Make a 2950 Switch act like a Hub

Unanswered Question

All, I recently installed a load balancing device for two internet T1's. Along with this I purchased the a failover for the device. In order for the primary and backup to talk to one another on a LAN switch (2950) it required broadcasting. Meaning the IP addresses they talk to one another with is a different subnet they the other devices on that switch. So the vendor says I need to enable broadcasting. The switch has to act like a hub .. How can I make the 2950 switch act like a hub? Thanks !!

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
paul.matthews Thu, 10/04/2007 - 05:09

I am not sure what they are asking for, so I will explain a little of the behaviour of a 2950 out of the box.

If a broadcast (at L2) is sent, it will be forwarded out of all ports.

If a packet to an unknown MAC address is sent, it will be forwarded to all ports.

When a device transmits, the switch will make a note of the source address, and add it to the list of known addresses. When a packet is sent to that mac address, it will then only be sent to the port that address was last seen on. The switch will remember an address for five minutes after it was last seen.

Could it have been multicasting they are talking about? Maybe of you mention the device, someone will have worked with one before and know what they mean.


Kevin Dorrell Thu, 10/04/2007 - 05:11

I don't think I understand what the vendor means. The switch is a layer-2 device, and knows nothing about layer-3 IP addresses and subnets except the one used to access its own management functions.

The switch will automatically pass MAC broadcasts to all ports in the VLAN. It will do the same for multicasts, with certain provisos.

You should consider defining a seperate VLAN on the switch for these two devices to isolate their traffic from the other devices on the switch, but even that is not essential. Do you know whether these two devices use MAC broadcasts or multicasts to talk to each other?

Kevin Dorrell


paul.matthews Thu, 10/04/2007 - 05:53

I am thinking of stonebeat type stuff that uses L2 Multicast MAN addresses ad per IP milticast (01005e type addresses) but does not do the IGMP Join) that need IGMP/CGMP turning off on some switches. I love it when network stuff is designed by people that don't understand networks...

Kevin Dorrell Thu, 10/04/2007 - 06:15

Oh right, in that case you would indeed need to disable IGMP snooping or CGMP. Pity is, I don't think you can do that per VLAN, so the other VLANs would also suffer a certain amount of multicast flooding.

BTW, the question of "can I turn my switch into a hub?" cropped up a couple of years ago before I took my sabbatical from NetPro. I have not tried it, but I wonder if you can persuade an RSPAN VLAN to do it. After all, and RSPAN VLAN is a VLAN with learning switched off. (If I had time) I would like to take a 2960, define an RSPAN VLAN on it, and then put a few access ports on the RSPAN VLAN and see if it works. (If I had time ... )

Kevin Dorrell


paul.matthews Thu, 10/04/2007 - 06:21

It would probably be cheaer to just buy a hub than to spend the time trying to figure it out!

Thanks for your replies !! The load balancing/failover units are XRoads EdgePro 3500's. I am not sure if it requires MAC bradcast or multicast for the secondary to assume the MAC address of the primary when failure occurs. At this point they still can not ping one another when plugged into the same network. (They ping one another as the heart beat) All devices on that network are 172.*.*.* and the Edgepro devices are and 2. Thanks again !!! Xroads support was not helpful ...

Kevin Dorrell Thu, 10/04/2007 - 06:23

Try no ip igmp snooping. That would flood the multicasts. (But for everyone else too!)

If that works, then try isolating the traffic by putting them in their own seperate VLAN:

vlan 35

 name EdgePro 3500


int F0/10

 switchport access vlan 35


int F0/11

 switchport access vlan 35

As an OT aside ...

I have seen load balancing schemes where the devices ARP for each other and get back a multicast MAC address, and one that should correspond to IP multicast. But these devices did IGMP correctly on the corresponding IP multicast group, so the switch did work. However, that did not work with a router, 'cos a router does not believe a multicast MAC address in an ARP response. The only solution there was a staic ARP antry in the router.

But that is by the way.

Kevin Dorrell



This Discussion