1-34) 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 processIf Host A is not on the same subnet with Host B, Host A first sends an ARP request to the gateway. Thetarget IP address in the ARP request is the IP address of the gateway. After obtaining the MAC addressof the gateway from an ARP reply, Host A sends the packet to the gateway. If the gateway maintains theARP entry of Host B, it forwards the packet to Host B directly; if not, it broadcasts an ARP request, inwhich the target IP address is the IP address of Host B. After obtaining the MAC address of Host B, thegateway sends the packet to Host B.ARP TableAfter obtaining the MAC address for the destination host, the device puts the IP-to-MAC mapping into itsown 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 entryA dynamic entry is automatically created and maintained by ARP. It can get aged, be updated by a newARP packet, or be overwritten by a static ARP entry. When the aging timer expires or the interface goesdown, the corresponding dynamic ARP entry will be removed.Static ARP entryA static ARP entry is manually configured and maintained. It cannot get aged or be overwritten by adynamic ARP entry.Using static ARP entries enhances communication security. You can configure a static ARP entry torestrict an IP address to communicate with the specified MAC address only. After that, attack packetscannot modify the IP-to-MAC mapping specified in the static ARP entry. Thus, communicationsbetween the protected device and the specified device are ensured.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 apermanent static ARP entry, you must configure a VLAN and an outbound interface for the entrybesides the IP address and the MAC address.z A non-permanent static ARP entry has only an IP address and a MAC address configured. If anon-permanent static ARP entry matches an IP packet to be forwarded, the device sends an ARPrequest first. If the sender IP and MAC addresses in the received ARP reply are the same as those
