Hi,

A good understanding. I too noticed this.

can you explain in this detail. when it is doubled

sairam

Sairam

When you ping the local interface on a serial P2P link the packets actually go to the other end of the link and back.

So when you ping B from A

echo request goes from A -> B

echo reply comes from B -> A

when you ping A from A

echo request goes from A -> B -> A

echo reply goes from A -> B -> A

This is why it takes twice as long.

Jon

Sairam,

As you know, ping operation actually generates two packets, one is ICMP echo request and one is ICMP echo reply.

Let's say on a P2P link, A end's IP is 10.1.1.1 and B end's IP is 10.1.1.2. When you ping B from A, the source IP is 10.1.1.1 and destination IP is 10.1.1.2. A sends ICMP echo request to B (one way trip delay). B responds back to A with ICMP echo reply (round trip delay).

When you ping from A's IP to A's IP, the source IP is 10.1.1.1 and destination IP is also 10.1.1.1. So A sends out ICMP echo request out of its interface (one way trip delay). B receives it and sees the destination IP is 10.1.1.1 and sends this ICMP echo request back to A (round trip delay). When A receives this ICMP echo request, A generates ICMP echo reply back to 10.1.1.1 and sends it out of its interface. B receives this echo reply and sends it back to A (round trip delay X 2).

This is the reason why when you ping the same IP as the source, the delay is round trip times 2.