Cisco – Connect 3 Cisco switches via ethernet ports

ciscospanning tree

We have been using 2 Cisco 2960-S 48 Port switches which were connected together using a regular Cat5e cable and everything was working fine, however we have purchased a 3rd switch of the same model and need to connect it to the other switches.

What would be the best way to connect all 3 switches?

The switches will be in a ring like this:

          switch 1
         /        \
        /          \
switch 2------------switch 3

On another site on an old thread I found, someone said this would cause a loop and cause problems.

Can anyone shed any light on if this config would work or cause any problems?

Any help greatly appreciated!

Best Answer

We have been using 2 Cisco 2960-S 48 Port switches which were connected together using a regular Cat5e cable and everything was working fine, however we have purchased a 3rd switch of the same model and need to connect it to the other switches.

What would be the best way to connect all 3 switches?

Generally, you connect all three switches in a "triangle". There is no problem with this configuration as long as you have some version of spanning-tree running.

I'm not sure what state your network is in right now, so I'm assuming you might not have spanning-tree running, and you have not connected the third switch. Generic steps to introduce the new switch:

Make sure the third switch is disconnected. Configure spanning-tree mode rapid-pvst on all switches.
Configure spanning-tree vlan 1-4094 priority 4096 on the switch closest to your Internet / WAN connection
Configure spanning-tree vlan 1-4094 priority 8192 on the other (running) switch
Configure spanning-tree portfast disable on all ports connecting to the other switches.

I gave you a very basic recipe to make this work... depending on the bandwidth needs of the clients, you might want to bundle multiple links between these switches with LACP or EtherChannel.

Related Solutions

HP Switching, DT Links to two STP switches

This isn't Distributed Trunking. The described topology is:

.-------.  .-------.
|       |  |       |
| 8200A |--| 8200B |
|       |  |       |
'-------'  '-------'
    |          |
    |          |
.-------.  .-------.
| sw e  |--| sw f  |
'-------'  '-------'

Since it's said that the 8200s are connected with "DT-LACP trunks" I assume the Inter-switch link required for DT is present between them. If these links are in the same VLAN and STP is not enabled, there is a loop.

This is what Distributed Trunking looks like:

.-------.  .-------.
|       |  |       |
| 8200A |--| 8200B |
|       |  |       |
'-------\  /-------'
    |    \/    |
    |    /\    |
.-------/  \-------.
| Sw E  |  | Sw F  |
'-------'  '-------'

The links on 8200s are configured for Distributed Trunk, and the link between them as Switch Interconnect link which makes the downlinks to each edge switch appear as one logical link. The uplinks on switches E and F are aggregated with LACP so each of them will see one logical link as well. There is no loop.

If there's a redundant link between switches E and F and it's in the same VLAN as the uplinks, a loop will form. STP is needed to counteract that, and root and path priorities must be configured so that STP blocks the redundant link.

Cisco – MAC Flapping on Cisco 2960G

Hunt for an L2 loop or a software misbehaviour.

Start at 2960G/Gi0/22, and go to the upstream router let's say R1. Search on which port is the @MAC: 00:16:3e:64:98:e2, let's say R1/Gi0/7. Go to the network equipment connect on R1/Gi0/7, and go ahead until you reach the final port which says it sees this @MAC.

If this isn't a physical connection to the same machine,

check for an L2 wireless loop (through a wireless access point),
check for a VLAN L2 loop (same VLAN trunked on 2 different ports),
check for a rogue repeater, switch or AP,
check for a ghost value at the CAM level (clear the port).

Best Answer

Related Solutions

HP Switching, DT Links to two STP switches

Cisco – MAC Flapping on Cisco 2960G

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