ARP

Created: Jan 8, 2019 02:33:24Latest reply: May 22, 2019 11:22:38 282 5 1 0
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for the above topology , why the destination mac address here is :00-00-00-00-00-00 , i think it should be FF-FF-FF-FF-FF-FF even this information in most of our huawei guides like HCNA R&S and HCNA security guide, please if any one has information about this , kindly share...

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mesbah  Visitor   Created Jan 8, 2019 14:55:07 Helpful(0) Helpful(0)

Both. Hamid.

The ARP request is a broadcast, so the Destination MAC will be FFFF.FFFF.FFFF. Of course, you aren't asking about this, but I'm listing it here for clarity. You are asking about the Target MAC field inside the ARP Payload.

The ARP Request is meant to find the unknown L2 address from from a known L3 address. As such, the Target IP address field is populated with the L3 address we are trying to resolve for... but the Target MAC field can not be populated, as the whole purpose of the payload is to discover the target's MAC address.

As such, the RFC does not specify what the Target MAC should be, just that it doesn't matter what it is:

[Sender of ARP Request] does not set ar$tha to anything in particular, because it is this value that it is trying to determine. It could set ar$tha to the broadcast address for the hardware (all ones in the case of the 10Mbit Ethernet)

ar$tha = ARP payload, Target Hardware Address -- aka, the official name for the field you are asking about

The RFC suggests to use the broadcast address (all F's), but does not designate that as a requirement. So implementations are free to do what they will. Most that I have come across will use all F's, or all 0's, as you have found.
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chenhui  Enthusiast Technician   Created Jan 8, 2019 08:56:05 Helpful(0) Helpful(0)

@Hamid_84102332 the arp packet which you described is the gratuitous ARP, it's used to checks for IP address conflicts, advertises a new MAC address, notifies other devices of an active/standby switchover in a VRRP group. By the way, the destination IP address of the gratuitous ARP request is the sender's IP address, but in your list the destination IP address doesn't match this, I'm not sure if you listed it right
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Hamid_84102332     Created Jan 8, 2019 12:03:53 Helpful(0) Helpful(0)

it is not gratuitous ARP , it is normal ARP ,so I guess it may be wrong in the HCNA guide or may there is an explanation for this point
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paradise1680     Created 4 days ago Helpful(0) Helpful(0)

thank you for sharing
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Sergio93  Enthusiast Technician   Created 2 days 11:22 Helpful(1) Helpful(1)

Hello,

The value of the destiation MAC in an ARP Request packet is always 0x0000-0000-0000 in Huawei VRP. To understand the entire process please follow the Address resolution overview: 
fig_dc_cfg_arp_100401.png

In Figure 1, Host_1 and Host_3 are on the same network segment, and Host_1 needs to communicate with Host_3.

  1. Host_1 searches its local ARP table for the ARP entry of Host_3. If the MAC address of Host_3 is found, Host_1 uses this MAC address to encapsulate the data packet into a frame and sends it to Host_3. If Host_1 does not find the MAC address of Host_3, Host_1 caches the data packet and broadcasts an ARP Request packet.
  2. Switch_1 receives the ARP Request packet and forwards it across the appropriate broadcast domain.
  3. Host_2 and Host_3 in that broadcast domain both receive the ARP Request packet. Because Host_3 finds that the destination IP address of the ARP Request packet is its own IP address, it adds the source IP and MAC addresses of the ARP Request packet to its own ARP table. It then unicasts an ARP Reply packet to Host_1.
  4. Switch_1 receives the ARP Reply packet and forwards it to Host_1. Upon receipt, Host_1 adds the MAC address of Host_3 to its ARP table. It then encapsulates the data packet into a frame and forwards it to Host_3.
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