3COM Switches, DHCP, MAC Addresses, Internet access 1

[jayc28]

Some time ago our company decided to go to DHCP IP addressing rather than using static. We also decided to stop using hubs and instead use 3COM switches. We thought this would help us out but now there are problems and I cannot tell if it is a 3COM switch issue or a DHCP issue.

Several computers cannot under any circumstances, access the Internet using DHCP. DHCP assigns automatically the IP address, DNS, and default gateway. All check out ok. They can browse the network, but when they try to get out to the Internet, they cannot. Others work just fine.

Here is another strange thing. Whatever IP addressed which was assigned to that workstation is the ONLY one that can be used, otherwise, access to the Internet is denied. For instance my workstation always used a lease for 192.168.4.210 - One day, DHCP assigned it 211 and it didn't work. I had to manually reserve 210 so it would work.

My question is, where is the blockage coming from? My switch database says there are 198 mac addresses in it. I know that is not true. There are at most, 40 PC's connected at any one time. I thought the aging process was supposed to remove a man address that didnt send any packets after 30 minutes, but apparently it is not doing that. Would the 3COM switch not allow the packets to pass through if it is receiving it from a mac address/IP address it does not recognize? Or is something happening on the DHCP side?

Any help would be greatly appreciated. If I have't been clear enough, let me know and I'll see if I can fill in some more details.

 

[motor135]

Have you tried clearing the ARP tables on the switch? I've seen instances (especially when hot-swapping patch cables from one switch to another) when clearing the ARP tables fixes some issues.

Doug

 

[mikeyt1513]

What are you using for NAT. It seems like you might have a problem with IP Addressing, not the switches.

 

[jayc28]

Thanks. You were right. It was a NAT issue and had nothing to do with the switches nor DHCP. I tried to tell this to our IT consultant some time ago, but he assured me that was not the issue. Now, router configuration is not my strong suit, but I have picked up enough over the last week to have a handle on what our problem is.

When our router was first configured it was set up to translate only 50 IP's. Well, that is why we are having trouble as we now have 55 client's that need to access the Internet. I know that the command to make the range of IP's for NAT is:

ip nat pool public xxx.xx.xx.xxx (start) xxx.xx.xx.xxx (finish) netmask 255.255.255.xxx

My question is, after I enter this in, what do I have to do? Does the router need to be restarted? Anything?

Also, how can I clear that ARP table out so that workstations aren't limited to what IP address they need to get out to the Internet.

Thanks!

 

[mikeyt1513]

It depends on what type of router you are using. I only understand how NAT works not how to configure it. But I do use NAT at my house and my little Linksys router has to be restarted, so I would say, yes restart it. As far as clearing the ARP table out, that should do it itself. Arp entries have a TTL, I believe, but don't quote me. So, what kind of router are you using, I will be able to help better.

 

[jayc28]

It's a Cisco 2600 router. Once I restarted it cleared the ARP table, and the changes worked great.

Thanks for your help, because I had somebody else insisting that it had nothing to do with the router.

 

[Crete]

I am having a lot of collisions on my 3Com 3300 switch.
Amber lights on all ports blinking.Please what could be the issue with this

 

[Crete]

I am having a lot of collisions on my 3Com 3300 switch.
Amber lights on all ports blinking.Please what could be the issue with this.
Crete

 

[Crete]

I am having a lot of collisions on my 3Com 3300 switch.
Amber lights on all ports blinking.Please what could be the issue with this.How do i configure the switch.
Crete

 

[mikeyt1513]

Give me some more background on your network configuration.


Thanks,

Mike