Flat Networks and the Challenges of Spanning Tree Protocol (STP)
I recently read Deep Dive: Flat Networks are the Future, from Network World, and it reminded me of another challenge with STP—network behavior when links fail.
When failures occur, the spanning tree needs to be redefined. This can take anywhere from five seconds with Rapid Spanning Tree (RSTP) up to several minutes with STP—and this convergence can vary unpredictably even with small topology changes. The demands for non-stop traffic flow increases with server virtualization, and consequently network convergence times have to shrink. STP does not provide an adequate solution for these requirements.
Finally, when a spanning tree is reconverging, broadcast storms can occur and result in network slowdown. All of these limitations of STP are why Layer 2 networks are typically kept small in the data center.
In contrast, consider the benefits of a Layer 2 network that:
- Is highly available
- Guarantees high-bandwidth utilization over equal-cost paths
- Does not stall traffic when links are added or removed due to failure or network reconfiguration
- Makes latency deterministic and is lossless
- Can transport IP and mission-critical storage traffic over the same wire
VM mobility can occur within a cluster of physical servers that are in the same IP subnet and Ethernet VLAN. This is required for the migration to be non-disruptive to client traffic as changes in the IP subnet are necessarily disruptive. The solution for flexible VM mobility is a more scalable and available Layer 2 network with higher network bandwidth utilization.
For a VM to migrate from one server to another, many server attributes must be the same on the origination and destination servers. This extends into the network as well, requiring VLAN, Access Control List (ACL), Quality of Service (QoS), and security profiles to be the same on both the source and destination access switch ports. Organizations could map all settings to all network ports, but that would violate most networking and security best practices. The distributed virtual switch in VMware vSphere 4 addresses some of these issues, but at the cost of consuming physical server resources for switching, added complexity in administering network policies at multiple switch tiers, and a lack of consistent security enforcement for VM-to-VM traffic.
Now, consider again a Layer 2 network that:
- Places no physical barriers in the way of VM migration
- Is aware of VM locations and consistently applied network policies
- Does not require manual intervention when a VM moves
- Removes the overhead of switching traffic from the hypervisor for maximum efficiency and functionality
- Supports heterogeneous server virtualization in the same network