6 Common Spanning Tree Mistakes and How to Avoid Them

Photo: slgckgc on Flickr

6 Common Spanning Tree Mistakes and How to Avoid Them

Let me start by saying that spanning tree is a Good Thing. It saves you from loops, which will completely shut down a network. But it has to be configured properly to work properly. I can’t count the number of times I’ve had a client call me, desperate with a terribly broken network, and I’ve responded, “Sounds like a spanning tree problem.”

There are many ways things can go wrong with spanning tree. In this article I’ve collected a few of the recurring themes.

  1. Not configuring spanning tree at all
  2. As I said, spanning tree is a good thing. But for some reason, a lot of switch vendors disable it by default. So out of the box, you might have to enable the protocol.

    Sometimes people deliberately disable spanning tree. The most common reason for disabling spanning tree is that the original 802.1D Spanning Tree Protocol (STP) goes through a fairly lengthy wait period from the time a port becomes electrically active to when it starts to pass traffic. This wait period, typically 45 seconds, is long enough that DHCP can give up trying to get an IP address for this new device.

    One solution to the problem is to simply disable spanning tree on the switch. This is the wrong solution.

    The right solution is to configure a feature called PortFast on Cisco switches. (Most switch vendors have a similar feature.) You configure the command “spanning-tree portfast” on all the ports connecting to end devices like workstations. They then automatically bypass the wait period and DHCP works properly.

    It’s important to only configure this command on ports that connect to end devices though. Ports connecting to other switches need to exchange spanning tree information.

  3. Letting the network pick your root bridge
  4. As the name suggests, spanning tree resolves loops in your network by creating a logical tree structure between the switches. One switch becomes the root of the tree, and is called the root bridge. All other switches then figure out the best path to get to the root bridge.

    If there are multiple paths, then on each switch, spanning tree selects the best path and puts all the other ports into a blocking state. In this way, there’s a single path between any two devices on the network, although it might be rather circuitous.

    Every switch taking part in spanning tree has a bridge priority. The switch with the lowest priority becomes the root bridge. If there’s a tie, then the switch with the lowest bridge ID number wins. The ID number is typically derived from a MAC address on the switch.

    The problem is that, by default, every switch has the same priority value (32768). So if you don’t manually configure a better (lower) bridge priority value on a particular switch, the network will simply select a root for you. Then Murphy’s Law applies. The resulting root bridge could be some tiny edge switch with slow uplinks and limited backplane resources.

    To make matters worse, a bad choice of root bridge can make the network less stable. If there’s a connectivity problem that takes any random switch off the network, spanning tree heals rather quickly. But if the root bridge goes down, or if the failure means that some switches no longer have a path to the root bridge, this constitutes a major topology change. A new root bridge needs to be selected. The entire network will freeze during this time and no packets can be forwarded.

    I always recommend making the core switch the root bridge. I also like to select a backup root bridge. If there are dual redundant core switches, then one is the root bridge and the other becomes my backup.

    Set the bridge priority on the primary root bridge to the best possible value—4096—and the backup root bridge to the next best value—8192. Why these funny numbers? Well, that’s a longer story that we don’t have space for here, but the lower order bits in the priority field have another purpose, so they aren’t available for use as priorities.

    spanning tree mistakes loops

    Spanning tree saves you from loops / Photo: darkday on Flickr

  5. Using legacy 802.1D
  6. The first open standard for spanning tree is called 802.1D. It’s one of the earliest standards in the IEEE 802 series of standards that includes the specifications for every type of Ethernet and Wi-Fi as well as a bunch of other protocols. It works well despite its age, and you’ll find this type of spanning tree on just about every switch. Any switch that doesn’t support 802.1D is only useful in small isolated environments, and should never be connected to any other switches.

    But there have been several important advancements to spanning tree since 802.1D. These
    improvements allow sub-second convergence following a link failure, as well as the ability to scale to larger networks and the ability to actually have different spanning tree topologies and different root bridges for different VLANs. So it makes a whole lot of sense to use them.

    Most modern Cisco switches default to a protocol called Per-VLAN RSTP. This stands for Rapid Spanning Tree Protocol. It automatically operates a separate spanning tree domain with a separate root bridge on every VLAN. In practice, it’s common to make the same switch the root bridge on all or most of the VLANs, though.

    The rapid feature or RSTP is what you’ll probably find most useful. This allows the network to recover from most failures in times on the order of 1 to 2 seconds. Multiple Instance Spanning Tree, or MST, is similar to RSTP. The main difference is that you can designate groups of VLANs that are all part of the same tree structure with a single common root bridge. However, I recommend using Per-VLAN RSTP in most cases because it’s easier to configure. Also, I’ve encountered some interoperabilty problems with MSTP between different switch vendors.

  7. Mixing spanning tree types
  8. It should be pretty clear from the descriptions of 802.1D, RSTP, and MST in the previous section that mixing them could get messy. The RSTP and MST protocols have rules for how to deal with this mixing, and in general it involves creating separate zones within the network for groups of switches running different flavours of spanning tree. This rarely results in the most efficient paths being selected between devices.

    The only really valid reason to mix spanning tree types is to allow the inclusion of legacy
    equipment that doesn’t support the more modern protocols. As time goes by, there should be fewer and fewer of these legacy devices, and the number of places where it makes sense to mix the protocols should becomes smaller.

    I recommend picking one, preferably RSTP or MST, and just using that in a consistent manner across all of your switches.

    spanning tree mistakes loops

    Photo: olle svensson on Flickr

  9. Using MST with pruned trunks
  10. Because MST allows a single spanning tree structure that supports multiple VLANs, you need to be extremely careful about your inter-switch trunks.

    I once had a client with a large complicated network involving many switches and many VLANs. They were running MST. For simplicity, they had designated a single MST instance, meaning that all VLANs were controlled by the same root bridge.

    The problem for this client arose when they decided that certain VLANs should only exist on certain switches for security reasons. All perfectly reasonable. So they removed the VLAN from the main inter-switch trunks, and added new special trunks just for these secure VLANs. And everything broke.

    MST considered all VLANs to be part of the same tree, and it selected which trunks to block and which to forward based on that assumption. But in this case, because some VLANs were only present on some trunks and other VLANs were present on the other trunks, blocking a trunk meant only passing some of the VLANs. Blocking the other trunk meant only passing the other set of VLANs. For the blocked VLANs there was simply no path to the root bridge at all.

    So, if you’re going to use MST, you need to either ensure that all VLANs are passed on all trunks, or you need to carefully and manually create different MST instances for each group of VLANs with special topological requirements. In other words, you have to do careful analysis and design the network properly. Or you could take the easy way out and run Per-VLAN RSTP.

  11. Conflicting root bridge and HSRP/VRRP
  12. Another common topological problem with spanning tree networks involves the way that Layer 2 and 3 redundancy mechanisms sometimes interact.

    Suppose I have a network core consisting of two Layer 3 switches. On each segment I want these core switches to act as redundant default gateways. And I want to connect all of the downstream switches redundantly to both core switches and make spanning tree remove the loops.

    In this scenario, the spanning tree root bridge for a particular VLAN might be on one of these core switches and HSRP/VRRP master default gateway on the other switch. Then an Ethernet frame originating on one of the downstream switches destined to the default gateway will need to take an extra hop, going first to the root bridge, and then to the secondary core switch that currently owns the default gateway IP.

    Normally this isn’t a problem, but imagine that I’m passing packets between two VLANs, both with Core Switch A as the root bridge and Core Switch B as the default gateway. Every packet must go up to Core Switch A, and cross the backbone link to get routed on Core Switch B.

    Then it has to cross the backbone link again to go back to Core Switch A to be delivered to its destination. All of the return packets must also cross the backbone link twice. This creates a massive traffic burden on the backbone link where every packet in both directions must cross twice. It also incurs a latency penalty as every packet needs to be serialized and transmitted twice. Even on 10Gbps links, this will typically cost a couple of microseconds in both directions, which could add up for particularly sensitive applications.

    Suppose instead that the default gateway was on the same switch as the root bridge. Now the packet goes up to the root bridge, Core Switch A, and gets routed between the VLANs and immediately switched out to the downstream device. It doesn’t cross the backbone at all in either direction.

Spanning tree is a terrifically important protocol. It allows us to build redundancy into inter-switch connections. It saves us from catastrophic loops when somebody accidentally connects things they shouldn’t.

It’s true spanning tree can be misconfigured with bad consequences, but this possibility shouldn’t discourage you from using it. The solution is to be careful and deliberate about your network design.

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Kevin Dooley

About Kevin Dooley

Kevin has 15+ years of experience as a network engineer. He has designed and implemented several of the largest and most sophisticated enterprise data networks in Canada and written several highly regarded books on networking for O'Reilly and Associates, including Designing Large-Scale LANs and Cisco IOS Cookbook. Kevin holds a Ph.D. in theoretical physics and numerous industry certifications.

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