["Giles Scott" <gscott@xxxxxxxxxxxxxxxxx>]

Some old switches have a limited size to the forwarding database (FDB) typically on old switches 1024 entries.

In the event of the forwarding database getting full packets tend to get flooded.

 

For example;

Synoptics 28K had a limit of 1024(this switch is very, very old).

Alteon ACEdirector switches have a limit of 1024 per port + 4K max for the switch.

 

If this is the case on some switches you can lower the FDB aging timer to try and keep the FDB from getting full.

Defaults are normally 5 minutes lowering to 60 seconds should help keep the database below 1024 entries.

 

I would think about redesigning you network to lower the size of the broadcast domain. I would think around a max of 500 clients per VLAN is a rough guide.

There are some very cheap L3 switches around at the moment.

 

Cheers

 

Giles Scott

 

 

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From: ethereal-users-bounces@xxxxxxxxxxxx [mailto:ethereal-users-bounces@xxxxxxxxxxxx] On Behalf Of Carsten Holfelder
Sent: Thursday, December 09, 2004 5:27 AM
To: ethereal-users@xxxxxxxxxxxx
Subject: [Ethereal-users] Broadcasts which shouldn't be broadcasts

 

Good Day,

 

I have noticed over time the network seems to slow down. So I have installed ethereal on my computer and noticed the following.

 

I have got a switched network with about 1600 devices connected to it.I noticed that I am able to sniff a lot of packets that should have been switched - Directed Traffic. When I check the data packets I noticed that there is a destination MAC Address in the data packet. I connected the packet sniffer to a different section on the network and it is the same. When I check on the switches the MAC Address is not in the ARP table. I have increased the aging time of the arps cache and it has made no difference. My main switches are Dell Poweredge 5324 and the rest are a mix between 3Com and Cabletron. The switches are not even sending a broadcast packet asking for the MAC Address of the Server or Computer. When I check the port where the server is connected to it doesn't show the MAC Address of the server.

 

The main problem is with a web based application that run a SQL server in the background. There are four IIS servers running with microsoft NLB (Network Load Balancing) between them. Two servers are connected in one switch and the other two in a anouther switch. The replys from the server to the clients are directed. It is only the traffic that is going to the servers that are being broadcast. The servers have got two nics in them. One is for the NLB which the clients access and the other is used for a maintenance connection.

 

I know that this is not about ethereal but a network problem, but I am hoping that there is someone who knows what the problem is and will be able to assist me to solve it.

 

Thanks

Carsten Holfelder

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