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per vlan spanning tree question

Posted on 2013-10-22
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Last Modified: 2013-10-24
Experts,

It's been a long time since I've had to deal with switches. I have a 3750 Stack (two 3750s) that is the root bridge for vlan x.

When I do a sh span vlan x, I get the below output:

Gi2/0/47            Desg FWD 4         128.101  P2p
Gi1/0/47            Root FWD 4         128.47   P2p


Both ports above trunk to a Nortel switch.

My question..

#1) It shows both ports as FWD. Does that mean both ports are actively sending traffic?

I believe the other side is just 1 Nortel switch. Would that Nortel switch block the port that connects to my cisco switch 2/0/47?  My thoughts are that it would be active on the port connected to cisco switch 1/0/47 since that is marked as root and thus the best way to get to the root.

#2) question 2: If the Nortel switch has other mechanisms for loop avoidance and STP being one of them, should I automatically enable STP on the NOrtel?
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Question by:trojan81
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7 Comments
 
LVL 50

Assisted Solution

by:Don Johnston
Don Johnston earned 668 total points
ID: 39594238
First off, connecting a Cisco to a non-Cisco is going to be... interesting.  I'm pretty sure that Nortel uses CST (Common Spanning Tree) either in 802.1d or 802.1w mode which is incompatible with Cisco's Per-VLAN Spanning-Tree.

Having said that, if the 3750 stack is the root, it would not have a root port (your output shows the G1/0/47 as a root port).  This would indicate an issue which needs to be addressed.
#1) It shows both ports as FWD. Does that mean both ports are actively sending traffic?
Yes, both ports are forwarding traffic.

The Nortel switch should be blocking one of those ports. But I suspect that it's not since they're running different STP types.
#2) question 2: If the Nortel switch has other mechanisms for loop avoidance and STP being one of them, should I automatically enable STP on the NOrtel?
IIRC, Nortel has spanning tree enabled by default. But it's not compatible with Cisco's PVST implementation.

The best approach is to use MST (802.1s) on the Cisco and Nortel switches.
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LVL 18

Assisted Solution

by:Akinsd
Akinsd earned 664 total points
ID: 39595045
You need to check to other end
There is a root port meaning the cisco switch is not the root bridge.

The 2nd port (Gi2/0/47 shouldn't be forwarding traffic, meaning there is a problem as DonJohnston mentioned.

IIRC, Nortel has spanning tree enabled by default. But it's not compatible with Cisco's PVST implementation.

You will need to check the other switch to find out which side is blocked. If both connect to the same switch, it means the other switch is the root bridge and root bridges do not block any of their ports.

Gi2/0/47            Desg FWD 4         128.101  P2p
Gi1/0/47            Root FWD 4         128.47   P2p

Open in new window


http://www.experts-exchange.com/Networking/Protocols/A_11315-Root-Port-Election-STP-or-RSTP.html

http://www.experts-exchange.com/Networking/Operating_Systems/A_11209-Root-Bridge-Election.html
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by:Craig Beck
ID: 39595809
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Author Comment

by:trojan81
ID: 39596259
thanks everyone. I will work on finding out the other side.

Don and akins, if both ports on the cisco stack are forwarding, wouldn't that lead to loss data assuming the other side is blocking their side of one of those ports?
So let's assume the other side of 2/0/47 is in blocking. If my stack is sending traffic both 1/0/47 and 2/0/47 for this VLAN, and the other side is simply blocking traffic on the 2/0/47 link, then that's loss data.
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Expert Comment

by:Don Johnston
ID: 39596637
if both ports on the cisco stack are forwarding, wouldn't that lead to loss data assuming the other side is blocking their side of one of those ports?
No. The function of spanning-tree is to block any redundant paths.  As long as one path is carrying traffic, there's no data loss.
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LVL 47

Accepted Solution

by:
Craig Beck earned 668 total points
ID: 39596700
In addition to what don and akinsd have said, the switch will send traffic based on what's in it's CAM table.  The CAM table contains MAC addresses which are learned and the port via which a particular MAC address is reachable.  This is the main concept which separates a switch from a hub.  There is no concept of STP on a hub.

When a port is blocked at one end no MAC addresses will be seen on that link, so the switch won't try to send traffic to a particular MAC address via that port.
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Author Closing Comment

by:trojan81
ID: 39599513
well done, gentlemen. Thank you.
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