GroupStudy.com GroupStudy.com - A virtual community of network engineers
 Home  BookStore  StudyNotes  Links  Archives  StudyRooms  HelpWanted  Discounts  Login
RE: ip broadcast-address [7:89919] posted 06/22/2004
[Chronological Index] [Thread Index] [Top] [Date Prev][Date Next] [Thread Prev][Thread Next]


================
You can use a broadcast address with ip helper. It's mostly effective if the
helper address is reachable via a local interface. If you expect the packet
to get routed across an internetwork and be forwarded as a broadcast onto
its destination network, there could be issues because other routers might
not have the same understanding of what broadcast means (could have
different subnet masks) and "no ip directed-broadcast" could be configured
(it's the default these day). But I digress.

Back to the main point, there are sometimes good reasons for forwarding an
incoming broadcast as a broadcast onto other local interfaces and you can do
this with ip helper-address. Provides some redundancy.

What address is being used for the helper address in your case? Is it a
unicast?
================

Yes, I agree about the redundancy in case there are more than one DHCP,
DC... etc.
but I think configuring multiple ip helper address works just as well.
BTW, the address being used for the helper address is a host IP, 
so the traffic will be unicast. 

===============
It's not more traffic. It's still just one packet. Sending as a broadcast
increases your chances that someone will respond. The downside is that it
bothers every interface in the broadcast domain.
===============

Hmm... my understanding of layer 3 broadcast traffic, 
(though I highly suspect that it is incomplete),
is that it is the packet that is destined to every IP address in that
segment.
And as you said all hosts(or interfaces configured with IP) 
will have to process(or drop) this packet.
How different is this from saying 'more traffic'?
I DO apologize if this question is simply too basic...

================
Perhaps the orignal configurer just wanted things nice and clean with
packets going to 172.16.255.255 (or whatever) instead of 255.255.255.255?
Maybe he was paranoid that the 255.255.255.255 would really go everywhere
(which they wouldn't.) Mabye there were some old servers that wouldn't
listen to packets to 255.255.255.255 but only to packets for the broadcast
address for the local LAN?

You could try changing it, but if it ain't broke, don't fix it. 
===============

I think you may be right.  
And I might just take your advice as well.  ;-)

Sean

P.S.  BTW, you probably won't remember, but I just wanted to thank you 
for a Qos paper you recommended to me a while back.
It helped me out alot and I never had a chance to thank you properly.





-----Original Message-----
From: Priscilla Oppenheimer [mailto:nobody@xxxxxxxxxxxxxx]
Sent: Tuesday, June 22, 2004 11:39 AM
To: cisco@xxxxxxxxxxxxxx
Subject: RE: ip broadcast-address [7:89919]


Kim, Sean (MGT) wrote:
> 
> 
> =============
> > I have a few routers that has this 'ip broadcast-address'
> > configured on ethernet and fastethernet interfaces.
> 
> Why? :-) You might want to find out why by talking to whoever
> it was that
> configured this.
> =============
> 
> I would love to... unfortunately this isn't an option...
> I hear the person who configured this parted with this company
> on bad terms...
> 
> =============
> > but it won't let the broadcast on any other interface... since
> > it is a router.
> 
> No, it would not forward the broadcast, as you say.
> 
> However, if you were using ip helper-address, configuring this
> command might
> have some effect if you want to forward to a broadcast address.
> Try it and
> see (in a lab! :-) You should easily be able to see the effect
> with Ethereal.
> ============
> 
> Yes, as one would expect, ip helper-address is configured for
> DHCP to work.
> I still don't seem to see the usage of this 'ip
> broadcast-address'
> My understanding is that ip helper-address turns broadcast into
> unicast

You can use a broadcast address with ip helper. It's mostly effective if the
helper address is reachable via a local interface. If you expect the packet
to get routed across an internetwork and be forwarded as a broadcast onto
its destination network, there could be issues because other routers might
not have the same understanding of what broadcast means (could have
different subnet masks) and "no ip directed-broadcast" could be configured
(it's the default these day). But I digress.

Back to the main point, there are sometimes good reasons for forwarding an
incoming broadcast as a broadcast onto other local interfaces and you can do
this with ip helper-address. Provides some redundancy.

What address is being used for the helper address in your case? Is it a
unicast?

> and route it to the ip address specified in the command.
> How would this 'ip broadcast-address' command affect this
> process?
> And why would someone want to forward the traffic to broadcast
> address,
> when he/she can make the network traffic a lot lighter with the
> unicast?

It's not more traffic. It's still just one packet. Sending as a broadcast
increases your chances that someone will respond. The downside is that it
bothers every interface in the broadcast domain.


> Besides, 'ip broadcast-address' command on all the interfaces
> are configured
> with the broadcast address of the network that are attached to
> the interface itself...

Perhaps the orignal configurer just wanted things nice and clean with
packets going to 172.16.255.255 (or whatever) instead of 255.255.255.255?
Maybe he was paranoid that the 255.255.255.255 would really go everywhere
(which they wouldn't.) Mabye there were some old servers that wouldn't
listen to packets to 255.255.255.255 but only to packets for the broadcast
address for the local LAN?

You could try changing it, but if it ain't broke, don't fix it. 

Priscilla


> 
> Sean
> 
> 
> 
> -----Original Message-----
> From: Priscilla Oppenheimer [mailto:nobody@xxxxxxxxxxxxxx]
> Sent: Tuesday, June 22, 2004 3:46 AM
> To: cisco@xxxxxxxxxxxxxx
> Subject: RE: ip broadcast-address [7:89919]
> 
> 
> Kim, Sean (MGT) wrote:
> > 
> > Hello,
> > 
> > I am having trouble understanding the neccessity of this
> > command.
> > 
> > ip broadcast-address
> 
> It's probably not necessary on modern networks. Back in the 80s
> and early
> 90s, there was some confusion about whether broadcast meant
> 0.0.0.0 or
> 255.255.255.255. Some operating systems used 0.0.0.0 because
> they misread
> the standards that say that zeros are used to mean "the
> network." The
> standards meant for zeros to be used in documentation, routing
> tables, and
> so on, but not as a destination address in a packet to mean all
> hosts.  Ones
> should be used for that.
> 
> There's also some confusion about whether broadcasts should be
> sent as all
> ones (255.255.255.255) or just all ones in the host field
> (172.16.255.255/16). It turns out it doesn't really matter
> because modern
> routers wouldn't forward 255.255.255.255 everywhere anyway.
> 
> 
> > 
> > I have a few routers that has this 'ip broadcast-address'
> > configured on ethernet and fastethernet interfaces.
> 
> Why? :-) You might want to find out why by talking to whoever
> it was that
> configured this.
> 
> > It only seems like to be specifying the broadcast address for
> > the particular interface.
> 
> Yes, for that interface, for traffic sourced by the router.
> 
> > But looking at some documents (which didn't have whole a lot
> of
> > info),
> > I found out that the default is 255.255.255.255.
> > If the default is 255.255.255.255, when broadcast packet
> > reaches the interface,
> > my guess is that the router will flood the network connected
> to
> > that interface,
> 
> No, a router wouldn't flood. When a packet for 255.255.255.255
> arrives from
> a LAN into the router's LAN interface, the router has to
> process it because
> it's a broadcast, but it doesn't flood it. It's already been
> "flooded" by
> the sender, in a sense, and by any switches in the broadcast
> domain. Every
> device in the broadcast domain must inspect the packet,
> including the
> router, but only switches would flood it.
> 
> > but it won't let the broadcast on any other interface... since
> > it is a router.
> 
> No, it would not forward the broadcast, as you say.
> 
> However, if you were using ip helper-address, configuring this
> command might
> have some effect if you want to forward to a broadcast address.
> Try it and
> see (in a lab! :-) You should easily be able to see the effect
> with Ethereal.
> 
> Priscilla Oppenheimer
> http://www.topdownbook.com/
> 
> 
> > 
> > Has anybody configured this and actually have it do something
> > meaningful?
> > 
> > Thanks for your help.
> > Sean
> > 
> > 
> >
> **********************************************************************
> > The information in this e-mail is confidential and may be
> > legally privileged. It is solely for the intended addressee.
> > Access to this e-mail by anyone else is unauthorized. If you
> > are not the intended recipient, any disclosure, copying,
> > distribution or any action taken or omitted to be taken in
> > reliance on it is prohibited and may be unlawful. KPMG does
> not
> > guarantee, warrant or represent that this email or any
> > attachments will be error free, virus free, nor that the
> > transmission will not be delayed, intercepted or interfered.
> **********************************************************************
> The information in this e-mail is confidential and may be
> legally privileged. It is solely for the intended addressee.
> Access to this e-mail by anyone else is unauthorized. If you
> are not the intended recipient, any disclosure, copying,
> distribution or any action taken or omitted to be taken in
> reliance on it is prohibited and may be unlawful. KPMG does not
> guarantee, warrant or represent that this email or any
> attachments will be error free, virus free, nor that the
> transmission will not be delayed, intercepted or interfered.
**********************************************************************
The information in this e-mail is confidential and may be legally
privileged. It is solely for the intended addressee. Access to this e-mail
by anyone else is unauthorized. If you are not the intended recipient, any
disclosure, copying, distribution or any action taken or omitted to be taken
in reliance on it is prohibited and may be unlawful. KPMG does not
guarantee, warrant or represent that this email or any attachments will be
error free, virus free, nor that the transmission will not be delayed,
intercepted or interfered.




Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=89982&t=89919
--------------------------------------------------
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html