We help IT Professionals succeed at work.

How to gauge if broadcast traffic is a problem due to subnet size?

230 Views
Last Modified: 2014-12-09
So I took over a network recently where they using a very large subnet (10.4.0.0/16).  Of course a subnet can never be that large, but I didn't change it as they had a lot of servers and other devices in production and didn't want to change subnets.  They are now using about 350 IP Addresses from this subnet.  Infrastructure is Completely Cisco.  All switches are Cisco 3560G's.  What is a safe number to grow this subnet before I should start another VLAN?
Comment
Watch Question

CERTIFIED EXPERT
Distinguished Expert 2018

Commented:
Same subject here.
Instructor
CERTIFIED EXPERT
Top Expert 2015
Commented:
This one is on us!
(Get your first solution completely free - no credit card required)
UNLOCK SOLUTION

Author

Commented:
So what number do I divide by the number of broadcasts:

GigabitEthernet1/0/4 is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet, address is 001b.d511.8d04 (bia 001b.d511.8d04)
  Description: TOP_3560G-48
  MTU 1530 bytes, BW 1000000 Kbit, DLY 10 usec,
     reliability 255/255, txload 3/255, rxload 4/255
  Encapsulation ARPA, loopback not set
  Keepalive not set
  Full-duplex, 1000Mb/s, link type is auto, media type is 1000BaseSX SFP
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output 00:00:19, output hang never
  Last clearing of "show interface" counters 00:15:56
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 15988000 bits/sec, 6113 packets/sec
  5 minute output rate 15661000 bits/sec, 5672 packets/sec
     6417526 packets input, 2451754699 bytes, 0 no buffer
     Received 427839 broadcasts (280131 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 280131 multicast, 0 pause input
     0 input packets with dribble condition detected
     5993425 packets output, 2412545171 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 PAUSE output
     0 output buffer failures, 0 output buffers swapped out
Don JohnstonInstructor
CERTIFIED EXPERT
Top Expert 2015

Commented:
6417526 packets input, 2451754699 bytes, 0 no buffer
     Received 427839 broadcasts (280131 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 280131 multicast, 0 pause input
     0 input packets with dribble condition detected

So in 15 minutes, you've received 6,417,526 packets of which 427,839 were broadcast. This puts the broadcast received on this port at 6%.

Author

Commented:
Thanks.  If I divide 6417526/427839 that equals 14.99.   How did you get 6%?  I must be missing something.
Don JohnstonInstructor
CERTIFIED EXPERT
Top Expert 2015

Commented:
427839/6417526=.06

Author

Commented:
Ok, thanks.  So its broadcasts/packets input.  

So if I did another trunk port:

 1668527 packets input, 376258519 bytes, 0 no buffer
     Received 620 broadcasts (615 multicasts)

It would be 620/1668527 which would equal approximately 3.72% broadcast traffic right?

Author

Commented:
Thanks

Gain unlimited access to on-demand training courses with an Experts Exchange subscription.

Get Access
Why Experts Exchange?

Experts Exchange always has the answer, or at the least points me in the correct direction! It is like having another employee that is extremely experienced.

Jim Murphy
Programmer at Smart IT Solutions

When asked, what has been your best career decision?

Deciding to stick with EE.

Mohamed Asif
Technical Department Head

Being involved with EE helped me to grow personally and professionally.

Carl Webster
CTP, Sr Infrastructure Consultant
Empower Your Career
Did You Know?

We've partnered with two important charities to provide clean water and computer science education to those who need it most. READ MORE

Ask ANY Question

Connect with Certified Experts to gain insight and support on specific technology challenges including:

  • Troubleshooting
  • Research
  • Professional Opinions
Unlock the solution to this question.
Join our community and discover your potential

Experts Exchange is the only place where you can interact directly with leading experts in the technology field. Become a member today and access the collective knowledge of thousands of technology experts.

*This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.

OR

Please enter a first name

Please enter a last name

8+ characters (letters, numbers, and a symbol)

By clicking, you agree to the Terms of Use and Privacy Policy.