spanning tree on 2960s

Posted on 2014-02-19
Last Modified: 2014-03-07
I have  2 2960s and they have 2 links connect to each other. I checked spanning tree and they both in the forwarding state. Shouldn't one be blocked? Thanks
Question by:leblanc
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Accepted Solution

ffleisma earned 200 total points
ID: 39872547
on the root bridge, depending on which is elected as the root bridge, you'll see both ports as forwarding as these ports are designated ports

if you try and look at the other switch, you should see one of the ports in blocking state and one in forwarding state, which is the root port.

also, have you configured the swithcport connections are trunks and not access ports?

Assisted Solution

ffleisma earned 200 total points
ID: 39872548
Spanning Tree
LVL 57

Expert Comment

ID: 39873127
If you look at both switches and the both show both ports in forwarding state, then you most likely have them configured in a Etherchannel.

When you do this logically you have one port even though you really have multiple physical ports.
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Author Comment

ID: 39874749
I see. I looked on the other switch and it is indeed blocked and I think this is a mistake because we want to use both links to maximize the throughput. I will need to configure etherchannel on my switch stack. Thx
LVL 57

Assisted Solution

giltjr earned 200 total points
ID: 39874825
Yes, to use both connections you need to configure them as a Etherchannel.

However, you won't get a true 50/50 split.   Etherchannel does not balance on per Ethernet frame or TCP Packet or UDP datagram.

The options for "port-channel load-balance"  are:


Default is src-mac.  We always change to src-dst-ip on devices that don't support src-dst-port.  I think the 2960 supports src-dst-ip, but it does not support src-dst-port.

Author Comment

ID: 39874876
Is  src-dst-ip or  src-dst-port better?
LVL 57

Assisted Solution

giltjr earned 200 total points
ID: 39874897
src-dst-port is better, but not all devices support it.

src-dst-ip says to balance based on src and dst IP address.

src-dst-port says to balance based on src ipaddress:port and dst ipaddress:port.

With src-dst-ip if two hosts have multiple tcp connections to each other, all of them will go over the same physical connection.

With src-dst-port  if two hosts have multiple tcp connections to each other, each connection can go across a different physical connections.
LVL 46

Assisted Solution

by:Craig Beck
Craig Beck earned 100 total points
ID: 39876462
If you're using trunks you might see that both links are up but some VLANs are blocked on each link, depending on how you configure STP per-VLAN on each switch.

Obviously you've got this one sorted, but I just thought it might help in the future.

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