3Com MSR 50 Series, Configuration Manual

Page: 551 / 2443

ARP Overview 551
Figure 156 ARP address resolution process
When Host A and Host B are not on the same subnet, Host A first sends an ARP
request to the gateway. The destination 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 encapsulates the packet and sends it to the gateway.
Subsequently, the gateway broadcasts the ARP request, in which the destination IP
address is the one of Host B. After obtaining the MAC address of Host B from
another ARP reply, the gateway sends the packet to Host B.
ARP Mapping Table After obtaining the destination MAC address, the device adds the IP-to-MAC
mapping into its own ARP mapping table. This mapping is used for forwarding
packets with the same destination in future.
An ARP mapping table contains ARP entries, which fall into two categories:
dynamic and static.
1
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.
2
A static ARP entry is manually configured and maintained. It cannot get aged or be
overwritten by a dynamic ARP entry. It can be permanent or non-permanent.
■
A permanent static ARP entry can be directly used to forward data. When
configuring a permanent static ARP entry, you must configure a VLAN and
outbound interface for the entry besides the IP address and MAC address.
■
A non-permanent static ARP entry cannot be directly used for forwarding data.
When configuring a non-permanent static ARP entry, you only need to
configure the IP address and MAC address. When forwarding IP packets, the
device sends an ARP request. If the source IP and MAC addresses in the
received ARP reply are the same as the configured IP and MAC addresses, the
device adds the interface receiving the ARP reply into the static ARP entry. Now
the entry can be used for forwarding IP packets.
n
Usually ARP dynamically implements and automatically seeks mappings from IP
addresses to MAC addresses, without manual intervention.
Target IP
address
192 .168 .1 .1
Target IP
address
192 .168 .1 .2
Host A
192.168.1.1
0002- 6779-0 f4c Host B
192.168.1.2
00a 0-2470 -febd
Target MAC
address
0000 -0000- 0000
Sender MAC
address
00a 0-2470 -febd
Target MAC
address
0002 -6779 -0f4c
Sender IP
address
192.168 .1.1
Sender MAC
address
0002 -6779 -0f4c
Sender IP
address
192.168 .1.2