From: Rick Burts (burts@xxxxxxxxxxxxxx)
Date: Sun Sep 09 2001 - 08:26:50 GMT-3
I think there is some confusion about what is usually termed "directed
broadcast" (and some about subnet-zero). Directed broadcast is a packet
to all stations (broadcast) on a particular subnet (for example
172.16.1.255 would be broadcast to all stations on subnet 1). Directed
broadcast was supported by default up to release 12.0 where the default
behavior changed. In current versions of code directed broadcast is
disabled by default and can be turned on if desired.
Your original question involved 172.16.255.255. This might look like
directed broadcast but it is not. 172.16.255.255 is an attempt to go to
every station on every subnet of the network and this is not supported at
all. Directed broadcast is sent to a particular subnet not to all
subnets.
Rick
Rick Burts, CCSI CCIE 4615 burts@mentortech.com
Mentor Technologies 240-568-6500 ext 6652
133 National Business Parkway 240-568-6515 fax
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On Wed, 5 Sep 2001, Jason Luo wrote:
> hi, group and Lionel
>
> thanks, Lionel.
>
> but i have the following topology to test the result.
>
>
> 172.16.x.x
> _____
> ____
> |
> |
> 1.1/24*(R1)*3.2/24-------------3.1/24*(R3)*2.2/24
> |
> /
> |
> /
> |
> /
> 1.2/24*(R2)*2.1/24 ---------------------------- /
> |
> -----
>
> All 3 routers are connected with serial line(back-to-back).
> No dynamic routing protocols configured and each router has 2 "C" routes.
> I used the "debug ip icmp" on every router.
> and I lauched the "ping 172.16.255.255" on each router.
> reply only can be seen on one of the 2 directly connected lines.
> If i ping with the source address of the ethernet interface(up-up), the is
> no any response.
>
> What is wrong with me??
> thanks so much again.
> Good luck
>
> jason
>
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
> Lionel Gourvitch
> Sent: 2001?9?5? 17:20
> To: Jason Luo
> Cc: ccielab@groupstudy.com
> Subject: Re: ping 172.16.255.255
>
>
> if you ping 172.16.255.255 and all your router have "ip subnet-zero"
> configured then all the devices on the "last subnet"( that contains the
> broadcast address 172.16.255.255) of 172.16.0.0 will do icmp reply. if "
> ip subnet-zero" is not configured then all the devices on 172.16.0.0 will
> do icmp reply.
>
> Hope it helps
>
> Lionel
>
> At 04:40 PM 9/5/2001 +0800, Jason Luo wrote:
> >Hi, group
> >
> >I have a question about the directed broadcast.
> >There is a network using various sub-network from the 172.16.0.0.
> >The length of subnet mask is various. (24 for lan and 30 for p-t-p)
> >Use "ping 172.16.255.255", what will happen??
> >Use "debug ip icmp" on every router, what can be seen?
> >Which switches should be opened or closed??
> >
> >Thanks alot!!!
> >
> >Jason
> >**Please read:http://www.groupstudy.com/list/posting.html
> **Please read:http://www.groupstudy.com/list/posting.html
> **Please read:http://www.groupstudy.com/list/posting.html
**Please read:http://www.groupstudy.com/list/posting.html
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