Why is the first 2 ping lost?

yrwins

R1#ping 5.6.3.2

Type escape sequence to abort
Sending 5, 100-byte ICMP Echos to 5.6.3.2, timeout is 2 seconds:
..!!!

R1#ping 5.6.3.2

Type escape sequence to abort
Sending 5, 100-byte ICMP Echos to 5.6.3.2, timeout is 2 seconds:
!!!!!

Why is the first 2 ping lost?

clarson

5.6.3.2 isn't in the routers mac address table. So, it needs to wait until the arp processes gets it the mac address of 5.6.3.2

yrwins

Hi Clarson. thanks for the reply .

james43026

The IP address 5.6.3.2 doesn't have an entry in the ARP table. So the first thing the router must do, is send an ARP request, which takes time, and causes the first 2 requests to timeout. The MAC address table is used for storing a mapping of MAC addresses to ports, and wouldn't impact this at all. As a missing entry in the MAC address table would just cause a unicast flood, which the packets would still end up at their destination regardless. By default the ARP table entry of a Cisco device has a timeout of 4 hours, which is why after the first attempt to ping, subsequent tries will usually yield 100% success.

yrwins

I just find out the book answer.
thank you so much.

Before a host can send ICMP (ping) packets to another device, it needs to learn the MAC address of the destination device so it first sends out an ARP Request. In fact, the first ping packet is dropped because the router cannot create a complete packet without learning the destination MAC address.