Velocity Reviews - Computer Hardware Reviews

Velocity Reviews > Newsgroups > Computing > Cisco > Arp Broadcast

Reply
Thread Tools

Arp Broadcast

 
 
Martin Leduc
Guest
Posts: n/a
 
      01-08-2008
Hi,

I would like to understand.

If I have a X.0.0.0/8 network mask and a station send a packet to the
broadcast X.255.255.255, the IP Protocol will send the Packets to all the
16581375 ip address in the network, even if the switch or the router arp
table will only contained 100 resolved ip address?

How can I evaluate the broadcast load over a subnet like the ARP Broadcast,
DHCP Broadcast or Virus Broadcast (the most important hihihihi).

Best Regards

Martin



 
Reply With Quote
 
 
 
 
Trendkill
Guest
Posts: n/a
 
      01-08-2008
On Jan 7, 7:41 pm, "Martin Leduc" <(E-Mail Removed)> wrote:
> Hi,
>
> I would like to understand.
>
> If I have a X.0.0.0/8 network mask and a station send a packet to the
> broadcast X.255.255.255, the IP Protocol will send the Packets to all the
> 16581375 ip address in the network, even if the switch or the router arp
> table will only contained 100 resolved ip address?
>
> How can I evaluate the broadcast load over a subnet like the ARP Broadcast,
> DHCP Broadcast or Virus Broadcast (the most important hihihihi).
>
> Best Regards
>
> Martin


While the arp would go to the address you have said above, a layer 3
arp would have to match the same network address on the router itself,
else it will not be propagated. In other words, if that node
broadcasts to 10.255.255.255, but the router had the network as a
10.0.0.0 255.255.255.0, then the arp wouldn't go anywhere.

Presuming the router has a matching subnet mask and address range,
then yes it will forward to that many addresses (presuming they are
up), but this is unrealistic unless you have a flat network with that
many addresses. An arp is only as big as the owning router's local
subnet, since it will not jump vlans or into other virtual/physical
interfaces. Also, an arp only goes out physical/virtual interfaces in
that same network, so unless you had that many switchports, the
question is also limited beyond your hypothetical situation.
 
Reply With Quote
 
 
 
 
Barry Margolin
Guest
Posts: n/a
 
      01-08-2008
In article <0Fzgj.4673$(E-Mail Removed)>,
"Martin Leduc" <(E-Mail Removed)> wrote:

> Hi,
>
> I would like to understand.
>
> If I have a X.0.0.0/8 network mask and a station send a packet to the
> broadcast X.255.255.255, the IP Protocol will send the Packets to all the
> 16581375 ip address in the network, even if the switch or the router arp
> table will only contained 100 resolved ip address?


Broadcasts are not sent to individual addresses. One packet is sent to
the LAN's link-level broadcast address; on Ethernet this is
ff:ff:ff:ff:ff:ff. NICs automatically recognize this address and
receive the packet as if it were addressed to them.

> How can I evaluate the broadcast load over a subnet like the ARP Broadcast,
> DHCP Broadcast or Virus Broadcast (the most important hihihihi).


Use a sniffer and filter for broadcast packets.

--
Barry Margolin, http://www.velocityreviews.com/forums/(E-Mail Removed)
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***
 
Reply With Quote
 
Thrill5
Guest
Posts: n/a
 
      01-09-2008
There are three different types of broadcasts, Layer 2 broadcasts, and two
types of layer 3 broadcasts and ARP broadcasts are layer 2. Layer 3
broadcasts are subnet broadcasts, an IP packet sent to 255.255.255.255 (and
is sent as layer 2 broadcast as well) is ONLY propagated on the local
subnet. The other type of Layer 3 broadcast is called a "directed
broadcast" which is sent to a specific subnet. A directed broadcast will be
sent with a layer 2 address of the default gateway, and it will be routed
just like any other packet until it gets to the router that has a directly
connected interface on that subnet. The router that receives a directed
broadcast will only send it out the directly connected interface if A) the
directly configured interface is configured to allow directed broadcasts
with an "ip directed-broadcast" command on that interface and B) the
interfaces subnet (as calculated by the interfaces IP address and subnet
mask) is an exact match. If both of these conditions are met, then the
packet is sent out the interface as a layer 2 broadcast.

In your example if you have an interface with an IP address of 10.0.0.1/8
and it receives a directed broadcast to 10.255.255.255, then it will send
the packet out that interface with a layer 2 destination address of
ff:ff:ff:ff:ff:ff. Only one packet is sent, not 16 million. Now if the
router has an interface with an IP address of 10.0.0.1/24 and receives a
directed broadcast to 10.255.255.255, it would either route the packet to
the next hop router if a 10.0.0.0/8 (or a default route) routing table entry
exists or drop the packet because it doesn't have a route.

To see how much broadcast traffic is being sent on an interface use the
"show interface" command, which will show you how many unicast and broadcast
packets have been sent.

I think you also need to do some research on Layer 2 and Layer 3 addressing.
In today's IP world, layer 2 addressing and how it works in conjunction with
Layer 3 protocols such as IP is not well known.


"Martin Leduc" <(E-Mail Removed)> wrote in message
news:0Fzgj.4673$(E-Mail Removed)...
> Hi,
>
> I would like to understand.
>
> If I have a X.0.0.0/8 network mask and a station send a packet to the
> broadcast X.255.255.255, the IP Protocol will send the Packets to all the
> 16581375 ip address in the network, even if the switch or the router arp
> table will only contained 100 resolved ip address?
>
> How can I evaluate the broadcast load over a subnet like the ARP
> Broadcast, DHCP Broadcast or Virus Broadcast (the most important
> hihihihi).
>
> Best Regards
>
> Martin
>
>
>



 
Reply With Quote
 
 
 
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Arp or Proxy Arp Darren Green Cisco 0 02-20-2009 09:38 PM
Why does ARP broadcast justin_ltg@yahoo.com Cisco 2 03-08-2006 09:04 PM
arp cache =?Utf-8?B?Y2IzOTk0MA==?= Wireless Networking 3 06-26-2005 01:22 AM
Loss of DNS/ARP responses from Linksys WAG54G nospam Wireless Networking 6 02-15-2005 05:30 PM



Advertisments