Hotel case study - Spanning Tree

Spanning Tree is an often ignored component of local area networks as it self configures to form a functional network. However, a "functional network" may not be an optimal network. Spanning Tree results in a single viable path through a switched Ethernet network. Optimising layer 2 (STP) involves trying to align common traffic paths with the structure of the Spanning Tree. Keep in mind that if you have a network where the flows are network where there are frequent flows between different pairs of devices you may always have a less than optimal traffic flow. Don't get too hung up on this, STP is an old protocol with limitations. We do the best we can with what we have.

To optimise Spanning Tree, you must look at the expected flows in the network. Think about what flows occur in a hotel. Where do guests do? What do staff do. I would suggest that virtually all guest activity involves the Internet. Perhaps some traffic is internal to the hotel; this could include billing, room service applications or streaming media for in-house movies. In terms of flow, all of this traffic heads out of the VLAN to somewhere else. Inbound guest traffic will originate from outside the VLAN (Internet / data centre). What this means is that the guest devices are almost exclusively sending and receiving traffic from there gateway. There is little host to host communication amongst guests. In fact, from a security perspective, we would prefer no direct interaction between guest devices.

So for our guests we see that flows are centred on the default-gateway. In the three layer hierarchy the default gateway resides at the distribution layer. Given that the Spanning Tree Protocol generates a tree that is optimally connected to a root-bridge, it makes sense that the root bridge for a VLAN is the same device as the default-gateway for that VLAN.

Our case study requirements make no specific mention of Spanning Tree. But if you look at the routing requirements and the location of the gateways, we can see how to proceed.

https://csn.murdoch.edu.au/mediawiki/index.php/Hotel_case_study_using_EVE_-_Overview

Routing Requirements

You must provide intervlan routing, such that all devices can ping one another.

    Configure intervlan routing on all DL switches.
        Users on level one should make use of DL1 as their default gateway.
        Users on level two should make use of DL2 as their default gateway.
        Use HSRP with the active router for each guest floor being on a different router.

We know from the above that the Level-1-Guest VLAN uses DL1 as the default gateway. Therefore DL1 should be the root-bridge for the Level-1-Guest VLAN.

Similarly DL2 should be the root-bridge for the Level-2-Guest VLAN.