Network engineers are all too familiar with the need for Layer 2 loop prevention and historically, that requirement has been fulfilled with the Spanning Tree Protocol, 802.1d, 802.1w, 802.1s, etc.
Spanning Tree Protocol or STP is a network protocol that builds a loop-free logical topology for ethernet networks and its basic function is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also allows a network design to include backup links providing fault tolerance if an active link fails. Unfortunately, many network engineers are also well aware of the headaches that come with planning and implementing a Spanning Tree Protocol solution.
Here’s a quick look at five alternatives you can use instead of Spanning Tree Protocol:
Route, Route, Route
By utilizing Layer 3 switches at the access layer where your edge devices reside, you can prevent Layer 2 loops by routing to the distribution layer. The routing protocols will inherently prevent Layer 3 loops, choose best path, and load share. Nice, right?
Local VLANs
If you have all the ports of any given VLAN (virtual LAN) isolated to a specific location, and do not have that VLAN span across multiple access layer devices via the distribution switches, that VLAN is not looped. A local VLAN solution, however, may not be feasible for all VLANs. For example, for ease of management and security, many administrators prefer having a guest VLAN or a VLAN for all printers span across the entire campus.
FlexLinks
Flexlinks simply have one of the two uplinks from an access switch to the distribution in standby mode, not utilized until the primary fails. FlexLinks, ironically, are not flexible and do not lend themselves to load sharing.
Multi-Chassis Link Aggregation
Multi-Chassis Link Aggregation or MC-LAG allows multiple switches to share a common switching fabric and in essence, appear to be a single switch. Therefore, the dual links from an access layer switch can be aggregated together across two different distribution layer switches acting as one. Standardization of MC-LAG has been slow and many vendors use their own proprietary methods.
Other Standards
There are two protocols being debated as the most implemented successor to the spanning tree protocol – TRILL (Transparent Interconnection of Lots of Links) and SPB (Shortest Path Bridging). Both protocols bring a Layer 3 routing philosophy to Layer 2. They are similar to a link state routing protocol where each switch is aware of every other switch in the network infrastructure.
In Conclusion
In some environments where redesigns are not an option and a Layer 2 implementation must be maintained, the Spanning Tree Protocol may be the best option. If this is the case, it is important that a more sophisticated, faster converging version of the Spanning Tree Protocol such as Rapid PVST (Rapid per VLAN Spanning Tree) or MST (Multiple Spanning Tree) be used, and that great care must be taken to ensure that the Spanning Tree Protocol is properly configured to provide an effective, fault-tolerant, and secure loop free network.
Training Resources:
Cisco Training
IT Training
