At 01:16 AM 4/13/01, Infotech wrote:
>Dear Group,
>
>I have a small query:
>
>Is it advisable to disable broadcast on all the ports of switches. I have 3
>catalyst series 6509/5500/5505 in my network running all the WINNT servers
>4.0
>with all win98/win95 as clients. With the help of CWSI I have observed that
>many a times the broadcast increases very much & I experience lot of
response
>delay in my network.
>can I use the command:
>
>switch> (enable)# set port broadcast 0%
Setting the threshold to zero would be a misuse of the Cisco broadcast
suppression feature. I'm not sure Cisco even lets you set it to zero? It
would also cause network problems since you need broadcasts for such
important functions as dynamic address assignment, address and name
resolution, service location, and service advertisement. ARP, DHCP, Cisco
Discovery Protocol, RIP, EIGRP, OSPF, IGMP, and NetBIOS are some of the
many protocols that make use of broadcasts and multicasts.
The goal of the broadcast suppression feature is to prevent broadcast
storms, not broadcasts in general. A broadcast storm occurs when bugs in a
protocol-stack implementation or in a network configuration cause a station
to send hundreds or thousands of broadcasts per second. In the worst case,
the broadcasts from one station result in other stations also sending
broadcasts, much like a storm that builds upon itself.
Broadcasts can cause problems because they interrupt every device in the
broadcast domain, causing a CPU interrupt and requiring processing. On slow
CPUs, broadcast storms can be a serious problem. Broadcast storms can wreak
havoc on 100-Mbps Ethernet LANs with slow computers, because misbehaving
devices have an opportunity to send broadcasts really quickly at 100 Mbps.
You can configure broadcast suppression on a switch port to keep excessive
broadcasts from causing performance degradation on devices or LANs
reachable from that port. Cisco implements broadcast suppression in
software or hardware, depending on the switch platform. Software broadcast
suppression uses a packet-based method. Hardware broadcast suppression uses
a bandwidth-based method.
When a packet-based method is used to measure broadcast activity, the
threshold parameter is the number of broadcast or multicast packets
received over a one-second time period. When a bandwidth-based method is
used, the threshold parameter is the percentage of total available
bandwidth used by broadcasts or multicasts. In either case, if the
threshold is reached, the switch port cuts off broadcast and multicast
packets for the rest of that second. Because packet sizes vary,
bandwidth-based measurement is more accurate and more effective than
packet-based measurement.
As Howard often points out, broadcast packets are usually short and don't
use much bandwidth. In addition, most normal applications don't broadcast
thousands of times in a second. (For a station to use up most of 100 Mbps
with 64-byte packets, it would need to send approximately 200,000 times per
second, which is obviously abnormal.) So, when broadcasts start to use a
lot of your available bandwidth in a one-second interval, this probably
indicates a serious problem.
So, to make a long story short, a proper setting for the threshold
parameter would probably be more like 75%. The value depends on your actual
applications and traffic types.
Priscilla
>Will this put any impact on my network like nodes won't be able to talk to
>server ....or problem in finding the servers. I have also created VLAN's in
>my
>network to limit the broadcasts. Should I doit on all the ports...
>
>many thanks in advance
>HP
>
>
>
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________________________
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
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