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no ip directed-broadcast
- Thread starter ChrisAC
- Start date
Hope this helps.
Broadcasting in the Internet
A broadcast is a data packet destined for all hosts on a particular physical network. Network hosts recognize broadcasts by special addresses. This section describes the meaning and use of Internet broadcast addresses. For detailed discussions of broadcast issues in general, see RFC 919, "Broadcasting Internet Datagrams," and RFC 922, "Broadcasting Internet Datagrams in the Presence of Subnets." The router support for Internet broadcasts generally complies with RFC 919 and RFC 922; however, the router does not support multisubnet broadcasts as defined in RFC 922.
The current standard for an Internet broadcast address requires that the host portion of the address consist of all ones. If the network portion of the broadcast address is also all ones, the broadcast applies to the local network only. If the network portion of the broadcast address is not all ones, the broadcast applies to the network or subnet specified.
Cisco routers support two kinds of broadcasting: directed broadcasting and flooding. A directed broadcast is a packet sent to a specific network or series of networks, while a flooded broadcast packet is sent to every network, as shown in . The packet that is incoming from interface E0 is flooded to interfaces E1, E2 and Serial 0. A directed-broadcast address includes the network or subnet fields.
Flooded Broadcast
For example, if the network address is 128.1.0.0, then the address 128.1.255.255 indicates all hosts on network 128.1.0.0. This would be a directed broadcast. If network 128.1.0.0 has a subnet mask of 255.255.255.0 (the third octet is the subnet field), then the address 128.1.5.255 specifies all hosts on subnet 5 of network 128.1.0.0, another directed broadcast.
The no ip directed-broadcast interface subcommand disables forwarding of directed broadcasts on the interface. The full syntax of this command follows.
ip directed-broadcast
no ip directed-broadcast
The default is to forward directed broadcasts. You disable the default condition with the no ip directed broadcast, and re-enable forwarding of directed broadcasts with the ip directed
broadcast command.
Internet Broadcast Addresses
The router supports Internet broadcasts on both local and wide area networks. There are at least four popular standard ways of indicating an Internet broadcast address. You can
configure a router host to generate any form of Internet broadcast address. The router can also receive and understand any form of Internet broadcast address. By default, a router uses
all ones for both the network and host portions of the Internet broadcast address (255.255.255.255). You can change the Internet broadcast address by using the ip broadcast-address
interface subcommand. Following is the full command syntax:
ip broadcast-address [address]
no ip broadcast-address [address]
The argument address is the desired IP broadcast address for a network. If a broadcast address is not specified, the system defaults to a broadcast address of all ones or
255.255.255.255.
Use the no ip broadcast-address command to remove the broadcast address.
To change from another broadcast address to the default broadcast address of 255.255.255.255, you must enter the following command:
ip broadcast-address 255.255.255.255
You cannot use a command in the format no ip broadcast-address x.x.x.x (where x.x.x.x is not 255.255.255.255) to return to the default broadcast address.
If the router does not have nonvolatile memory, and you want the specify the broadcast address to use before it has its configuration, you can change the Internet broadcast address by setting jumpers in the processor configuration register. Setting bit 10 causes the router to use all zeros. Bit 10 interacts with bit 14, which controls the network and subnet portions of the broadcast address. Setting bit 14 causes the router to include the network and subnet portions of its address in the broadcast address
Broadcasting in the Internet
A broadcast is a data packet destined for all hosts on a particular physical network. Network hosts recognize broadcasts by special addresses. This section describes the meaning and use of Internet broadcast addresses. For detailed discussions of broadcast issues in general, see RFC 919, "Broadcasting Internet Datagrams," and RFC 922, "Broadcasting Internet Datagrams in the Presence of Subnets." The router support for Internet broadcasts generally complies with RFC 919 and RFC 922; however, the router does not support multisubnet broadcasts as defined in RFC 922.
The current standard for an Internet broadcast address requires that the host portion of the address consist of all ones. If the network portion of the broadcast address is also all ones, the broadcast applies to the local network only. If the network portion of the broadcast address is not all ones, the broadcast applies to the network or subnet specified.
Cisco routers support two kinds of broadcasting: directed broadcasting and flooding. A directed broadcast is a packet sent to a specific network or series of networks, while a flooded broadcast packet is sent to every network, as shown in . The packet that is incoming from interface E0 is flooded to interfaces E1, E2 and Serial 0. A directed-broadcast address includes the network or subnet fields.
Flooded Broadcast
For example, if the network address is 128.1.0.0, then the address 128.1.255.255 indicates all hosts on network 128.1.0.0. This would be a directed broadcast. If network 128.1.0.0 has a subnet mask of 255.255.255.0 (the third octet is the subnet field), then the address 128.1.5.255 specifies all hosts on subnet 5 of network 128.1.0.0, another directed broadcast.
The no ip directed-broadcast interface subcommand disables forwarding of directed broadcasts on the interface. The full syntax of this command follows.
ip directed-broadcast
no ip directed-broadcast
The default is to forward directed broadcasts. You disable the default condition with the no ip directed broadcast, and re-enable forwarding of directed broadcasts with the ip directed
broadcast command.
Internet Broadcast Addresses
The router supports Internet broadcasts on both local and wide area networks. There are at least four popular standard ways of indicating an Internet broadcast address. You can
configure a router host to generate any form of Internet broadcast address. The router can also receive and understand any form of Internet broadcast address. By default, a router uses
all ones for both the network and host portions of the Internet broadcast address (255.255.255.255). You can change the Internet broadcast address by using the ip broadcast-address
interface subcommand. Following is the full command syntax:
ip broadcast-address [address]
no ip broadcast-address [address]
The argument address is the desired IP broadcast address for a network. If a broadcast address is not specified, the system defaults to a broadcast address of all ones or
255.255.255.255.
Use the no ip broadcast-address command to remove the broadcast address.
To change from another broadcast address to the default broadcast address of 255.255.255.255, you must enter the following command:
ip broadcast-address 255.255.255.255
You cannot use a command in the format no ip broadcast-address x.x.x.x (where x.x.x.x is not 255.255.255.255) to return to the default broadcast address.
If the router does not have nonvolatile memory, and you want the specify the broadcast address to use before it has its configuration, you can change the Internet broadcast address by setting jumpers in the processor configuration register. Setting bit 10 causes the router to use all zeros. Bit 10 interacts with bit 14, which controls the network and subnet portions of the broadcast address. Setting bit 14 causes the router to include the network and subnet portions of its address in the broadcast address
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