LAN Broadcasting

[arifspectrum]

How to check that which ports of a switch are affected to broadcast and is there any tool available to capture/find out the broadcast traffic? plz help

[Kevin Dorrell]

If you do a "show int", it will tell you the number of broadcasts transmitted on each interface.  I find the best way to examine the broadcasts is to connect WireShark or snoop to an unused port.  Broadcasts, by definition, are sent to all ports on the VLAN.

Kevin Dorrell

Luxembourg

[arifspectrum]

Thnaks for your reply.

Show Interface command output

GigabitEthernet1/0/5 is up, line protocol is up (connected)

  Hardware is Gigabit Ethernet, address is 5057.a835.1305 (bia 5057.a835.1305)

  Description: "Admin-Bld_2-Access connection"

  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation ARPA, loopback not set

  Keepalive not set

  Full-duplex, 1000Mb/s, link type is auto, media type is 1000BaseLX SFP

  input flow-control is off, output flow-control is unsupported

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input 00:00:17, output 00:00:00, output hang never

  Last clearing of "show interface" counters never

  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 1087000 bits/sec, 128 packets/sec

  5 minute output rate 261000 bits/sec, 171 packets/sec

     30618912 packets input, 36910343707 bytes, 0 no buffer

     Received 87486 broadcasts (61433 multicasts)

     0 runts, 0 giants, 0 throttles

     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

     0 watchdog, 61433 multicast, 0 pause input

     0 input packets with dribble condition detected

     39848450 packets output, 4996613567 bytes, 0 underruns

     0 output errors, 0 collisions, 2 interface resets

     0 unknown protocol drops

     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

Here I found 87468 broadcast packets on this interface. my question is for what value I can say that this interface is affected by broadcast?

[Kevin Dorrell]

They are all broadcasts.  The counter is cumulative, so you should take two reading spaced by, let us say, one hour, and calculate the difference.  That is the number of broadcasts the port has received.  What is a reasonable value?  It depends on what the port is used for.  My worst VLAN is running at about 20 broadcasts per second average.

Another useful form of the command is show interface count or show interface G1/0/5 count.  That presents the information in tabular form, and gives more detail.  There are other keywords you can add to give you more specific counters.

Kevin Dorrell

Luxembourg