Ethernet Bridge, STP/RSTP STP/RSTP and Provider Bridge Configuration
STP/RSTP and Provider Bridge
Configuration
Spanning-Tree Protocol (STP) is a link management protocol used in Ethernet bridged
networks to provide path redundancy while preventing undesirable loops in the network. This
is done by verifying that only one active path exists at any one time between two stations
since multiple active paths between stations cause loops in the network. Rapid Spanning-Tree
Protocol (RSTP) evolved on the basis of STP and provides faster recovery of connectivity
after an outage.
Another standard, IEEE 802.1ad (Provider Bridge), further extends STP/RSTP usage by
enabling differentiation between STP/RSTP messages from the Customer Bridges and those
from the Provider Bridges. This is done through the allocation of different MAC address
space according to the type of bridge (Customer or Provider).
Spanning tree algorithm-aware bridges exchange configuration messages periodically. The
configuration message is a multicast frame called BPDU (Bridge Protocol Data Unit) or
Hello message. According to the BPDU, these STP-aware bridges will construct a loop free
network with a tree architecture.
NOTE: The only difference between STP and RSTP implementation, is defining the addressing space
which allows the coexistence of fully separated Customer and Provider loopless topologies.
Actelis ML devices support STP/RSTP in accordance to either Customer Bridge (IEEE
802.1d) or Provider Bridge (IEEE 802.1ad) standards, where STP/RSTP BPDU Address is
configurable per Network Element:
IEEE 802.1d uses the reserved MAC 0x01-80-0C-00-00-00 for STP/RSTP BPDU.
IEEE 802.1ad uses the Reserved MAC 0x01-80-0C-00-00-08 for STP/RSTP BPDU.
ML device STP/RSTP configuration can be set at bridge level and at port levels. By default,
STP is disabled at bridge level and enabled at port levels.
The two reserved MACs are additionally controlled by L2CP application. When L2CP is
configured to DROP or TUNNEL, the reserved MAC, STP application is not triggered. The
STP BPDU behavior that is described below, is valid only when L2CP control (port level
configurable) for the chosen reserved MAC (0x01-80-0C-00-00-00 or 0x01-80-0C-00-00-08)
is set to PEER handler. PEER handler accepts BPDU locally on NE, and performs according
to the application configuration.
Table 18: STP Configuration Description
Bridge and Port level STP is
Disabled
Does not Participate in STP. BPDUs are dropped.
Bridge and Port level STP is
Enabled
Participates in STP. BPDUs are accepted and answered.
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