In this post I’ll talk about the migration of our backbone links from layer two to layer three links. Cisco recommends using layer three links between the distribution and the core switches for various reasons:
- Faster Convergence.
- Equal load balancing on redundant links.
- loops will be taken care by routing protocol not STP.
Another objective of this project is to localize the VTP domain for each building’s network. Currently, all vlans are distributed through one VTP domain. The enterprise backbone is running STP instances for 129 vlans.
The figure shows the network, that is the connectivity between distribution and core layer switches. All the links connecting the Core switches to distribution switches are layer two links. Making the campus a wide broadcast domain, manual pruning is taking place, but yet the management vlan is spanning throughout the network, and new distribution switches are not pruning properly. Many of the redundant links are blocked by STP as a step to mitigate layer two loops.
One of the major issues with this design is that vlans are spanned throughout the network. The core switches already have exceeded the running spanning tree instance recommendation. The current design consist of user vlans that are confined to the distribution switch, (although spanning tree is running network wide for these vlans.), the management vlan with ip scheme of 10.xx.0.0/16 is spanning throughout the enterprise from core switch to the access point.
In part two, i would discuss how we have done the change. and the issues that we faced.