SG300 STP disables single uplink to router

khargoosh · ‎09-30-2015

Hi all, thanks in advance for your advice.

We have 2 SG300-28PP switches with the core switch uplink to a Juniper SRX210 appliance. Both switches have VLANs 1 (native) 36 and 52. The configuration is basically as below:

Switch 2 >> LAG (1U, 36T, 52T) >> Switch 1 >> Uplink (1U) >> SRX210 port ge-0/0/1

I discovered this issue after the following test:

When I connect my laptop directly to SRX210 ge-0/0/1 I had internet access as expected. However, when my laptop was connected to either switch, I no longer had internet connectivity.

I discovered that STP was disabling the uplink port to the SRX - even though it was only a single cable! I had originally configured MSTP on the switches to separate the spans for each VLAN, but as per above I ended up connecting Switch 2 to Switch 1 through a LAG supporting all VLANs as required.

The first thing I tried was to disable STP globally, and as soon as I did this, everything worked as expected (internet connectivity on VLAN 1 on both switches).

I can't wrap my head around what happened here. In this scenario, why should this result occur due to STP?

Eventually we will uplink VLAN 36 and 52 to additional ports on the SRX210. What is the correct/standard way to configure STP here such that they will coexist happily? There will be no L3 connection between the 3 VLANs until they reach the SRX.

Cheers

devils_advocate · ‎10-01-2015

Is Switch 1 currently in L3 mode and acting as the default gateway for VLAN's 36 and 52?

Is the Uplink to the SRX210 a Trunk or an Access Port (on the Switch 1 side)?

khargoosh · ‎10-01-2015

Hi devils_advocate, thanks for your assistance.

Both switches are configured at L2, the SRX210 acts as default gateway for VLAN1 and will act as default gateway for VLANs 36 and 52.

I had the same thought, but I tried using both Trunk and Access ports for the Switch 1 uplink to the SRX210, the result with both configurations was STP disabling the uplink. Again there was only the single uplink for VLAN1 anyways (the uplink was configured trunk/access 1UP).

Any other thoughts?