HP FlexNetwork NJ5000, User Manual

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195
All hosts on this subnet can receive the broadcast request, but only the requested host (Host B)
processes the request.
3.
Host B compares its own IP address with the target IP address in the ARP request. If they are
the same, Host B:
a.
Adds the sender IP address and sender MAC address into its ARP table.
b.
Encapsulates its MAC address into an ARP reply.
c.
Unicasts the ARP reply to Host A.
4.
After receiving the ARP reply, Host A:
a.
Adds the MAC address of Host B into its ARP table.
b.
Encapsulates the MAC address into the packet and sends the packet to Host B.
Figure 194 ARP address resolution process
If Host A and Host B are on different subnets, Host A sends a packet to Host B, as follows:
1.
Host A broadcasts an ARP request to the gateway. The target IP address in the ARP request is
the IP address of the gateway.
2.
The gateway responds with its MAC address in an ARP reply to Host A.
3.
Host A uses the gateway's MAC address to encapsulate the packet, and then sends the packet
to the gateway.
4.
If the gateway has an ARP entry for Host B, it forwards the packet to Host B directly. If not, the
gateway broadcasts an ARP request, in which the target IP address is the IP address of Host B.
5.
After the gateway gets the MAC address of Host B, it sends the packet to Host B.
ARP table
An ARP table stores dynamic and static ARP entries.
Dynamic ARP entry
ARP automatically creates and updates dynamic entries. A dynamic ARP entry is removed when its
aging timer expires or the output interface goes down. In addition, a dynamic ARP entry can be
overwritten by a static ARP entry.
Static ARP entry
A static ARP entry is manually configured and maintained. It does not age out and cannot be
overwritten by any dynamic ARP entry.
Static ARP entries protect communication between devices, because attack packets cannot modify
the IP-to-MAC mapping in a static ARP entry.