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broadcast address

Under what circumstances would you have to configure a specific broadcast address rather than using the default . On some of our equipment a specific broadcast address was put in XXX.XXX.XXX.255 rather than using the default of . what would be the implications of this if any on performance or other problems it might cause . Would you have to configure a specific broadcast address when using vlsm or just use the default ?


Re: broadcast address

Currently, the most popular way, and the default, is a broadcast address consisting of all ones (, although the IOS can be configured to generate any form of IP broadcast address.

To change the broadcast address you use the command "ip broadcast-address". The destination address can be set to any desired address. Thus, the destination address may change as the datagram propagates through the network. After a decision has been made to send the datagram out on an interface, the datagram is handed to the normal IP output routines and is, therefore, subject to access lists, if they are present on the output interface.

Directed broadcasts only apply to a specific subnet. A directed broadcast address includes the network or subnet fields. They can also be forwarded past routers with the interface command ip directed-broadcast. By default, IP directed broadcasts are dropped; they are not forwarded. Dropping IP directed broadcasts makes routers less susceptible to denial-of-service attacks.

Broadcasts serve a necessary role of passing the data to a large number of hosts, but they also have the negative side affect of consuming the CPU of all hosts as the packets must be processed. Generally broadcasts are seen as a necessary evil, and efforts are made to limit them. Multicasting is often the prefered more efficient method. It is rare to change the default broadcast, and network designs should limit them. Protocols that rely heavily on them are seen as inefficient and alternative protocols are sought.

Hope it helps.



Re: broadcast address

Thanks Steve , I'm just wondering why we seem to have it both ways ,some subnets do not have a broadcast address configured thus it is and some seem to have one configured XXX.XXX.XXX.255 . I know when you configure one it can screw up the DHCP function , we ran into this awhile back and took us a while to figure it out . thanks for your explanation .

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