Default Gateway

[Mohammed Ismail Shareef]

Hi All,

I have noticed that i have a communication between both sites when i had configured the default gateway in the PC's which are not in the same segment as their IP address is.

As far as i know, whenever the destined traffic is in different network, its simply goes to default gateway and from there its gateways responsibility to forward the packet correctly.

So basically my doubt is how does the PC sends the packet to default gateway when it is not in the same segment ?

Cloud anyone explain how this is technically happening ?

Whats happening in arp reqeust and reponse and  is it something called source routing ?

Regards

@Mohammed

[Jon Marshall · Accepted solution]

It works because your routers will have proxy arp enabled on the interfaces connecting to the clients.

What happens is your client still sends an arp for the default gateway even if it is the wrong subnet and because the arp is a broadcast then the routers sees this.

If the router knows how to get to the IP address that the arp is for it responds with the mac address of the interface facing the client and so the client then sends the packet to the router and it forwards it on to the other router.

If you want to test this out first do a "sh ip int <x/y>" on the router and you should see proxy arp enabled.

Assuming it is you can then disable proxy arp ie under the interface "no ip proxy-arp", clear the arp cache on the client machine and then it shouldn't work.

Jon