Broadcast

[kuldeeprawat]

Hi,

I am new to this question and answer solutions.

I wanted to know that we always says router/L3 doesn't supports broadcasts,

it drops the all 1's packets.

Whereas when I come to the real scenario whenever broadcast storm happens it

affects the whole network. So what's the benefit of the router/L3 Switch.

Another thing incase of RIP and IGRP both sends their updates via broadcast

packets. It means router does support broadcast........

Tell me if I am wrong I have a big confusion regarding this.......

Please let me know.

[scottmac]

By default, routers do not pass broadcasts. Routers define the endpoints of a brodacast domain.

Because some networking devices / processes relay on a broadcast to propagate information (for example, DHCP), Cisco provides an "IP Helper" that can be enabled on an interface to convert the broadcast into a unicast aimed at the required server (IP Helper xx.xx.xx.xx would forward the broadcast to "xx.xx.xx.xx" as a noral unicast - so it will pass through any subsequent routers).

Routing protocols like RIP and IGRP do use broadcasts (RIP V2 uses multicasts as well) .... but they only need to send the information to the neighbor router(s). The neighbor router will accept that information, add it to its own routing table (if appropriate) then send the entire table onward to the next neighbor... so the information doesn't really pass through the router, it gets added to the local pile of routes then sent on.

So routers do not normally pass broadcasts or multicasts, but can be enabled to do so if there is some need (like DHCP, Netbeui, etc.).

Good Luck in your studies

Scott

[kuldeeprawat]

Thanks a lot Scott for the answer.

So it means routers doesn't forward the broadcast by default. IF we enable ip helper then it send the broadcast.

And incase in RIP router itself generate the broadcast and neighbors receives it.

But if the broadcast packet is coming from the local lan then it will drop the packet.

Tell me if i am wrong till now.....

Another last thing...

Incase of broadcast storm the broadcast packets sends in the entire domain. Then why Layer3 device or router doesn't stop these packets????????

Thanks a lot and waiting for this reply....

[scottmac]

You have most of it right...

IP Helper only forwards specific types of boradcasts (broadcasts from DHCP to get addresses, for example). By default, IP Helper will forward (I forget) something like eight specific broadcast types. Any other broadcasts are ignored and blocked from traversing the router to the next segment.

You can narrow down the types of broadcasts passed by DHCP as well (it may be a security issue).

Also, the broadcast is not forwwarded as a broadcast, it's forwarded as a unicast to a specific destination (the ip address listed in the IP helper command). You are also allowed multiple IP Helper addresses (you can have one IPH do only DHCP, one IPH forward NetBeui, etc).

AS you mention, RIP will generate broadcasts with route information. The neighboring router will receive it ... and it stops right there. The neighbor router will generate it's own route table info and broadcast it to the next neighbor. So, it's not passing a broadcast, it receives one from interface A, then generates another one out interface B.

It will also generate a broadcast back out interface A, but it not include the routes it learned from the router at the other end of the A connection.

Any broadcast not handled by IP Helper or a routing protocol will be blocked and dropped ... including packets contained in a broadcast storm. If the broadcast storm consists of packets of a type that IP Helper is configured to forward, then the broadcasts are sent to the specific host as unicasts (lots of 'em).

Even though the router is blocking broadcasts, it will still take processor time and memory to handle them ....they are still a load on the router.

The broadcast storm is still propagated on the LAN segment - switches (by default) will forward / flood broadcasts and multicasts. The router can't take the broadcasts off the local segment, it can only stop them from being propagated to the next segments.

Some (commercial-grade) switches will have processes for broadcast storm control, and permit you to set the thresholds for "normal" broadcast traffic versus "excessive" broadcast traffic.

Good Luck

Scott

[kuldeeprawat]

Once Again thanks for your valuable input

hmm got you scott you are right.

I am using 2948 and 3548 Layer2 Switches, where i have the option to limit the broadcast storm, but that is possible only when i access them through web browser through command line there is no command comes to set the threshold.

OK If i want to set the threshold through http then what is recommended by cisco as normal and excessive.

Thanks Scott

[irshad.saifi]

Hi Scott,

I am trying to enable broadcast control in my network.

I am using layer2 2950 and 3550 switches.

someone please tell me what is the ideal packets per second must be set on a particular interface.

by default it's rising 500 and falling 250.

When i set it to default the ports were automaticalls shuts....

Need help from Cisco...

Thanks

Irshad