network connections form loops and STP

 To build a loop-free topology, switches (bridges) determine the root bridge and compute the port roles. To do this, the bridges use special data frames called Bridge Protocol Data Units (BPDUs) that exchange bridge IDs and root path cost information. BPDUs are exchanged regularly, typically at two second intervals, and enable switches to keep track of network topology changes and to start and stop forwarding on ports as required. Hosts should not send BPDUs to the switch ports and to avoid malfunctioning/malicious hosts from doing so, the switch can filter or block BPDUs. If you enable BPDU filtering on a port, BPDUs received on that port are dropped but other network traffic is forwarded as usual.  If you enable BPDU blocking on a port, BPDUs received on that port are dropped and the port is shut down.

Port Roles in RSTP

Root Port (one per bridge): The forwarding port on each bridge which is on the best path to reach the root bridge.

Designated Port: The forwarding port for each LAN segment that leads away from the root bridge.

Alternate Port: An alternative path to the root bridge on a particular LAN segment, which is part of a bridge other than the one that has  a designed port for the LAN segment.  Alternate port is the second best root port.

Backup port: A backup/redundant port for the segment that already has one designated port. This port leads away from the root port.

Disabled: A port which is manually disabled and is not a part of STP.

Port States in RSTP

Discarding: No data is exchanged over the port.

Learning: Frames are not forwarded, but the MAC address table is populated.

Forwarding: Fully functional.

Switches in RSTP expect a BPDU every 2 seconds (hello time) and if they do not receive a BPDU for 6 seconds (3 hello time intervals), it is considered to be a link failure. This is significantly faster than the STP link failure detection time of 20 seconds, dictated by the max age timer. RSTP can actively confirm if a port can safely be transitioned to the forwarding state without having to rely on the timer mechanism. Ports can be configured as edge ports if they are attached to a LAN that has no other bridges connected to it. Such a port can transition directly to the forwarding state, but it loses the edge port status as soon as it receives a BPDU. RSTP achieves rapid transition to the forwarding state on edge ports and point-to-point links (operating in full-duplex mode) but not on shared links (i.e., ports connected to a shared medium, hence operating in. half-duplex mode)

If network connections form loops and STP is disabled, packets are forwarded indefinitely across the switches, causing degradation of network performance. STP supports limited Layer 2 multipathing and can result in sub-optimal utilization of available network links. Therefore, a fabric of switches does not rely only on RSTP within the boundaries of the network. Pluribus Networks recommends the use of RSTP for ad hoc networks that inter-operate in a heterogeneous, multi-vendor switch environment.

More about : network security level


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