Spanning Tree Protocol Essentials for the CCNA Exam

Author: John Pherson

Abstract

The Spanning Tree Protocol (STP) was developed to solve the problems caused by loops in a bridged, or switched, topology. This white paper discusses how the STP is addressed in the CCNA exam. We can use the terms bridge and switch interchangeably in this white paper because they act in a similar fashion; switches are essentially very fast multi-port bridges.

The underlying cause of bridging loops is the fact that switches flood broadcast frames and frames with unknown destination MAC addresses. When there are loops in the topology, switches receive copies of the same frame on multiple ports which confuses the MAC table. It also causes switches to multiply the number of frame copies, since each switch floods the frames it is receiving. Each switch then starts receiving multiple copies of the frames and flooding the copies. This leads to broadcast storms and duplicate unicast frames on the network.

STP stops this process by first identifying a bridge, called a root bridge, to serve as a reference point for the topology. Once the root bridge is selected, each non-root bridge determines the lowest-cost path back to the root, and identifies the port associated with that path as the root port. Once each bridge determines its root port, it then evaluates any other paths to the root that will cause loops. For each of these paths a single bridge, called the designated bridge, is selected to provide connectivity for that segment back to the root. Any other switches on the same segment will be non-designated, and the ports on these bridges will be put in blocking state.

Blocking state does not mean the port is shut down; it simply means that no frames are allowed to be sent or received through that port. Spanning tree information continues to be received via that port from the designated bridge on that segment, which allows STP to communicate a change in topology should one occur.

The Bridge ID (BID)

Table 1. IEEE Spanning Tree Bridge ID

Each switch has a spanning-tree bridge ID composed of two parts, the priority and the MAC address. The MAC address is burned into the switch at the time of manufacture, and can't be changed. But the bridge ID as a whole can be modified by changing the priority assigned to the switch. Since STP always prefers the switch with the lowest bridge ID, lowering the priority value of a switch makes it more likely to become the root, and makes it more likely to be the designated switch for a segment. The spanning-tree vlan 1 root [primary, secondary, # value] command modifies the priority setting.

The default priority setting is 32768.

Table 2. Extended System-ID Bridge ID

Cisco's implementation of spanning-tree is done, by default, at the VLAN level. Each VLAN runs its own instance of spanning-tree. This is called PVST (per-vlan spanning-tree). To support this, Cisco modified the bridge ID to include a 12-bit VLAN ID field in the bridge ID which is called the Extended System ID. The rationale for this was that the priority field did not need the granularity provided by a 16-bit field, but instead could get by with the top four bits of the field, leaving 12 bits to express a VLAN ID. This is consistent with the VLAN ID field in a dot1q frame, which also has a 12-bit VLAN ID field. It means then, that the priority bits have decimal values of 4096, 8192, 16384, and 32768. Using this form of the bridge ID means that priorities are always expressed as some multiple of 4096.

Path and Port Cost

Table 3. Port Costs

The costs used by STP are derived from the above table, which assigns cost based on the port speed. A switch directly attached to the root looks at the port by which it is attached, and derives its path cost to the root from the speed of that port. These switches then advertise this path cost to their downstream neighbors. Each downstream switch adds the path cost advertised by the upstream switch to the cost associated with the local port receiving the information. The sum of these costs becomes their own path cost back to the root.

The table shows the current cost scheme used by IEEE (traditional 802.1d) Spanning Tree and Rapid Spanning- Tree protocols. This cost scheme supersedes an earlier cost scheme that ran out of headroom as port speeds became faster. It has also been superseded by a more recent cost scheme used on a variant of spanning-tree called Multiple Spanning-Tree, which is outside the scope of the CCNA content. You should be familiar with the costs listed above.

Related Courses

ICND1 - Interconnecting Cisco Network Devices 1
ICND2 - Interconnecting Cisco Network Devices 2
BCMSN - Building Cisco Multilayer Switched Networks v3.0