correct again!

I think you've got it. :-)

Author

Commented:
This was a difficult concept for me to get.

I may have understood it more quickly if the rule was stated.

Auto-summarization will change the advertised routes to their classful network mask when:

Auto-summary is ON and the advertisement is across classful network boundaries IE the classful network of the advertised route is different than the classful network of the ip address on the interface sending the advertisement?

Just making sure that I fully understand this.

I could ask why but I should probably open another question for that to award more points!