EIGRP AUTO-SUMMARY

[laxmicisco]

I have configured the EIGRP 200 in all routers and Auto-summary is enabled .But the Thing is all my loopbacks are summarizing as 145.1.0.0/16 as pointing to a null 0 .But all my Physical  interfaces ip are summarizing to the  140.1.0.0/16 pointing to null 0 .But the thing is that why i am receiving the individual prefix with summary prefix for all my interfaces .
My Loopback series are -145.1.0.0/16

 R1,R2,R3
========
Router eigrp 200
network 145.1.0.0 0.0.255.255
network 140.1.0.0 0.0.255.255
auto-summary

R1#show ip route eigrp

Gateway of last resort is not set

145.1.0.0/16 is variably subnetted, 3 subnets, 3 masks
D 145.1.0.0/16 is a summary, 01:04:30, Null0
140.1.0.0/16 is variably subnetted, 18 subnets, 3 masks
D 140.1.0.0/16 is a summary, 01:04:30, Null0
D 140.1.23.0/24
[90/25861120] via 145.1.12.2, 01:04:30, FastEthernet0/0

[Cristian Matei]

Hi,

   That is what auto-summary does in EIGRP: it picks up all prefixes injected into EIGRP and summarise it to class A,B,C in oder to be advertised to EIGRP neighbors with a Null0 route. The null0 route is also for loop prevention.

Regards,

Cristian Matei.

[laxmicisco]

hi,


But why it is sending me this below route ??

D 140.1.23.0/24
[90/25861120] via 145.1.12.2, 01:04:30, FastEthernet0/0

it could have sent the Loopabck prefix as /32 also .

Right ??.


Thanks

[laxmicisco]

The Key point is here , if it is Advertising me the individual route as  /24 and also the summary   as /16 for 140.1.23.0/24, Then why not for Loopback interface as /32 and /16  .
i am receiving the exact  route for this below network .But why not for loopback ??


 D 140.1.23.0/24
[90/25861120] via 145.1.12.2, 01:04:30, FastEthernet0/0

But for loopback its advertising me only the summary route .

[Cristian Matei]

Hi,

   That is because we summarise across network boundaries and it makes sense. What i mean by this is that for example R1 will NOT summarise and advertise the 140.1.0.0/16 to R2 and R3, because the interconnect between the routers (140.1.12.0/24 and 1401.13.0/24) is included in the summary of 140.1.0.0/16 and thus it doesn't summarise cause it't not across major network boundaries. Think abut it, how could R1 tell to R2 and R3 that behind him is the whole 140.1.0.0/16, since the interconnect in between falls in the respective summary? That's a BIG lie, isn't it?

  So in the updates sent between R1, R2, R3, the interconnects are never summarised, so you should have on R1 an EIGRP route for 140.1.23.0/24, on R2 an EIGRP route for 140.1.13.024 and on R3 an EIGRP route for 140.1.12.0/24.

Regards,

Cristian Matei.