| Core 1 Sw |=============| ILM 1 Sw |
|___________| 1Gb |__________|---
||1Gb ||1Gb ||
|| || ||
|| || ||1Gb
|| || ||
| Core 2 Sw | 1Gb | ILM 2 Sw |
** All connections are 2 x 1Gb links port channelled together.
Core 1 = Root Bridge
Core 1 Priority = 8192
Core 2 Priority = 16384
Objective: All data from ILM2 should be forwarded to ILM1 before being sent across to Core1 to be routed. The link from ILM2 to Core2 should be the failover link.
Using default priorities (32768) when we connected ILM1 and ILM2, the links from ILM1 to Core1 and ILM2 to Core2 were forwarding and the interface from ILM2 to ILM1 blocking. This is not ideal for our situation. The default path cost for the link from ILM2 to ILM was 3 and the default path cost of the link from Core2 to ILM2 was 2. To force traffic the way we wanted it to go, we set the priority of the link from Core2 to ILM2 to 4. This then caused the link from Core2 to ILM2 to go into Blocking status and the link from ILM2 to ILM1 into Forwarding status (which is what we wanted). In keeping with our standard, we set the priority of both ILM1 & ILM2 to 49152. This is the priority used on other server switches in our environment. This however caused a problem. The interface from ILM2 to ILM1 changed to blocking status and the link from Core2 to ILM2 went into Forwarding status. We set the Priority on the link from Core2 to ILM2 to 5 to get it to block but at 4 it would forward.
Question: I read through the document 'Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst Switches' from the Cisco website and near the end it states that 'The port with the lowest path cost to the root bridge becomes the root port. The root port is always in the forwarding state.' Could someone please explain why then, in my situation mentioned above, did STP forward on the higher path cost (4) and block on the lower one (3)?? It was not until we raised the higher path cost to 5 that it started blocking. Are any other values taken into account??