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Recall that routers block
broadcasts and prevent broadcast storms --- a situation in
which a single broadcast triggers an onslaught of other broadcasts
ultimately leading to a disruption in network services. Large flat
networks are notorious for their bouts with broadcasts storms.
An example of broadcasting is
when a client sends a broadcast to find a server. If the server is
not on the same network, the broadcast will not get through to the
server. Helper addresses facilitate connectivity by forwarding these
broadcasts directly to the target server.
Client hosts interact with a variety
of network support servers, such as a domain name server (DNS),
BOOTP/Dynamic Host Configuration Protocol (DHCP) server, or TFTP
server. At startup time, the clients often do not know the IP
address of the server, so they broadcast to find it. Sometimes the
clients do not know their own IP address, so they use BOOTP or DHCP
to obtain it. If the client and server are on the same network, the
server will respond to the client's broadcast request. From these
replies, the client can glean the IP address of the server and use
it in subsequent communication.
But what if the server is not on the
same physical medium as the client? Remember that a destination IP
address of 255.255.255.255 is sent in a link-layer broadcast (FFFFFFFFFFFF).
By default, routers will never forward such broadcasts and you would
not want them to. A primary reason for implementing routers is to
localize broadcast traffic. You do, however, want clients to be able
to reach the appropriate server(s). You use helper addresses for
this purpose.
Helper commands change broadcast
addresses to a unicast address (an address of a single device on the
network) so that the broadcast message can be routed to a specific
destination, rather than everywhere. It is important to note that
every broadcast (as specified or defaulted in the configuration you
will see later) is sent to all helpers, whether or not the helper
will actually be able to help for a certain port.
Consider the design shown in the main
figure. The client boots up and sends a broadcast to
locate the server. The broadcast is stopped at the router. The
router then reissues the broadcast as a unicast that is directed to
the BOOTP server.
Note: Router-Broadcast Handling
is defined in RFC 1812 and provides background information on
router-broadcast handling for IPv4
Routers:
Forwarding of Link Layer
Broadcasts
A router must not forward any packet
that the router received as a Link Layer broadcast, unless it
is directed to an IP Multicast address. In this latter case, one
would presume that Link Layer broadcast was used due to the lack of
an effective multicast service. A router must not forward any packet
which the router received as a Link Layer multicast unless the
packet's destination address is an IP multicast address. A router
should silently discard a packet that is received via a Link Layer
broadcast but does not specify an IP multicast or IP broadcast
destination address. When a router sends a packet as a Link Layer
broadcast, the IP destination address must be a legal IP
broadcast or IP multicast address. See the RFC (1812) for the
complete text.
Helper Addressing
What if a client needs to reach a server and does not know the
server's address? In these situations, the client broadcasts to find
the server. If a router separates the client and server, the
broadcast will not get through. Helper addresses facilitate
connectivity by forwarding these broadcasts directly to the target
server.
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Lab
Activity |
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In
this lab, you will learn how to use the IP
Helper Address command to pass broadcasts
from a network. |
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