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Question on Broadcasts
typesh
Member Posts: 168
in CCNA & CCENT
Hey everyone, I needed some helping figuring out something to do with broadcasts.
Here is what I am thinking of:
A router has 2 interfaces: Fa1 and Fa2.
Out Fa1 is the subnet 192.168.1.0/20
Out Fa2 is the subnet 192.168.1.16/20
Suppose PC1 is connected out Fa1 and wants to send a message to all the nodes out Fa2. The broadcast address out Fa2 is 192.168.1.31. However, Routers drop broadcasts. So how can PC1 send a message that hits all nodes on Fa2 if the router will drop messages addressed to 192.168.1.31 (since it is a broadcast address)?
Thank you.
Edit:
Also, I just thought of something else. If PC1 wanted to send a message to reach all nodes out Fa2, it would have to ARP for the MAC Address. But how can it ARP when the message is intended for multiple recipients?
Here is what I am thinking of:
A router has 2 interfaces: Fa1 and Fa2.
Out Fa1 is the subnet 192.168.1.0/20
Out Fa2 is the subnet 192.168.1.16/20
Suppose PC1 is connected out Fa1 and wants to send a message to all the nodes out Fa2. The broadcast address out Fa2 is 192.168.1.31. However, Routers drop broadcasts. So how can PC1 send a message that hits all nodes on Fa2 if the router will drop messages addressed to 192.168.1.31 (since it is a broadcast address)?
Thank you.
Edit:
Also, I just thought of something else. If PC1 wanted to send a message to reach all nodes out Fa2, it would have to ARP for the MAC Address. But how can it ARP when the message is intended for multiple recipients?
Comments
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OptionsColbyG Member Posts: 1,264Each interface on a router is a broadcast domain. Broadcasts won't come in one interface of a router and leave another interface. You also can't have multiple routed interfaces in the same subnet.
Your networks are a little cooky btw. -
Optionstypesh Member Posts: 168You also can't have multiple routed interfaces in the same subnet.
Hey,
Not sure I understand...
These are different subnets ... the .0 subnet, and the .16 subnet. So for the purposes of routing, aren't they considered 2 different networks? -
OptionsColbyG Member Posts: 1,264Hey,
Not sure I understand...
These are different subnets ... the .0 subnet, and the .16 subnet. So for the purposes of routing, aren't they considered 2 different networks?
Oh, so then it looks like you meant to use a /28 mask, not a /20. In your post, those subnets are definitely the same network.
Broadcasts will not traverse a router. That should answer your question. -
Optionsnetworker050184 Mod Posts: 11,962 ModHey,
Not sure I understand...
These are different subnets ... the .0 subnet, and the .16 subnet. So for the purposes of routing, aren't they considered 2 different networks?
No they are not two different subnets if you are using a /20 mask. I think you are looking for a /28.
Damn you Colby!An expert is a man who has made all the mistakes which can be made. -
Optionsdynamik Banned Posts: 12,312 ■■■■■■■■■□Your subnet mask should be /28 though.
Edit: epic response time fellas! -
Optionstypesh Member Posts: 168Ahhhh! Can't believe I did that!
Yes, it should be a /28 mask!!
My apologies.
Lol thanks for the split-second responses!Oh, so then it looks like you meant to use a /28 mask, not a /20. In your post, those subnets are definitely the same network.
Broadcasts will not traverse a router. That should answer your question.
Thanks! So then considering we are using a /28 mask, how can PC1 send a message to all the nodes out Fa2 since the broadcast won't traverse the router... -
Options1number9 Member Posts: 13 ■□□□□□□□□□actually i think there is a way to set routers to forward broadcasts. i dont know how but i think i remember my Cisco 4 teacher saying it can be done.
nevermind I was probably thinking of forwarding DHCP broadcasts. -
OptionsColbyG Member Posts: 1,264actually i think there is a way to set routers to forward broadcasts. i dont know how but i think i remember my Cisco 4 teacher saying it can be done.
nevermind I was probably thinking of forwarding DHCP broadcasts.
You can forward some broadcasts with the ip-helper command. -
Optionstypesh Member Posts: 168Hey thanks for the reply. I figured out the ARP part. Stupid mistake. If sending a message to someone outside your subnet, then the PC would ARP for the default-gateway's MAC.
So if PC1 wanted a message to get to all nodes on Fa2, the the ip helper command would be the only way to allow this? -
Optionsnetworker050184 Mod Posts: 11,962 ModHey thanks for the reply. I figured out the ARP part. Stupid mistake. If sending a message to someone outside your subnet, then the PC would ARP for the default-gateway's MAC.
So if PC1 wanted a message to get to all nodes on Fa2, the the ip helper command would be the only way to allow this?
You probably are looking for something along the lines of multicast. Is there a certain scenario you are trying to figure out or is this just a general questions? We might be able to help you out with something specific.An expert is a man who has made all the mistakes which can be made. -
Optionsmikej412 Member Posts: 10,086 ■■■■■■■■■■If your router is configured to allow ip directed broadcasts, you may be able to send the message to the IP subnet broadcast address.:mike: Cisco Certifications -- Collect the Entire Set!
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Optionstypesh Member Posts: 168Hey thanks for the responses.
networker050184:
It's not really something specific I am working on... I am just thinking about scenarios-- walking through routing logic/switching logic in my mind. I was thinking about LAN broadcasts where a host sends out a broadcast which reaches everyone on its own subnet. Then I started thinking "what if this host wanted to reach everyone on a different subnet". I couldn't reason it out in my head. Maybe this is not a practical situation in real life. I haven't come across any examples of it in my books (that I can remember). -
OptionsHeero Member Posts: 4862 options
1. Put the PC on the other subnet.
2. Implement a multicast configuration. Multicast was developed for just this kind of situation.
The ip-helper address is able to take a broadcast and forward it as a unicast to a specific host on another subnet, not really viable for a large number of recipeints.