H3C Technologies H3C S7500E Series Switches - Manual (Page 15)

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3) Host B compares its own IP address with the destination IP address in the ARP request. If they
are the same, Host B saves the source IP address and source MAC address in its ARP table,
encapsulates its MAC address into an ARP reply, and unicasts the reply to Host A.
4) After receiving the ARP reply, Host A adds the MAC address of Host B to its ARP table.
Meanwhile, Host A encapsulates the IP packet and sends it out.
Figure 1-2 ARP address resolution process
If Host A is not on the same subnet with Host B, Host A first sends an ARP request to the gateway. The
target IP address in the ARP request is the IP address of the gateway. After obtaining the MAC
address of the gateway from an ARP reply, Host A sends the packet to the gateway. If the gateway
maintains the ARP entry of Host B, it forwards the packet to Host B directly; if not, it broadcasts an ARP
request, in which the target IP address is the IP address of Host B. After obtaining the MAC address of
Host B, the gateway sends the packet to Host B.
ARP Table
After obtaining the MAC address of a host, the device puts the IP-to-MAC mapping into its own ARP
table. This mapping is used for forwarding packets with the same destination in future.
An ARP table contains ARP entries, which fall into one of two categories: dynamic or static.
Dynamic ARP entry
A dynamic entry is automatically created and maintained by ARP. It can get aged, be updated by a new
ARP packet, or be overwritten by a static ARP entry. When the aging timer expires or the interface
goes down, the corresponding dynamic ARP entry will be removed.
Static ARP entry
A static ARP entry is manually configured and maintained. It cannot get aged or be overwritten by a
dynamic ARP entry.
Using static ARP entries enhances communication security. After a static ARP entry is specified, only a
specific MAC address is associated with the specified IP address. Attack packets cannot modify the
IP-to-MAC mapping. Thus, communications between devices are protected.
Static ARP entries can be classified into permanent or non-permanent.
z A permanent static ARP entry can be directly used to forward packets. When configuring a
permanent static ARP entry, you must configure a VLAN and an outbound interface for the entry
besides the IP address and the MAC address.