How does a L2 pppoe connection work without a default gateway ?

[Odys (CSC)]

Since a ping packet doesn't need a default gateway over a pppoe link at L2, how does the communication look like between a local device and a remote device let's say google dns server ?

[Jon Marshall · Accepted solution]

I didn't realise you were taking about PPP.

There is still some confusion though. A ping between a client and remote device still involves L3 because it needs to be routed so what your senior said may have been somewhat misleading.

I think what you are referring to (as per your original question) is that the default gateway does not need to be in the same IP subnet and with PPP that is correct. So on a PPP link either end of the link could be using a completely different IP address from different subnets and it works fine.

Rather than simply repeat what is covered in much more detail (and better than I could) in another thread have a read of this link and see if it helps -

https://supportforums.cisco.com/discussion/11365321/dsl-modemip-and-defaultgateway

Jon

[Richard Burts]

Garry

I am glad to see this post with your new question, which follows up on your previous post. As we said in that post if you are using pppoe then for ppp the address of your gateway does not need to be in the same range as your IP address. So if you try to ping to dns at google you would forward the ping packet to your gateway (which might not be in the same range as your IP address). Your gateway would receive that ping packet and would do normal IP forwarding toward Google DNS. Google DNS would send a response and the Internet routing table would bring the packet to your provider. Your provider (your pppoe peer) knows to forward to your IP address, even if it is in a different range than its IP address. So the ping response gets back to you.

Jon

Thank you for posting the excellent link to the explanation that Peter provided about pppoe. It is a very good explanation and I hope that Garry will find it helpful.

HTH

Rick