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Connecting Two Smart Switches with Two Cables: Smart or Stupid?

Posted on 2004-04-10
Medium Priority
Last Modified: 2007-12-19
After years of networking experience, I'm doing a real brain fart on a basic question...

Scenario: I want to join two LANs together that are on opposite sides of the building. Each LAN has its own smart switch. I've run two Cat 5e cables across the building's ceiling, terminating in each LANs patch bay. I ran two cables for backup and redundancy, not 'cause I have too much traffic.

Question: If I patch both cables into the respective smart switches, am I adding backup and redundancy, in case one port or cable fails? Or am I creating a routing problem? (I know that if this was a hub, then I wouldn't want to do this. But I'm blanking on the behaviour of smart switches.)

Additonal info: The LANS are on the same subnet and share WAN and server resources. The switches are 10/100.
Question by:TerrellITC
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LVL 79

Accepted Solution

lrmoore earned 252 total points
ID: 10799265
Not a problem. Yes, back and redundency.
No, no routing problems.
Simple answer is Spanning Tree, enabled by default on most switches.
Spanning Tree prevents Layer 2 bridge loops, but provides failover. One link will be in blocking mode, one link in forwarding mode. If the "live" link goes down for whatever reason, the other link will become active.
LVL 27

Assisted Solution

pseudocyber earned 248 total points
ID: 10799453
Alternatively, you could trunk the two of them and possibly have an active/active trunk and if one fails, the link is still up.  Depends on your switches.

Author Comment

ID: 10799597
Thanks lrmoore,

Is Spanning Tree something that is setup in the switches configuration?

Thanks pseudocyber,
But I'm unfimiliar with your usage of "trunk the two of them".
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LVL 79

Expert Comment

ID: 10800754
Most switches have it already on as default, and no configuration is necessary.
What kind of switches do you have?

Some switches can combine two or more uplinks into what appears as one connection to double the bandwidth and provide failover/redundency at the same time. Cisco calls it EtherChannel, others call it trunking.

Author Comment

ID: 10801480
The switches are a HP Procurve 4000M and a Dell PowerConnect 3024.
LVL 27

Expert Comment

ID: 10803124
For trunking/etherchannel/bonding/etc. look - in the documentation, look for support of 802.1q.  Many times, connecting multiple switches - the best design is to use two connections which are active/active.  This means traffic can go over either link and ideally is loadbalanced.  If one fails, the link to the switch stays up.

There are a few other things you can play with with spanning tree - you can have two connections open but one in sort of a "standby".  If it fails, the other one is already learned and ready to go.


Expert Comment

ID: 10804576
EtherChannel and trunking are two different things.  EtherChannel allows you to bond two or more ethernet links together to increase bandwidth.  Trunking allow you to run multiple vlans over one link.  Trunking protocols include isl and 802.1q.


LVL 27

Expert Comment

ID: 10804588
You're right Pascall - sorry to confuse people with 802.1q - it is of course VLAN trunking.

However, I'm comfing from a Nortel environment and Nortel calls their equivalent to Etherchannel - Multi Link Trunking (MLT).

Author Comment

ID: 10830507
Okay. In the middle of the day, I plugged the second cable into the two switches and brought the network to a halt. Problem went away when I unplugged the second cable.

Do I need to do anything like reboot the switches after making the connections?
LVL 27

Expert Comment

ID: 10831949
I've done that before.

You need to make sure Spanning Tree is running, or you have ether channel/trunking configured, before you plug in the second cable - if not, you create a loop which will crash a network.

Perhaps it would be best to schedule this for after hours ...

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