Understanding Spanning Tree

Terry Slattery
Principal Architect

I’m studying for my CCIE recertification and was reading up on Rapid Spanning Tree (RSTP) operation. Cisco has a really nice white paper on its operation, with a lot of good pictures and excellent description of how it works and how it interoperates with the older 802.1d STP protocol: Understanding Rapid Spanning Tree Protocol (802.1w)

RSTP automatically falls back to the 802.1d operating mode when it encounters the older switches. This implies that important VLANs that need fast convergence should be built using newer switches that support RSTP. Move the older switches that you still want to use to less critical parts of your network.

There’s also a nice white paper on 802.1s, Multiple Instance Spanning Tree (MIST): Understanding Multiple Spanning Tree Protocol (802.1s)

MIST relies on RSTP internally, so it is useful to also be aware of how RSTP works. The MIST document doesn’t go into the level of detail that the RSTP document does in describing the transactions between switches running the protocol.
There are a number of potential configuration problems when MIST is used with other switches that run a non-MIST protocol (PVST, RSTP). Because of these potential problems, if you’re planning a switch to MIST, make sure all switches in the spanning tree run it. The best way to have a stable network is to avoid potential problems.

In thinking about where I might want to use MIST, I thought of the Cisco Solutions Reference Network Design (SRND) guides. The current Cisco thinking (I’m not up to speed on other vendor recommendations – leave a comment if you know what they are) is to do routing at the access layer, or certainly at the distribution layer. The result is that VLANs would be constrained to a wiring closet (think subnet per closet). This should limit the number of VLANs handled by a given topology and the limited topology would create more stable VLANs (fewer places for trouble to occur and smaller spanning tree domains make them easier to troubleshoot when they do occur). So, why would you implement MIST? Please leave a comment if you’ve implemented it and are willing to describe why.



Re-posted with Permission 

NetCraftsmen would like to acknowledge Infoblox for their permission to re-post this article which originally appeared in the Applied Infrastructure blog under http://www.infoblox.com/en/communities/blogs.html


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Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.


Virgilio “BONG” dela Cruz Jr.

CCDP, CCNA V, CCNP, Cisco IPS Express Security for AM/EE
Field Solutions Architect, Tech Data

Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.


John Cavanaugh

CCIE #1066, CCDE #20070002, CCAr
Chief Technology Officer, Practice Lead Security Services, NetCraftsmen

John is our CTO and the practice lead for a talented team of consultants focused on designing and delivering scalable and secure infrastructure solutions to customers across multiple industry verticals and technologies. Previously he has held several positions including Executive Director/Chief Architect for Global Network Services at JPMorgan Chase. In that capacity, he led a team managing network architecture and services.  Prior to his role at JPMorgan Chase, John was a Distinguished Engineer at Cisco working across a number of verticals including Higher Education, Finance, Retail, Government, and Health Care.

He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.