What is a bridge interface; why is STP important?

Short answer

A bridge is a software switch that connects more than one interface to the same L2 segment (broadcast domain); frames are forwarded between the members according to the MAC address, and the member interfaces become part of a single network. In TR7, a Bridge is created on the Network → Interfaces screen by selecting Type = Bridge in the Add Interface form and specifying the members in the Bridge Members field; the members must be in the same route table as the bridge. STP (Spanning Tree Protocol, IEEE 802.1D) is the protocol that prevents, by blocking some ports, the loops that form when the same L2 segment is connected to over more than one path; if a loop forms, broadcast frames circulate endlessly (broadcast storm) and the network becomes unusable. There is no STP setting in TR7's Bridge form; TR7 creates the bridge without running STP. For this reason, it is made sure that no loop will form on the switch ports to which the bridge members are connected, and loop protection is provided by the STP configuration on the switch side.

Where It Is Used in TR7

Where	Field / label	What for
Network → Interfaces → Actions → Add Interface	Type = Bridge, Bridge Members	Connecting interfaces to the same L2 segment
Bridge interface → Add IP	Add IP	TR7 having an address on the bridged network
Network → Interfaces → Network Topology view	Interfaces connected to the bridge (Network topology view)	Seeing which interface is a member of which bridge
WEB-CLI	network interface add (type bridge, --bridgeMemberIds) (network interface)	Creating the bridge from the command line

Bridge Rules in TR7

• Members must be in the same route table as the bridge; otherwise the record is rejected with Must be in the same namespace.
• An interface can be a member of only one bridge at a time; Bond/LACP members and protected interfaces (those that carry a management access IP, the cluster sync interface) cannot be added to a bridge.
• A member interface cannot be moved to another route table (… is a bridge member); when the bridge is deleted, the members are released (Bridge member interfaces will be released: …).
• The form has no fields such as STP, bridge priority or port cost.

Common Misconceptions

• "A bridge performs routing between two networks." A bridge works at L2; it makes its members a single network segment. Routes are used for forwarding between networks.
• "Loop protection is turned on in TR7." TR7 creates the bridge without running STP; if two members are connected to the same switch network, a loop can form. Protection is provided by STP on the switch side.
• "Bridge and Bond do the same job." Bond/LACP combines ports that go to the same destination into a single connection; Bridge brings different segments together in a single broadcast domain (What is the difference between Bond and LACP?).

How it is done in the documentation

• Add Interface · Bridge
• Add Interface · Bridge Members
• Add Interface · Best Practices

Related questions

• What is an interface; what is the difference between a physical (ETH) interface and a virtual interface?
• What is the difference between unicast, multicast and broadcast?
• What is the difference between Bond and LACP?