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Ping packet lost by ARP

trane.m
Beginner
Beginner

Hello,

First post, sorry if it's misplaced.

 

So i read that the first packet from the first ping between a switch and some other device is lost, due to the switch having no entries for the MAC address in the ARP table. But i can't figure out exactly what is going on, for that to be the result?

 

You tell the switch to ping x.x.x.x and the switch looks in the ARP table for an entry matching the destination IP. When it has no mathcing entry, will it send the packet as a broadcast packet, resulting in no one answering, because there's no devices configured with that address?

 

Is the packet lost because the switch starts to "do another job" as in sending an ARP request, instead of the ping packet?

 

I hope someone can help me by telling exactly what happens. Thank you in advance

2 ACCEPTED SOLUTIONS

Accepted Solutions

balaji.bandi
VIP Guru VIP Guru
VIP Guru

Just addition to other post, there is good learning discussion here :

 

https://learningnetwork.cisco.com/s/question/0D53i00000Kt2Im/about-ping-and-arp-table

 

BB

***** Rate All Helpful Responses *****

How to Ask The Cisco Community for Help

View solution in original post

Welcome to the community. You say "First post, sorry if it's misplaced." This part of the community is one of several where your question is appropriate. 

Your question about the loss of the first ping packet is not uncommon for someone who is learning about networking. 

You ask "When it has no mathcing entry, will it send the packet as a broadcast packet" No it will not send the packet as a broadcast. This behavior is found when the switch is doing layer 2 forwarding (if the switch receives a frame to forward and the destination mac address is not in the mac table then the switch does forward the frame to all ports in that vlan - except for the port on which it was received) but is not implemented for layer 3 processing.

You also ask "Is the packet lost because the switch starts to "do another job" " It is not that the switch starts to do another job. The switch has a packet that it needs to transmit but it can not transmit it because it has no destination mac address. This triggers the switch to send an arp request. Assuming that the destination device does exist in the vlan/subnet it should send an arp reply and the switch will have the destination address for the second packet. But the first packet fails.

 

HTH

Rick

View solution in original post

4 REPLIES 4

Georg Pauwen
VIP Master VIP Master
VIP Master

Hello,

 

as I understand it, the first ping usually 'fails' because it times out.

 

I think the link below explains it pretty well...

 

https://www.pathsolutions.com/resources/why-does-the-first-ping-usually-fail/#:~:text=The%20reason%20the%20first%20ping,delay%20is%20usually%20too%20long.

balaji.bandi
VIP Guru VIP Guru
VIP Guru

Just addition to other post, there is good learning discussion here :

 

https://learningnetwork.cisco.com/s/question/0D53i00000Kt2Im/about-ping-and-arp-table

 

BB

***** Rate All Helpful Responses *****

How to Ask The Cisco Community for Help

Welcome to the community. You say "First post, sorry if it's misplaced." This part of the community is one of several where your question is appropriate. 

Your question about the loss of the first ping packet is not uncommon for someone who is learning about networking. 

You ask "When it has no mathcing entry, will it send the packet as a broadcast packet" No it will not send the packet as a broadcast. This behavior is found when the switch is doing layer 2 forwarding (if the switch receives a frame to forward and the destination mac address is not in the mac table then the switch does forward the frame to all ports in that vlan - except for the port on which it was received) but is not implemented for layer 3 processing.

You also ask "Is the packet lost because the switch starts to "do another job" " It is not that the switch starts to do another job. The switch has a packet that it needs to transmit but it can not transmit it because it has no destination mac address. This triggers the switch to send an arp request. Assuming that the destination device does exist in the vlan/subnet it should send an arp reply and the switch will have the destination address for the second packet. But the first packet fails.

 

HTH

Rick

I am glad that our explanations have been helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.

HTH

Rick
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