3Com 4007, Руководство по внедрению, Страница: 374 / 684

374 C
HAPTER
16: IP R
OUTING
target protocol address, the receiving device places its MAC address in
the target hardware address field and sends the packet back to the
source hardware address.
When the originating host or router receives this ARP reply , it places the
new MAC address in its ARP cache next to the corresponding IP address.
See Figure 41.
Figure 41 Example of ARP Cache Updated with ARP Reply
After the MAC address is known, the host or router can send the packet
directly to the next hop.
Important
Considerations
Keep the following things in mind about this protocol:
■
Enter a static ARP entry when the ARP resolution does not result in an
ARP entry in the cache. For example, some applications do not
respond to ARP requests and, consequently, specific network
operations may time out for lack of address resolution.
■
Enter a static ARP entry in a test environment if your test analyzer
cannot respond to an ARP request.
■
Setting an ARP cache age time of zero (no aging) is useful in the
middle of lengthy tests so that ARP requests do not have to be issued.
If you do set an ARP cache age time of zero, be aware that the ARP
cache can quickly grow in size and consume module resources. In this
case, be sure to flush the ARP cache after your tests are complete.
■
You can keep ARP cache entries if you refresh the ARP cache;
otherwise, the Multilayer Switching Module removes the entries after
they reach their defined age time.
ARP cache
158.101.1.1
158.101.2.1
158.101.3.1
IP address MAC address
00308e3d0042
0080232b00ab
0134650f3000