HP J3245A Installation And Configuration Manual page 137

Advanced Concepts
Automatic Broadcast Control (ABC)
To learn host D's MAC address, host A sends a broadcast ARP request.
Because the switch does not yet know the location of host D, it floods the
request out all ports. However, the switch also learns from the ARP request
the location of host A and stores this information in its ARP cache. Host D
receives the ARP request (as will all other hosts connected to the switch), and
responds with a unicast packet through the switch to host A. The switch
monitors this response, learns the location of host D, and stores this informa-
tion in its ARP cache. Thus, the switch now knows the address information
for both host A and host D. Now, hosts A and D can send unicast packets to
each other because they have learned each other's addresses. Suppose that
host C now wants to communicate with host A. C sends a broadcast ARP
request to the switch. Because the switch already has A's address information,
it does not flood C's ARP request out all ports, but instead sends a proxy ARP
reply to C that tells C the address information for host A. Host C can now send
unicast packets directly to host A. From these packets, host A will learn host
C's addressing information and be able to respond with unicast packets
addressed to host C. The result is reduced network traffic because host C's
broadcast ARP request was not flooded on the switch's ports. Similarly, for
IPX networks, the switch learns service and route information from SAPs and
RIPs respectively, and maintains SAP and RIP tables that contain the
addresses of known servers. Using this data, the switch sends proxy responses
to NSQ requests for these servers instead of flooding the requests on all ports.
N o t e
The switch sends proxy ARP replies to hosts (ARP initiators) that are on a
different port than the target host. However, the switch does not send a proxy
ARP reply when both the initiator and the target host are on the same port.
For example, the switch does not send a proxy ARP reply for host B (figure
7-17) in replying to an ARP request from host A.
The switch does not translate encapsulation types (such as 802.2 to SNAP in
IPX). As a result, if a host client sends an NSQ request for a server, the switch
will always send a proxy response containing the address of a server support-
ing the same encapsulation type. If the switch has not learned of a server using
the same encapsulation type as the host client, then the switch will flood the
host client's NSQ request to all ports. However, if a local server supporting
the same encapsulation type exists on a port from which the NSQ request is
received, the switch will not forward the request to other ports.
If Automatic Broadcast Control (ABC) is configured and more than one port
is monitored, then broadcast packets may be duplicated on the monitor port.
Reducing RIP and SAP Broadcast Traffic. You can also configure ABC to
limit IP RIP and IPX RIP and SAP broadcasts, which can further reduce
broadcast traffic on your network. RIP and SAP broadcasts are normally
7-31