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It is important to
consider how you want to get the broadcast, in a controlled
way, to the appropriate server(s). Such considerations depend on the
location of the server(s). In practice, there are several ways that
server location is implemented (see Figure ):
- A single server on a single remote
medium
Such a medium may be directly
connected to the router that blocks the broadcast, or might be
several routing hops away. In any case, the all-ones broadcast needs
to be handled at the first router it encounters and sent to the
server.
- Multiple servers on a single
remote medium, sometimes called a "server farm"
Different kinds of servers (for
example, DNS and TFTP for Cisco AutoInstall) could exist on the same
medium. Or perhaps redundant servers of the same type are installed
on the same medium. In either case, you want a directed broadcast on
the server farm subnet so the multiple devices can see it.
- Multiple servers on multiple
remote media
Here, a secondary DNS server could
exist on one subnet and the primary DNS server on another subnet.
For fault tolerance, client requests need to reach both servers.
IP Helper Address Commands
Two commands are used to
configure helper addresses:
- ip helper-address
- ip forward-protocol
The ip helper-address Command
Use the ip helper-address
command to configure an interface where broadcasts are expected or
can be received. The specified address can be the unicast address of
a remote server or a directed broadcast address.
ip helper-address address
If an ip helper-address is
defined, forwarding for eight default UDP ports is enabled
automatically. The default ports are TFTP (69), DNS (53), Time (37),
NetBIOS name service (137), NetBIOS datagram service (138), BOOTP/DHCP
server (67), BOOTP/DHCP client (68), and TACACS (49).
These same eight ports are
automatically forwarded if you define an ip helper-address
and ip forward-protocol udp.
The
ip forward-protocol Command
Use the ip forward-protocol
command to specify which type of broadcast packet is forwarded.
ip forward-protocol {udp [port] | nd
| sdns}

To forward only one UDP port (whether
a default-forwarded port, another UDP port, or a custom port), you
must use ip forward-protocol udp [port] for the port(s) you
want to forward and then specify no ip forward-protocol udp
[port] for the default ports you do not want forwarded.
There really is no easy way to
forward all UDP broadcasts. Essentially, you would need to specify
all the UDP ports in the ip forward-protocol command.
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