From: Henk de Tombe (henk.de.tombe@qi.nl)
Date: Sun May 29 2005 - 05:48:13 GMT-3
Hi,
>> But when a router or a host generate an arp or any other kind of
broadcast, which address is used when?
I've configured R8 and R14 on a ethernet segment. The ip subnet on the
ethernetsegment is 192.168.1.0/24. I've enabled debug arp and executed the
command clear arp. Output is listed below:
R8#sh arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 192.168.1.1 - 000b.be49.2ac0 ARPA FastEthernet0/0
Internet 192.168.1.2 0 000d.295f.63c1 ARPA FastEthernet0/0
R8#
R8#deb arp
ARP packet debugging is on
R8#
R8#clear arp
*Mar 1 00:35:33.152: IP ARP: sent req src 192.168.1.1 000b.be49.2ac0, dst
192.168.1.2 000d.295f.63c1 FastEthernet0/0
*Mar 1 00:35:33.152: IP ARP: sent rep src 192.168.1.1 000b.be49.2ac0, dst
192.168.1.1 ffff.ffff.ffff FastEthernet0/0
*Mar 1 00:35:33.156: IP ARP: rcvd rep src 192.168.1.2 000d.295f.63c1, dst
192.168.1.1 FastEthernet0/0
*Mar 1 00:35:33.156: IP ARP: creating entry for IP address: 192.168.1.2,
hw: 000d.295f.63c1
You can see that ffff.ffff.ffff is used for the broadcast address.
Next I've configured RIPv1 on the segment. And enabled deb ip rip to see
some updates:
R8#clear ip ro *
R8#
*Mar 1 00:55:15.265: RIP: sending request on FastEthernet0/0 to
255.255.255.255
*Mar 1 00:55:15.281: RIP: sending request on FastEthernet0/0 to
255.255.255.255
*Mar 1 00:55:17.273: RIP: sending v1 flash update to 255.255.255.255 via
FastEthernet0/0 (192.168.1.1)
The router used 255.255.255.255 as broadcast address for sending RIPv1
updates.
FastEthernet0/0 is up, line protocol is up
Internet address is 192.168.1.1/24
Broadcast address is 255.255.255.255 <<<<<<<<<<<<<
You can change the broadcast address so RIPv1 updates are sent to whatever
broadcast address you want:
R8(config-if)#
R8(config)#int fa0/0
R8(config-if)#ip broadcast-address 192.168.1.254
R8(config-if)#do sh ip int
FastEthernet0/0 is up, line protocol is up
Internet address is 192.168.1.1/24
Broadcast address is 192.168.1.254
R8#deb ip rip
RIP protocol debugging is on
R8#clear ip ro *
R8#
*Mar 1 01:00:03.598: RIP: sending request on FastEthernet0/0 to
192.168.1.254
*Mar 1 01:00:03.610: RIP: sending request on FastEthernet0/0 to
192.168.1.254
*Mar 1 01:00:05.605: RIP: sending v1 flash update to 192.168.1.254 via
FastEthernet0/0 (192.168.1.1)
Regards,
Henk
-----Oorspronkelijk bericht-----
Van: nobody@groupstudy.com [mailto:nobody@groupstudy.com] Namens Vazman
Verzonden: woensdag 25 mei 2005 03:03
Aan: ccie2be
CC: Ccie Be; ccielab@groupstudy.com
Onderwerp: Re: Broadcast address
Thanks for that info. But when a router or a host generate an arp or any
other kind of broadcast, which address is used when?
Thanks.
On 5/24/05, ccie2be <ccie2be@nyc.rr.com> wrote:
> Typically, routers don't send or forward broadcast traffic. For a
> router to do so, it's behaving in a non default manner.
>
> Various host protocols, on the hand, do use broadcasts for a variety
> of
> functions: arp, dhcp, and depending on the desktop OS and underlying
> transport protocols a whole slew of other reasons. Windows based
> hosts used to generate a lot of broadcast traffic although I don't
> know how true that still is.
>
> Off-hand, the only time I can think of when a router generates a
> directed broadcast is if it's configured with a helper-address which
> uses a directed-broadcast address parameter or if the router is
> configured to behave like a host. For example, you can configure a
> router interface as a dhcp client.
>
> A router also uses arp so that will generate a broadcast.
>
> HTH, Tim
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of Ccie Be
> Sent: Tuesday, May 24, 2005 4:43 PM
> To: ccielab@groupstudy.com
> Subject: Broadcast address
>
> Lets say a router has an ethernet interface with IP 192.168.1.1/24.
> When would it use the broadcast address 192.186.1.255 vs
> 255.255.255.255. Any good examples would be helpful.
>
> Thanks.
>
>
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