This document explains the Spanning Tree Protocol (STP) root guard feature. This feature is one of the STP enhancements that Cisco created. This feature enhances switched network reliability, manageability, and security.
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This document is not restricted to specific software and hardware versions.
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The standard STP does not provide any means for the network administrator to securely enforce the topology of the switched Layer 2 (L2) network. A means to enforce topology can be especially important in networks with shared administrative control, where different administrative entities or companies control one switched network.
The forwarding topology of the switched network is calculated. The calculation is based on the root bridge position, among other parameters. Any switch can be the root bridge in a network. But a more optimal forwarding topology places the root bridge at a specific predetermined location. With the standard STP, any bridge in the network with a lower bridge ID takes the role of the root bridge. The administrator cannot enforce the position of the root bridge.
Note: The administrator can set the root bridge priority to 0 in an effort to secure the root bridge position. But there is no guarantee against a bridge with a priority of 0 and a lower MAC address.
The root guard feature provides a way to enforce the root bridge placement in the network.
The root guard ensures that the port on which root guard is enabled is the designated port. Normally, root bridge ports are all designated ports, unless two or more ports of the root bridge are connected together. If the bridge receives superior STP Bridge Protocol Data Units (BPDUs) on a root guard-enabled port, root guard moves this port to a root-inconsistent STP state. This root-inconsistent state is effectively equal to a listening state. No traffic is forwarded across this port. In this way, the root guard enforces the position of the root bridge.
The example in this section demonstrates how a rogue root bridge can cause problems on the network and how root guard can help.
In Figure 1, Switches A and B comprise the core of the network, and A is the root bridge for a VLAN. Switch C is an access layer switch. The link between B and C is blocking on the C side. The arrows show the flow of STP BPDUs.