how to reach 2 wan nodes that has LANs with same network ID

Unanswered Question
Nov 4th, 2009

hi mates :)

pls see the attached figure i draw for you shows my network.

- Location A and Location B is working OSPF right now without any problems.

- we want to add a new WAN node(C) but they want computers to have the same network ID on two WAN nodes (B and C)

- is it possible to reach from A to B and C ?

- how can i achieve that? what kind of configuration should i do in order to reach two.

- would static routing between A and C solve the problem? (we can not change OSPF between A and B)

-or is it impossible to reach from A to B and C as long as B and C LANs have same network ID?

i'll appreciate all your help,

ty in advance for helping :)

I have this problem too.
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Giuseppe Larosa Wed, 11/04/2009 - 03:32

Hello Ohal,

the new location should use a different IP subnet.

the design should be reviewed because it is insane.

otherwise you should use NAT or some L2 transport tecnique like L2TPv3 but:

these have requirements on device types:

NAT can be done only on SW based routers or C6500 with sup720.

L2TPv3 can only be done on sw based routers.

But all this has complexity and performance penalties.

if this is a new site it is better to use a different IP subnet.

for L2TPv3 see

vlan based L2TPv3

for NAT: you need to assign two IP subnets to represent the common subnet in the different sites

siteC should see net as equivalent to

first site should use something like when attempting to access other site's ip address.

Each router should nat its own block

ip nat pool mypool netmask

access-list 10 permit

int loop5

ip address

int lan 0

ip nat inside

int wan0

ip nat outside

ip nat inside source list 10 pool mypool

router ospf xx

network area YY

something similar should be done in other site

hope to help


ohalnet53 Wed, 11/04/2009 - 07:14

ty for your fast response but i am not sure if that solves my issue.

lets say

there is a pc with on location B


there is a pc with on location C,

how can router 2 understand which IP belongs to B, which IP belongs to C and send the packet accordingly ?

sohailian Tue, 11/10/2009 - 20:40

Hi Ohalnet

Can u further tell that u specify only 1, 2 and3 etc on serial interface, is it in the same network that u assign on LAN like ip address for location A serial interface will be or is it another network ID, i need the class or full address that you used or if it is public addressa nd you can not specify over here then just tell me that it is not the same class as on LAN and it is related to another class then it will be easy for me to completely do the full lab and will tell you the result.


fanlongkf Tue, 11/10/2009 - 07:56

Hi,why are we doing this ?

Obviously , Routing loops Will be generated in that network ..I think I can not help you anything about arouse the dead scheme ,I just suggest you avoid this thinking ...

hope this helps ..

Long Fan


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