Re: Eigrp reading network statements wrong.

GeneBeamer · ‎04-14-2011

This is what I am seeing on my router...

router eigrp 100
network 10.1.16.0 0.0.0.255
network 10.1.17.0 0.0.0.255
network 10.1.64.0 0.0.0.255
network 10.1.249.0 0.0.0.3
network 10.1.249.16 0.0.0.3
network 10.1.250.0 0.0.0.255
network 10.1.253.1 0.0.0.0
network 172.17.0.0 0.0.7.255 - - Wrong subnet
network 172.17.44.0 0.0.0.7 - - Wrong Subnet
network 172.17.50.0 0.0.0.255
network 172.17.200.0 0.0.3.255
eigrp router-id 10.1.253.1

The Vlan setup for 172.17.0.0 is with the subnet of 255.255.252.0 but thats not what the wildcard says.....

The Vlan setup for 172.17.44.0 is with the subnet of 255.255.252.0 but thats not what the wildcard says......

So when I do a sho ip route this is what I get for both.....

sho ip route 172.17.44.1
Routing entry for 172.17.44.0/22
Known via "connected", distance 0, metric 0 (connected, via interface)
Redistributing via eigrp 100
Routing Descriptor Blocks:
* directly connected, via Vlan401
Route metric is 0, traffic share count is 1

sho ip route 172.17.0.1
Routing entry for 172.17.0.0/22
Known via "connected", distance 0, metric 0 (connected, via interface)
Redistributing via eigrp 100
Routing Descriptor Blocks:
* directly connected, via Vlan100
Route metric is 0, traffic share count is 1

How is EIGRP reading these as a /22 when the network statements say one is a /21 and the other is a /29 ?? I am so confused and I don't know if I need to fix these and what this issue maybe causing... Any input would be Awesome... This router is setting in a multi-router enviroment.

Martin Parry · ‎04-14-2011

Hi

The router is seeing these as /22 as they are directly connected and it knows what the subnet mask is as it is locally configured on the device itself.

Hope this helps

Martin

GeneBeamer · ‎04-14-2011

I thought that the only thing telling the router to forward routes would be the network statement.. However, because it is directly connected it looks at the vlan information instead of the EIGRP statement?

Martin Parry · ‎04-14-2011

Hi

The network statements within EIGRP config are not telling the local router how to forward traffic, they are telling the router what to advertise out to it's neighbours.

The router will decide how to route traffic based on the information within it's routing table. If you do a show ip route and look at the output you will see all the networks / subnets that the router is aware of. These entries will be learnt in a number of ways and any locally originated networks eg: those residing on interfaces on the local device (VLAN SVIs in your case) will be preferred over any other potential entry.

Hope this helps

Martin

Antonio Knox · ‎04-14-2011

It looks like you have some auto summarization issues here. Did you include the 'no auto-summary' command in your router eigrp 100 configuation and omit it from the posted config?

GeneBeamer · ‎04-14-2011

Yes i did leave out the auto summary statement from the posted..

router eigrp 100
network 10.1.16.0 0.0.0.255
network 10.1.17.0 0.0.0.255
network 10.1.64.0 0.0.0.255
network 10.1.249.0 0.0.0.3
network 10.1.249.16 0.0.0.3
network 10.1.250.0 0.0.0.255
network 10.1.253.1 0.0.0.0
network 172.17.0.0 0.0.7.255
network 172.17.44.0 0.0.0.7
network 172.17.50.0 0.0.0.255
network 172.17.200.0 0.0.3.255
eigrp router-id 10.1.253.1
no auto-summary

The problem I have with what you suggestion Martin Parry is this output from another router when i do sho ip route. This router is telling others I have the 172.17.44.0 /22 network when it DOES but it's not in the EIGRP statement. ( Also note that EIGRP isnt a strong area of mine ).

sho ip route 172.17.44.1
Routing entry for 172.17.44.0/22
Known via "eigrp 100", distance 90, metric 31232, type internal
Redistributing via eigrp 100
Last update from 10.24.249.x on Vlan802, 2d01h ago
Routing Descriptor Blocks:
* 10.24.249.17, from 10.24.249.x, 2d01h ago, via Vlan802
      Route metric is 31232, traffic share count is 1
      Total delay is 220 microseconds, minimum bandwidth is 100000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 12/255, Hops 3

Antonio Knox · ‎04-14-2011

According to your output these routes are redistributed by the routing protocol.

sho ip route 172.17.44.1
Routing entry for 172.17.44.0/22
Known via "connected", distance 0, metric 0 (connected, via interface)
Redistributing via eigrp 100
Routing Descriptor Blocks:
* directly connected, via Vlan401
Route metric is 0, traffic share count is 1

sho ip route 172.17.0.1
Routing entry for 172.17.0.0/22
Known via "connected", distance 0, metric 0 (connected, via interface)
Redistributing via eigrp 100
Routing Descriptor Blocks:
* directly connected, via Vlan100
Route metric is 0, traffic share count is 1

Is there a 'redistribute connected' command in your EIGRP config? That would fully advertise the connected networks whether they have smaller scoped network statements or not.

GeneBeamer · ‎04-14-2011

Not on this router however on another network there is a redistribute static metric 1 1 1 1 1 command with a statement of network 172.17.44.0 0.0.3.255

Martin Parry · ‎04-14-2011

Hi

Sorry for the delay, I've been on the tube.

The network statement within EIGRP does not specify what is actually advertised but more specifies what interfaces will participate and be advertised.

For instance if you had four interfaces with ip addresses as follows:

172.17.0.0 /24

172.17.1.0 /24

172.17.2.0 /24

172.17.3.0 /24

You could insert a network statement of

Network 172.17.0.0 0.0.3.255

This would mean that the configured ip subnets all interfaces within this network statement would be advertised to the neighbour routers along with their subnet masks. The router would still advertise the individual subnets as opposed to the aggregated /22. So the receiving router would learn the 4 subnets

If however you had 5 interfaces Such as:

172.17.0.0 /24

172.17.1.0 /24

172.17.2.0 /24

172.17.3.0 /24

172.17.4.0 /24

And inserted the same network statement into EIGRP:

Network 172.17.0.0 0.0.3.255

Only the first 4 subnets would be advertised. Not the 5th.

Hope this helps

Martin

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tj.mitchell · ‎04-14-2011

EIGRP Network statements state what interface to send hellos on and what interfaces to advertise into the protocol. It does NOT state what the subnet mask will be as that is grabbed from the actual interface/s that are assocated with the network statement.

When I build an EIGRP network my network statements are for the host ip address associated to the interface that I want to adverise out. Not the entire subnet. Example:

interface VLAN10

ip add 10.10.10.1 255.255.255.0

interface VLAN11

ip add 10.10.11.1 255.255.255.0

router eigrp 10

network 10.10.10.1 0.0.0.0

network 10.10.11.1 0.0.0.0

What gets advertised to the neighbor routers are the /24 networks, not the /32 networks. I do this for granularity and control over the interfaces and networks that will be advertised to the rest of the infrastructure.

Does that make sense?

Antonio Knox · ‎04-14-2011

That is correct.

Antonio Knox · ‎04-14-2011

Gene,

TJ is right on the money! To paraphrase in a way that fits your scenario,

This your interface vlan401 config:

interface Vlan401

ip address 172.17.44.1 255.255.252.0

!----output omitted--------

And your EIGRP config says:

router eigrp 100

network 172.17.44.0 0.0.0.7 <------The command in question*

*This command dictates that any connected interface host-addressed in the 172.17.44.0/29 subnet (172.17.44.1 - 7) will advertise the network configured to it.

What you may have expected from the config was for the network 172.17.44.0/29 to be advertised per the network statement.

Martin Parry · ‎04-14-2011

Hi

I apologise if my response was not clearly written, but this is what I meant to get across. Sorry for any confusion

Martin

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