EIGRP Question

Unanswered Question
Feb 28th, 2010

I've got limited router experience and was wondering if someone could explain the affects of this routing statement:

router eigrp 100

network 10.0.0.0 0.0.0.3

network 204.238.51.0

auto-summary

Thanks in advance.

I have this problem too.
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Reza Sharifi Sun, 02/28/2010 - 16:09

DJ214 wrote:

I've got limited router experience and was wondering if someone could explain the affects of this routing statement:

router eigrp 100

network 10.0.0.0 0.0.0.3

network 204.238.51.0

auto-summary

Thanks in advance.


Hi DJ,

network 10.0.0.0 0.0.0.3 this advertises only 10.0.0.1 and 10.0.0.2 this is equal to a /30

network 204.238.51.0 this advertise a /24 subnet this is equal to 0.0.0.255

HTH

Reza

Jon Marshall Sun, 02/28/2010 - 16:14

DJ214 wrote:

I've got limited router experience and was wondering if someone could explain the affects of this routing statement:

router eigrp 100

network 10.0.0.0 0.0.0.3

network 204.238.51.0

auto-summary

Thanks in advance.

router eigrp 100

starts an eigrp routing process on the router. The process number ie. 100 is important in that it must match with other routers for a neighborship to be formed.

network 10.0.0.0 0.0.0.3
network 204.238.51.0

the network statements tell the router which interfaces to run EIGRP on. So

10.0.0.0 0.0.0.3  actually =

10.0.0.1 -> 10.0.0.2  with a broadcast address of 10.0.0.3.

So if you have an interface with an address of 10.0.0.1 or 10.0.0.2 then EIGRP will be run on this interface ie. the router will try and form a neighborship with any other routers on the same subnet. As there are only 2 available addresses ie. .1 and .2 then there can only be one other router on this subnet.

204.238.51.0 - EIGRP will be run on any interface that has an address in the 208.238.51.1 -> 208.238.51.254 range and again the router will try to form an EIGRP neigborship with any other router that has an address in the same subnet.

By starting EIGRP on these interfaces these networks will also be advertised to neighboring routers.

auto-summary

usually you turn this off in EIGRP configurations ie. "no auto-summary" is a more common occurence.

auto-summary summarises on classful boundaries eg.

R1 ( 172.16.5.0/24) R2 (10.5.1.0/24 ) R3

If auto-summary is on R2 advertises 172.16.0.0/16 to R3 and advertises 10.0.0.0/8 to R1.

If auto-summary is turned off R2 advertises 172.16.5.0/24 to R3 and 10.5.1.0/24 to R1.

Jon

Kevin Dorrell Mon, 03/01/2010 - 03:46

I'm going to be a bit more strict and pedantic here, just for the record.  Sorry.

The network 10.0.0.0 0.0.0.3 tells the EIGRP which interfaces to include in EIGRP, and to include in the search for other EIGRP routers.  Ignoring the address 10.0.0.0 for the moment, in fact if any of your interfaces have the address 10.0.0.1 or 10.0.0.2 or 10.0.0.3, then EIGRP will be activated on that interface, whatever its mask.  That is, EIGRP will look for other EIGRP routers on that interface, and it will also advertise that network.

To illustrate the difference, consider an interface that has ip address 10.0.0.3 255.255.255.0.  The network statement network 10.0.0.0 0.0.0.3 will activate EIGRP on that interface, and it will advertise the network 10.0.0.0/24.  What I mean is that the wildcards in the network statement do not actually affect the mask of the prefix that is advertised.

I ignored 10.0.0.0 for the moment because it is a major network base address, and so cannot normally be used as an IP address for an interface.

Kevin Dorrell

Luxembourg

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