Loopback IP or WAN IP to check issues ?

[Anup Sasikumar]

Hello All , 

When checking if you have any packet drops to a link , Should we ping to WAN IP or Loopback IP ( in a typical PE-CE setup ) ?
Does it make any difference ?

Please help !

Anup

[Richard Burts · Accepted solution]

Anup

If you are checking for packet drops then it is a matter of how many packets are received compared to how many are sent. I do not see that it would make any difference whether you were pinging the connected interface or a loopback, other than the fact that with pinging the connected interface the route would automatically be in the routing table and with the loopback you would have to create a route. But once the route is created then the behavior would be the same whether it was connected interface or loopback.

HTH

Rick

[Anup Sasikumar]

Thanks for the information , Rick ! 

But does the situation change if I am checking for incrementing errors on the physical WAN interface ?

[Richard Burts]

Thanks for the rating.

I do not see that the situation would change in any way if you are checking for incrementing errors on the physical WAN interface. What you are testing is the relationship of packets sent vs responses received. And as long as the destination causes packets to be sent out the right interface it should not matter whether you are using a loopback or the physical interface address.

Having said that, it does occur to me to consider a possible scenario where your router has two outbound interfaces that connect to the remote device (two parallel links), which would normally result in two equal paths toward the loopback interface which would result in load sharing. It could alter the results of the ping test when you ping to a loopback on the remote router. So perhaps I should modify my previous response and say that the results may be more accurate when you ping the physical interface.

HTH

Rick

[Anup Sasikumar]

Thanks a lot for clearing the confusion , Rick ! 

Cheers :)

Anup

[Richard Burts]

Anup

I am glad that I was able to help. Your follow up question helped me realize that my first response was based on an assumption (that there would be a single path) and that sometimes that assumption might not be correct. I am glad that we came to a better understanding. Thanks for the rating.

HTH

Rick