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MSTP Related query


Consider a network topology of three nodes Node-A, B, and C connected to each other in a ring using 1 Gbps links. Also all these nodes are connected using 100 Mbps links.

The network connections are as below:

- Node-A has connectvity with Node-B with one 1 Gbbps link and one 100 Mbps link.

- Node-B has connectvity with Node-C with one 1 Gbbps link and one 100 Mbps link.

- Node-C has connectvity with Node-A with one 1 Gbbps link and one 100 Mbps link.

Is it possible to have different forwarding behavior for 1 Gig ring and 100 Mbps ring using two MST instances configured on Node-A, B and C with all the nodes configured in same region with similar MST-I to VLAN mapping?

With default configurations on MST instances, on one Node only one 1 Gig link was in forwarding state and three links (one 1 Gbps and two 100 Mbps) links went into discarding state. Due to this for 100 MBps ring, one node went out of reach for VLANs that were mapped to the MST instance.

By changing the MSTI configurations for instance (Port Priority and Port Path Cost for 100 Mbps and 1 Gbps) mapped to 100 MBps ring VLANs, we could get both the rings operative and there was only one 1 Gig port and 1 100 Mbps port went into discarding state.

My questions are as below:

1. Cant this requirement be achived with default configuration MSTI configurations?

2. Will above configuration be able to handle  any link failurs? Meaning on any link faiure, all the nodes will be reachable to each other for the VLANs on 100 Mbps and 1 Gbps rings.