Networking 101: Understanding Spanning Tree - Page 2

Learn the basics of the Spanning Tree Protocol (STP) and its drawbacks and alternatives with the ENP primer.

By Charlie Schluting | Updated May 29, 2014 / Posted Aug 14, 2007
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Updated by Paul Rubens

Rapid STP

In 2001, certain vendors started introducing rapid spanning tree, a modified version of the spanning tree algorithm that reduces outages. It's fully compatible with older devices that only know the old spanning tree algorithm and reduces the 30-50-second outage time to less than ten in most cases, so use it if you can.

Note: RSTP works by adding an alternative port and a backup port. These ports are allowed to immediately enter the forwarding state rather than passively wait for the network to converge.

VLANs and PVST

STP can cause problems with VLANs if one of the physical links happens to be a VLAN trunk. That's because with only one spanning tree, it's possible the link with the VLAN trunk will need to be blocked. That could result in no connectivity for a particular VLAN to the rest of its LAN. To solve this, enable per-VLAN spanning trees (PVST).

With PVST enabled, a bridge will run one spanning tree instance per VLAN on the bridge. If a trunk link contains VLANs 1, 2, and 3, it can then decide that VLANs 1 and 2 should not take that path, but still allow VLAN 3 to use it.

Spanning tree drawbacks

One of the drawbacks of STP is that even though there may be many physical or equal-cost multiple paths through your network from one node to another, all your traffic will flow along a single path that has been defined by a spanning tree. The benefit of this is that traffic loops are avoided, but there is a cost. Restricting traffic to this unique path means blocking alternative, and sometimes more direct, paths.

That means that your full potential network capacity can never be realized. (It is possible to use multiple simultaneous spanning trees for separate VLANs, as mentioned above, but the traffic in any given VLAN will still not be able to use all your available network capacity.)

In the past this has been acceptable, but with the increasing use of virtualization technology in many data centers, there is a need for a more efficient and reliable routing infrastructure that can handle the very high I/O demands of virtualized environments.

Spanning tree alternatives: TRILL and NPB

Transparent Interconnection of Lots of Links (TRILL) is a routing protocol network standard which:

  • Uses shortest path routing protocols instead of STP.
  • Works at Layer 2, so protocols such as FCoE can make use of it.
  • Supports multihopping environments.
  • Works with any network topology, and uses links that would otherwise have been blocked.
  • Can be used at the same time as STP.

The main benefit of TRILL is that it frees up capacity on your network which can't be used (to prevent routing loops) if you use STP, allowing your Ethernet frames to take the shortest path to their destination. This in turns mean more efficient utilization of network infrastructure and a decreased cost-to-benefit ratio.

These benefits are particularly important in data centers running cloud computing infrastructure. TRILL is also more stable than STP because it provides faster recovery time in the event of hardware failure.

In addition to TRILL, there is another, very similar, competing network standard called Shortest Path Bridging (NPB).

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