WAN SUBNET

[birenderrana]

Can we have two different subnets on the same WAN link?
If not what's the reason?
If yes then how we can do that?
Thanks in advance.  

[AjitKumar]

Hi @birenderrana 

This seems like a difficult idea.

This is my understanding though.

A Link/Device/IP/Interface etc can be part of one network at a given time - That is how they are identified. If you need this to be reachable from multiple Subnets - Router helps us with this. In this scenario too - the Link/Device/IP/Interface shall be Unique.

[balaji.bandi]

Can you explain more what exactly you trying to achieve - why you need 2 subnets on the WAN link. you can have secondary IP configured on an interface, again that is based on the requirement?

also, provide environment information

what is the device?

what IOS code?

[birenderrana]

Hi Balaji  

Thanks for your response but it's just my general query. 

[paul driver]

Hello
Yes, you can, and how to use them would depend on the wan device connecting the the isp,
1)If your router has multiple interfaces then these two public subnets could be applied to two separate physical interfaces of your router to give you two egress point from your lan.

2) Or you may have just the one physical wan interface so you could tag the two public subnets towards the ISP (if the ISP is accepting tagging) using sub-interfaces on your wan rtr

3) Lastly you could just use the single physical wan interface and have a primary and secondary addressing assigned to it.

[birenderrana]

Hi Paul  ,

I was considering this type of scenario  .

Router1/GigbaitEthernet0/0 - 192.168.1.1/24 <--WAN LINK--> Router2/GigabitEthernet0/0 - 192.168.2.1/24

[Georg Pauwen]

Hello,

see the answer below. This is not a problem, as long as your WAN interface arps for the IP address of the other side...

[Georg Pauwen]

Hello,

just in case you mean this:

Router1/GigbaitEthernet0/0 - 192.168.1.1/24 <--WAN LINK--> Router2/GigabitEthernet0/0 - 172.16.1.1/24

where both ends of the WAN link are in a different subnet: this works if you define the route below on both routers. The WAN interface will ARP for any address on the other side, it does not have to be in the same subnet:

ip route 0.0.0.0 0.0.0.0 GigabitEthernet0/0

[birenderrana]

Hi Georg ,

Thanks for your answer .

I was able to ping across .

But can you tell me what's the logic behind this  ?

[Georg Pauwen]

Hello,

the 'logic' is that the interface arps for the IP address of the other side, and if it gets a response, that is where it will send the traffic. The IP address of the other side can be any address...

You can turn on 'debug arp' (do a 'clear arp-cache' first to see how this works...

[birenderrana]

Please let  me know if i am correct or not  ?

So is it once the router is able to get the ARP entry it will form the arp header which it will get with the help of default route mentioned .

[paul driver]

Hello

the only problem with setting an interface as the next hop is it will arp for ALL external addressing not just it isp wan address  so it can be very resource intensive to the rtr.

Can you elaborate on your current setup please as i see that addressing are non routable private subnets so why is it you have two address ranges ?