Broadcast Frames

[Zap]

The purpose of a broadcast frame is still a bit confusing to me. Under what conditions or circumstances does a computer send out a broadcast frame? What does the broadcast frame tell all of the other hosts in the collision domain?

[Ed]

A broadcast frame is one that is sent by a device with an address that will be acknowledged by all other devices on a collision domain. A broadcast is a UDP transmission meaning that it is sent without the expectation of an acknowledgement of receipt.

An example of a broadcast transmission is a DHCP Discover packet. This is where a computer is looking for the presence of a DHCP Server. It broadcasts out to the entire network because it is unaware of the actual address of the DHCP Server. In this case there would be no acknowledgement of receipt, but rather the DHCP Server would send back a separate packet to continue communications with the original computer.

I hope that helps,
Ed

[Zap]

So when the DHCP server responds, the switch will record it's MAC address in the MAC address table, correct? The aging parameter will determine how long that address will be stored and then that information will be dropped after the aging time has elapsed, correct? If a host needs to contact the DHCP server, say in a couple of hours, will it need to send out another broadcast frame?

Do you see where I am going with this? I want to understand the process, start to finish. Am I right to summarize then that a broadcast frame is sent by a host when its requires service, but the Switch will only store the MAC address of the server that is replying to the host for a limited time of say 300 seconds? And after those 300 seconds elapse, if that same host needs to contact that same DHCP server again it will send out another "request for service" in the form of another broadcast frame?


A broadcast frame is one that is sent by a device with an address that will be acknowledged by all other devices on a collision domain. A broadcast is a UDP transmission meaning that it is sent without the expectation of an acknowledgement of receipt.

An example of a broadcast transmission is a DHCP Discover packet. This is where a computer is looking for the presence of a DHCP Server. It broadcasts out to the entire network because it is unaware of the actual address of the DHCP Server. In this case there would be no acknowledgement of receipt, but rather the DHCP Server would send back a separate packet to continue communications with the original computer.

I hope that helps,
Ed

[Ed]

Zap,

You keep mentioning a "switch" in your response, but you should have probably been saying "host". Yes, a host will enter an unknown MAC address into its cache when it receives a communication from that address. It does stay in the cache for a certain aging period. On hosts running Microsoft operating systems it will stay in the cache for up to 2 minutes unless there is an additional communication at which point in stays cached for an additional 2 minutes, and continues this process for up to a maximum of 10 minutes. However, this really has nothing to do with your original question. Broadcast communications are not necessarly a "requet for service". There are many situations in which broadcast communications are used. I only used DHCP as one example.

It seems as though you are merging pieces from different networking concepts together. This is somewhat typical for many people when they are first learning about computer networking. Try to keep individual concepts separate and with time and practice it will all come together. A broadcast transmissions is simply when one host sends out a communication which will be heard by all other hosts on the same broadcast domain. (Sorry, I mistakenly said collision domain in my last reply.)

Ed

[Zap]

When Chris was talking about too many host broadcasts slowing down the Switch, I was trying to visualize the process and what the resolution would be. Perhaps in the ICND2 lessons this will become clearer.

One more question I have for you. If I were to purchase a used router and switch to use for my CCNA training program, does the age of the equipment make a great deal of difference? I'd sure hate to buy something I cannot use, or pass on to someone who is studying for their CCNA cert.

I appreciate your help with my question on broadcast frames. The amount of data one needs to undrstand and retain can be overwhelming at times.

[David Davis]

Hi Zap,
Yes, the age of the equipment will make a difference for Cisco certification studies...
However, I suspect that it won't make a huge difference for the CCNA studies.
The main difference that older equipment will cause is that you won't be able to run the latest and greatest IOS (the router or switch OS). By not having that, you may be preventing from practicing newer IOS features which could be on the test.
To me, it just depends how far you want this equipment to serve you in your cisco certifications...
If you were to buy a current router, I would likely recommend one of the 800 series routers but I would find one used on ebay instead of buying it new-
http://www.cisco.com/cisco/web/solutions/small_business/products/routers_switches/800_series_isr/index.html

I hope that helps!

-David Davis, CCIE, vExpert, VCP

[Zap]

Yes, you were very helpful, thank you. I believe I have a much better understanding of it Now. It would have been beneficial had Chris mentioned the various types of broadcasts.