High broadcast frames on VLAN interfaces, is this normal?

[david.bradley]

My question is regarding broadcasts on a VLAN interface. I have a high proportion of broadcasts on my MSFC2 VLAN interface, see below.

My theory is because MLS is enabled, unicasts on a whole are being L3 switched by the switching engine, not hitting the MSFC. The broadcasts won't be creating an XTAG and will be moving to layer 3 hitting the router's VLAN interface. The broadcasts to unicast ratio is high because the only unicast packets being processes are the ones with no MAC mapping.

So is this true?

here's an example sh int:

Queueing strategy: fifo

Output queue :0/40 (size/max)

5 minute input rate 4000 bits/sec, 5 packets/sec

5 minute output rate 0 bits/sec, 0 packets/sec

2270 packets input, 198598 bytes, 0 no buffer

Received 502 broadcasts, 0 runts, 0 giants, 0 throttles

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

278 packets output, 16966 bytes, 0 underruns

0 output errors, 0 interface resets

0 output buffer failures, 0 output buffers swapped out

Thanks,

Dave

[editorstudio]

Dear Dave;

It seems that almost 1/4 of your incoming packets are Broadcast !!

in this case (L3 Switching) you must have a high CPU load and delayed packet forwarding .

if that's right you have a Broadcast Storm that may cause you network to Force Down !

First check your network for STP mismatch ( use sh pross CPU & sh ru )

Second , Do you have a unmanaged Aironet 1100 in your network ? check it .

I still searching for more reasons can cause this problem .

Good Luck

Yours Truly

N.Baradaran

Senior Admin of Central Library - IUST University

[lgijssel]

This is an output that should be looked at in the correct perspective.

It is a fact that a lot of the background traffic on a LAN is broad/multicast. Mostly ARP, STP and routing updates. On an interface with low or zero traffic, it appears as if there are only broadcasts flying around.

It appears to me as if this is what you observed. In abslute terms, the amount of traffic is very low and there should be no need to worry about it.

Regards,

Leo