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    Spanning Tree Protocol

    How the Spanning Tree protocol organizes an Ethernet network

    Article from ADMIN 22/2014
    Ethernet is so popular because it simply works and is inexpensive. However, the administration side looks a bit more complicated: For the network to run smoothly, the admin might need to make important decisions about the Spanning Tree protocol.

    Organizations today often divide their local networks using switches, which operate Layer 2 of the OSI model, rather than the (Layer 3) routers for which the TCP/IP network protocol system was originally designed. Switches are inexpensive and quite effective for reducing traffic and subdividing the network, but, in fact, a switch is still operating at the Ethernet level and doesn't have access to the time-to-live (TTL) settings, logical addressing, and sophisticated routing protocols used with routers.

    Networks that are subdivided with multiple switches could theoretically experience a situation where an Ethernet frame loops continuously around the network, forwarded endlessly through the circle of switches  – but they don't. One of the main reasons why they don't is the Spanning Tree protocol, which was specifically designed to address this problem of Ethernet loops and has been adopted (and adapted) by many of the biggest switch vendors.

    Spanning Tree prevents network loops from occurring and causing broadcast storms that would very quickly overload a network segment. The Spanning Tree protocol performs this magic by disabling certain connections between switches. Modern switches use a spanning tree to determine routes through the switched network – and to close off routes that could potentially cause a loop. The devices are designed to map these routes automatically, but if you need to do some troubleshooting or address performance issues on your Ethernet network, it helps to have some basic knowledge of Spanning Tree.

    Understanding Spanning Tree

    Figure 1 shows three examples (A, B, C) of topologies that would lead to the collapse of a network without a spanning tree blocking some switch ports.

    Figure 1: The Spanning Tree protocol prevents loops in network topologies.

    Variant A in Figure 1 offers no technical advantages and usually only arises from carelessness. Variants B and C, however, are often desired in order to increase the bandwidth between two switches (Variant B) or to achieve redundancy in the event of a cable failure caused by external influences – for example, construction work between two buildings – (Variant C). The block by the spanning tree in Variant B initially lifts bandwidth aggregation.

    Switch manufacturers have developed different, sometimes mutually incompatible, solutions, such as EtherChannel, Port Channel, virtual PortChannel (Cisco), and Trunk (HP). These technologies bundle multiple physical Ethernet connections to create a single logical connection (variant D in Figure 1).

    Spanning Tree perceives this logical connection, say, as a 2Gb connection rather than two separate 1Gb connections and therefore does not block either of the two. Variant C is easy to achieve, but it is not optimized. It would be a good idea to change the physical cabling to replace the triangular connection.

    For each VLAN (i.e., logical network segment), a separate spanning tree is computed. The Spanning Tree protocol is primarily known today by its three most popular variants: the Rapid Spanning Tree (RST) protocol, the Rapid per VLAN Spanning Tree (RPVST) protocol, and the Multiple Spanning Tree (MST) protocol; the original Spanning Tree variant suffers from high convergence times in complex topologies, thus leading to failures of 30 to 50 seconds.

    The spanning tree table is computed in three main steps:

    • Election of the root bridge
    • Election of root ports
    • Election of the designated ports

    A switch port can assume different roles and states (see the box titled "Roles and States of Switch Ports"). The Rapid Spanning Tree variant bundles the states Disabled, Blocking, and Listening as Discarding.

    Roles and States of Switch Ports

    In a spanning tree, each port on a switch assumes one of four possible roles:

    • Root port – Active switch port whose upstream points toward the root bridge. Each switch has one root port at the most.
    • Designated port – Active port pointing away from the root bridge (downstream).
    • Alternate port (only with Rapid Spanning Tree) – Also points to the root bridge but is not active (Blocked).
    • Backup port (only with Rapid Spanning Tree) – A connection to another switch that can be reached more quickly via another switch port; again inactive (Blocked).

    In addition to these roles, a port can inhabit four states: Blocking, Listening, Learning, and Forwarding. The additional Disabled state calls for a port to discard all packets. Packets are also discarded in the Blocking state, in which a port exclusively receives and processes Bridge Protocol Data Units (BPDUs) that carry information about the network and the spanning tree. In the Listening and Learning states, a port forwards BPDUs. In the Learning state, the port learns the addresses of other network nodes. Only in Forwarding mode will a port also transmit data packets.

    The root bridge is the reference point for the entire spanning tree and computes the paths and settings for the tree. The root bridge is elected on the basis of the Bridge ID, which is composed of three components:

    • Bridge priority
    • System ID extension
    • MAC address

    The bridge priority is configurable in steps of 4096. The system ID and the MAC address are not freely selectable; MAC address changes are possible but not recommended. The higher the value of the bridge priority, the less useful the switch is as the root bridge. A switch with a bridge priority of 0 will most likely become the root bridge, unless another switch in the network also has this value.

    If more than one switch has the same minimum bridge priority, the system ID extension and the MAC address quickly settle the election of the root bridge. The candidate with the lowest values wins. The system ID extension corresponds to the VLAN number.

    Older switches often have lower MAC addresses than their more recent successors, because many manufacturers assign MAC addresses in ascending order. A lower address leads to the risk of the oldest and potentially weakest switch becoming the root bridge. Because spanning tree calculations are time-consuming on large networks, a powerful switch should take over the role of the root bridge. The vendor design guides recommend setting up the root bridge at aggregation and distribution level, to achieve short convergence times in case of failures (Figure 2).

    Figure 2: The root bridge of a network should reside in the aggregation/distribution level of the network hierarchy.

    On Cisco switches, the bridge priority is configured using spanning-tree vlan <VLAN> priority <prior>, or alternatively, with the root bridge macro spanning-tree vlan <VLAN> root [primary **| secondary]. This macro automatically configures the priority based on the current root bridge in the network, but it runs only once when called, rather than permanently in the background.

    A Revolt Against the Root Bridge

    The exchange of information about the spanning tree topology between different switches is handled by BPDUs (Bridge Protocol Data Unit). Each switch port sends and receives BPDUs. However, this property allows remote attackers to discover and change the topology with forged BPDUs. Countermeasures include BPDU Guards and Filters (see the box titled "BPDU Guards and Filters").

    BPDU Guards and Filters

    • BPDU guard disables a switch port when it receives a BPDU packet. The cause can be an attack or unauthorized connection of a switch. Optionally, BPDU Guard automatically enables a disabled switch port with a timer after a certain time. BPDU Guard should be configured on each access switch port. This is done in the interface with spanning-tree enable bpduguard or globally with spanning-tree portfast bpduguarddefault.
    • BPDU filtering prevents a switch sending BPDUs on a switch port. This feature should also be enabled on every access port by typing spanning-tree bpdu-filtering enable in the interface, or globally with spanning-tree portfast default bpdufilter.

    Even after the root bridge is selected, and the entire spanning tree is active, additional switches can join the network. For example, you might wish to install a new horizontal distribution switch. If the new device has a lower bridge priority, it becomes the new root bridge. This change could affect the entire network topology and possibly lead to suboptimal paths and performance bottlenecks.

    Protection against accidental or malicious changes to the root bridge are prevented by Root Guard [1]. Figure  3 shows the starting situation in which Switch D is added to the network. If this switch is a lower bridge priority than the previous root bridge, the topology changes, as shown in Figure 4. To prevent this change, configure Root Guard on the port on Switch C that points in the direction of Switch D. Root Guard disables the switch port once it receives a BPDU with too low of a bridge priority.

    Figure 3: If the new Switch D has a lower bridge priority than Switch A …
    Figure 4: … the Spanning Tree network topology changes.

    Cost, Cost, Cost

    Connection costs play an important role in choosing the root ports and designated ports. The spanning tree standard costs for different bandwidths are shown in Table 1. However, the predefined values lead to a 40Gb and a 100Gb connection having the same spanning tree cost as a port channel comprising two 10Gb connections, in which the cost is calculated as the total bandwidth of bundled connections. To respond more accurately in such situations, you can configure the spanning tree costs for a connection manually for each port.

    Table 1

    Connection Costs

    Bandwidth Costs
    10 Mbps 100
    16 Mbps  62
    100 Mbps  19
    200 Mbps  12
    622 Mbps  6
    1 Gbps  4
    10 Gbps  2
    20+ Gbps  1

    Figure 5 shows a topology example with four 100Mbps switches, in which Switch 4 becomes the root bridge. Because all of the connections have a speed of 100Mbps, the cost for each port is 19, as shown in Table 1. The only exception is the port channel between Switch 3 and Switch 4, which offers a total of 200Mbps and therefore has a cost value of 12.

    Figure 5: For 100Mb lines between all ports, the value of 19 applies to all route costs, except for the bundled cables between Switches 3 and 4.

    Based on these costs, it follows that the root port on Switch 1 (abbreviated in the figure to RP) points in the direction of Switch 3. The opposite switch port becomes the designated port (abbreviated DP) because the cost is only 31 (19+12) on this path, compared with 38 (19+19) via Switch 2.

    If there were a port channel between Switch 3 and Switch 4, two equally expensive routes would lead from Switch 1 to the root bridge (Switch  4). In this case, the bridge IDs would be used to calculate the path, where the lower ID is authoritative.

    All switch ports on the root bridge (Switch 4) are designated ports, as well as all ports that are opposite to a root port. In the connection between Switch 1 and Switch 2, the port roles are determined – with the possible options Designated and Blocked – based on the cost of path to the root bridge. For Switch 2, the cost is 19, but Switch 1 has a cost of 31. Thus, the ports between Switch 1 and 2 all become blocked or designated ports. This applies in kind to the connection between Switches 2 and 3: the path cost of Switch 3 to the root bridge is 12; thus, the port pointing in the direction of Switch 2 becomes the designated port.

    All of these spanning tree calculations result in blocks between Switches  1 and 2, and between 2 and 3, as shown in Figure 5. Communication between 1 and 2 is therefore routed through Switches 3 and 4.

    The topology shown in Figure 5 demonstrates that the location of the root bridge in the network plays an important role. In addition to optimal routes, admins also need to consider the need to recompute the spanning tree in case of switch failures; for optimal speed, the root bridge should be a switch that is as powerful as possible.

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