Yesterday, I have got the following e-mail from one of my blog readers ...
Let me introduce myself, I work in medium size company and we began to sell Dell Networking stuff to go along with VxRail. We do small deployments, not the big stuff with spine/leaf L3 BGP, you name it. For a Customer, I had to implement this solution. Sadly, we are having a bad time with STP as you can see on the design.
Is there a way to be loop-free ? I thought about Multi Domain VLT LAG but it looks like it is not supported in OS10.
I wonder how you would do this. Is SmartFabric the answer ?
Well, first of all, thanks for the question. If you ask me, it all boils down to specific design factors - use cases, requirements, constraints, assumptions.
So let's write down design factors
- Multi-site deployment
- A small deployment with a single VLT domain per site.
- Robust L2 networking for VxRail clusters
- Dell Networking hardware with OS10
- Networking for VMware vSphere/vSAN (VxRail)
- No more than a single VLT domain per site is required
- No vSphere/vSAN (VxRail) Clusters are Stretched across sites
Option 1 - Stretched L2 Loop-Free across sites
- Stretched L2 across sites allows workload (device, VM, container, etc.) migrations across sites without L2 over L3 network overlay (NSX, SmartFabric, etc.) and re-IP.
- Topology is not scalable for more TOR access switches (VLT domains), but this ok with the design factors
- Topology optimally requires 8 links across sites. Optionally, can be reduced to 4 links.
- Only two routers. One per site.
- Stretched L2 topology across sites also extends L2 network fault-domain across sites, therefore broadcast storms, unknown unicast flooding, and potential STP challenges are the potential risks.
- This topology has L3 trombone by design - https://blog.ipspace.net/2011/02/traffic-trombone-what-it-is-and-how-you.html. This drawback can be accepted or mitigated by NSX distributed routing.
OPTION 2 - L3 across sites with L2/L3 boundary in TOR access switches
- Better scalability, because other VLT domains (TOR access switches) can be connected to core routers. However, this benefit is not required by the design factors above.
- Topology optimally requires 4 links across sites. Optionally, can be reduced to 2 links. This is less than Option 1 requires.
- Each site is local fault-domain from L2 networking point of view, as L2 fault-domain is not stretched across sites. L2 faults (STP, broadcast storms, unknown unicast flooding, etc.) are isolated within the site.
- More complex routing configuration with ECMP and dynamic routing protocol like iBGP or OSPF
- Four routers. Two per site.
- L3 topology across sites restricts workload (device, VM, container, etc.) migrations across sites without L2 over L3 network overlay (NSX, SmartFabric, etc.) or changing the IP address of migrated workload.
Conclusion and Design Decision
- Do I have VMware NSX in my toolbox or not?
- What is the skillset level of network operators (Dynamic Routing, ECMP, VRRP) responsible for the operation?
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