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how much broadcast is too much??

Posted on 2014-10-23
Medium Priority
Last Modified: 2014-11-11
I am running HP Procurve switches on my network, have about 50 or so switches. My core is Procurve 8206 with mulitple vlans. I have a question regarding broadcast. How much is too much? I recently have voice quality problem so i set QOS on voice and will looking at all interfaces, i see high broadcasts but not sure what is high or acceptable. Is there a guide line or best rule of thumb?

Question by:officertango
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Accepted Solution

Predrag earned 750 total points
ID: 40401117
Link to info source
Table 2-4. Maximum Devices per VLAN

Layer 3 Protocol      
Maximum Number of Devices

IP ------------- 500
IPX ----------- 300
AppleTalk - 200
NetBIOS --- 200
Mixed ------ 200

It is important to note that the numbers specified in Table 2-4 are guidelines only and were defined by Cisco several years ago. Today, CPUs are literally tens of times faster, and these limits may be extended. If you are looking to exceed the limits defined in Table 2-4, monitor the network regularly to ensure the network is performing appropriately. If you find that network performance is poor, consider splitting your network into more VLANs to reduce the number of broadcasts and increase network performance. As a rule of thumb, broadcast and multicast traffic should not account for more than 20 percent of the network utilization.
But, you can significantly reduce broadcast by setting ports as edge ports on access level, so there will be no MAC aging when hosts are on or off. Reason on reducing MAC aging from 5 minutes to 15 sec is response for STP topology change. And turn on IGMP snooping for multicast.

Author Comment

ID: 40401181
Can you elaborate on setting ports as edge port on access level? Igmp snooping?I am not following you. I have hp procurve switches on my network.
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Expert Comment

ID: 40401215
Portfast basics and configure - but since HP have many different procurve switches and different OS on them you need to check for your exact models.
Side affect that it is not described on link above, but on link below (page 14) is that Default MAC aging is 5 minutes, but when you turn on or off host STP sends topology change notification that will also change temporary MAC aging on 15 seconds. So every time some PC is turned on or off if portfast is not configured on that switch port- switch forgets where are all hosts that were not communicating at that 15 sec interval.
Read page 14 - but as much as I could read portfast is on HP procurve enabled by default, not as on Cisco switches where it is disabled by default. But, again, check documentation of your procurve access switches.

IGMP snooping is for multicast (it is not for broadcast but save bandwidth) but I put these two in same basket in my head since behavior is similar :) )
In a network where IP multicast traffic is transmitted for various multimedia applications, you can use the switch to reduce unnecessary bandwidth usage on a per - port basis by configuring IGMP (Internet Group Management Proto­col controls). In the factory default state (IGMP disabled), the switch simply floods all IP multicast traffic it receives on a given VLAN through all ports on the at VLAN (except the port on which it received the traffic)
IGMP snooping procurve documentation.
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Assisted Solution

by:Don Johnston
Don Johnston earned 750 total points
ID: 40401754
The rule of thumb for broadcast traffic is 20% of total traffic.  

Clear your counters and then check them after an hour.  If it's not over 20%, that's not your problem.

Author Closing Comment

ID: 40436540

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