Switch port not getting traffic

[bern46]

Hello everyone,

I'm kind of new on networking and I'm trying to understand this behaviour:

I have two switches let's say S1 and S2, port 1 and 2 of each switch is connected to the other one with link aggregation. Then port 3 of both S1 and S2 it's connected to a core switch.

I checked the port that cames from S2 to the core switch, and the input/output rates are 0 while the rates in the port that cames from S1 are normal. This port is not configured as a backup of the other so I cannot understand what's happening here...  I mentioned the link aggregation because I don't really know if it has something to do with this...

Any help would be much appreciated.

[Flavio Miranda]

Hi

 If I understood right your topology, this port that you dont see traffic is blocked by the Spanning-tree.

Can you run "show spanning-tree interface X"

[bern46]

Hello, thanks for the fast reply.

Port S1 (with traffic):

Vlan Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0001 Root FWD 4 128.19 P2p
VLAN0005 Desg FWD 4 128.19 P2p
VLAN0021 Desg FWD 4 128.19 P2p
VLAN0030 Desg FWD 4 128.19 P2p

Port S2 (Without traffic)

Vlan Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0001 Desg FWD 4 128.21 P2p
VLAN0005 Desg FWD 4 128.21 P2p
VLAN0021 Desg FWD 4 128.21 P2p
VLAN0030 Desg FWD 4 128.21 P2p

[MHM Cisco World]

Yes that clear that S2 is Root 
and Core SW is BLK port not S1.
this is why you don't see traffic.
check Core Port you will see it BLK.

[Flavio Miranda]

Yes, the port will be blocked. You can run the same on the Core and check.

What you can do is, if possible:

Put this two switch in stack mode, then, you can use both ports to Core and you dont need link aggregation

Or you can configured those portas as Layer 3 IF the switch suports it

[bern46]

Sorry I think it was a misunderstood:

The two that I previously attached were both ports of the CORE switch (One going to S1 and the other one to S2)

S1 and S2 are 3com devices and I don't know how I could check STP or if they have this protocol.

[Flavio Miranda · Accepted solution]

They probably have and thats why your problem is just one port with zero traffic, otherwise could be worst. 

 The 3com switch must have HTTP mamagement. Must be easier to see the spanning tree. 

But you dont have much to do with this topology. To avoid loop, one uplink must be blocked.

 The option you have is that I mentioned before. 

 Stack

Layer 3

[bern46]

Hi,

I was already checking from the HTML interface, and I cannot see STP under device or network section. 

So about what you mention, the second link is totally useless right now if switches are not stacked or the link is configured as a backup, right ? 

The traffic from core to S2 is going through S1 and then to S2 from the link aggregation ports (configured as trunk ports between the two switches) 

Thanks for all the help, now I think I have everything more clear but I think I'll need to take a look at STP...

[MHM Cisco World]

If your previus stp you share for core 

Then s1-s2 is blk and there is something not stp.

Traffic from core must pass directly from core to S1/S2 as your  stp not blk any port.

That my opinion.

[MHM Cisco World]

depend on where is root of STP
if Root is  S1 or S2 then STP block the port 
if the core is Root of STP then there is something elsa happened.

[balaji.bandi]

how is your network diagram looks like ? as per the information they are forwarding mode, but in the path some where blocking may be, so if you can show us more information how your topology to understand the issue here