Definitions of Managed Objects for Bridges
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Extracted elements (29)
A separate Timeout data type (1/100 s units) was defined instead of using the BPDU wire units (1/256 s) to avoid ambiguity and value distortion caused by repeated unit conversions across management protocol boundaries. A specific algorithm (multiply-then-divide with floor/ceiling) is mandated.
dot1dBasePortCircuit (an OBJECT IDENTIFIER) was added to disambiguate ports that share the same dot1dBasePortIfIndex value, e.g., multiple ports each corresponding to one X.25 virtual circuit on a single interface. Ports with a unique ifIndex set this to {0 0}.
dot1dSrPortLargestFrame and dot1dTpPortMaxInfo cannot reuse ifMtu because ifMtu is defined as the size available to the network layer, which may differ from the MAC-layer INFO field size when multiple encapsulation layers are used.
Several IEEE 802.1d management objects were omitted because they duplicate MIB-II objects: BridgeName≡sysDescr, BridgeUpTime≡sysUpTime, PortAddresses≡ifPhysAddress, PortName≡ifDescr, PortType≡ifType, and port Uptime≡ifLastChange. BridgeIdentifier and PortIdentifier are derivable by combining MIB fields.
The MIB was deliberately kept minimal by applying six criteria: start small; require objects be essential for fault or configuration management; consider evidence of current use; limit total object count; exclude objects derivable from other MIBs; and instrument at most one counter per critical section per layer.
dot1dStpProtocolSpecification distinguishes DEC LANbridge 100 STP (decLb100(2)) from IEEE 802.1d (ieee8021d(3)). Future incompatible IEEE STP versions will require new enumeration values, so implementations should treat unknown values conservatively.
Port numbers have no mandatory relationship to interface numbers. In the simple case a port number equals the corresponding interface number, but multiple ports may map to the same interface (e.g., X.25 virtual circuits). Port numbers are in range 1..dot1dBaseNumPorts.
The Bridge MIB counters and objects apply only to the subset of interface traffic belonging to bridged protocols. In a device that also performs IP routing, only non-IP frames fall within the bridging domain and are counted.
Implementation of the dot1dBase group is mandatory for all bridges. It contains objects applicable to all bridge types regardless of bridging mode.
dot1dBase group (dot1dBridge 1) contains dot1dBaseBridgeAddress, dot1dBaseNumPorts, dot1dBaseType, and dot1dBasePortTable — the core objects applicable to all bridge types including transparent, source route, and SRT bridges.
dot1dSr group (dot1dBridge 3) is optional and implemented by source route and SRT bridges. dot1dSrPortTable contains per-port objects for hop count, local/target segment numbers, bridge number, largest frame size, STE span mode, and frame counters.
dot1dSrPortLargestFrame specifies the maximum INFO field size (LLC and above, not including MAC framing) for source routing ports. Valid values defined by 802.5 source routing are: 516, 1500, 2052, 4472, 8144, 11407, 17800, and 65535 octets.
dot1dSrPortSTESpanMode controls Spanning Tree Explorer frame handling per port: auto-span(1) delegates to dot1dStpPortState, disabled(2) silently discards all STE frames, forced(3) always accepts and propagates STE frames for manual spanning tree configuration.
dot1dSrPortTable tracks per-port source routing statistics: specInFrames/specOutFrames (specifically routed), apeInFrames/apeOutFrames (all-paths explorer), steInFrames/steOutFrames (spanning tree explorer), and discard counters for segment mismatch, duplicate segment, and hop count exceeded.
dot1dStatic group (dot1dBridge 5) is optional and implemented by bridges performing destination-address filtering. dot1dStaticTable is indexed by (dot1dStaticAddress, dot1dStaticReceivePort) and specifies the allowed output port bitmask and entry lifetime.
dot1dStaticAllowedToGoTo is an OCTET STRING bitmask where each bit represents one bridge port (first octet = ports 1–8, MSB = lowest port number). A bit value of 1 permits forwarding to that port; the bit for the receive port is irrelevant.
dot1dStaticStatus controls entry lifetime with values: other(1), invalid(2) (write to delete), permanent(3) (survives reset), deleteOnReset(4), and deleteOnTimeout(5) (ages out).
dot1dStp group (dot1dBridge 2) is optional and implemented only by bridges supporting the Spanning Tree Protocol. It covers global STP state (priority, root, timers) and a per-port table with state, path cost, and designated bridge information.
dot1dStpPortTable provides per-port STP management: port number, priority, state, enable/disable, path cost, designated root (BridgeId), designated cost, designated bridge (BridgeId), designated port (2-octet OCTET STRING), and forwarding transition counter.
dot1dTp group (dot1dBridge 4) is optional and implemented by transparent and SRT bridges. It includes dot1dTpLearnedEntryDiscards, dot1dTpAgingTime, the forwarding database table (dot1dTpFdbTable), and a per-port frame counter table.
dot1dTpFdbTable is the forwarding database for transparent bridges, indexed by dot1dTpFdbAddress (MacAddress). Each entry carries a port number and a status: other(1), invalid(2), learned(3), self(4), or mgmt(5).
newRoot trap (enterprise dot1dBridge, trap type 1) is sent by a bridge soon after it is elected as the new Spanning Tree root, e.g., upon expiration of the Topology Change Timer.
STP bridge-level timer objects: dot1dStpBridgeMaxAge (range 600–4000 in 1/100 s), dot1dStpBridgeHelloTime (100–1000), and dot1dStpBridgeForwardDelay (400–3000) are read-write and represent the values this bridge would advertise if it became root. An agent MAY return badValue for non-whole-second values.
topologyChange trap (enterprise dot1dBridge, trap type 2) is sent when any configured port transitions from Learning to Forwarding or from Forwarding to Blocking. It is not sent if a newRoot trap is sent for the same transition.
Security issues are not discussed in this memo. The document provides no guidance on access control for read-write MIB objects such as STP timers, port enable/disable, aging time, or static filtering entries, all of which could be used to disrupt bridging if modified by unauthorized parties.
dot1dStpPortState defines six STP port states: disabled(1), blocking(2), listening(3), learning(4), forwarding(5), and broken(6). The bridge places a malfunctioning port into broken(6); administratively disabled ports report disabled(1).
BridgeId is OCTET STRING (SIZE 8): the first two octets (network byte order) hold the priority value, and the last six octets hold the MAC address used to uniquely identify the bridge in the Spanning Tree Protocol.
MacAddress is defined as OCTET STRING (SIZE 6) in 'canonical' order (least-significant-bit first per IEEE 802.1a), even though 802.5 transmits MAC addresses most-significant-bit first. 16-bit addresses are represented by setting the upper 4 octets to zero.
Timeout is INTEGER in units of 1/100 seconds used for all STP timer objects in this MIB. Conversion to BPDU units (1/256 s) uses floor((n*256)/100); reverse conversion uses ceiling((b*100)/256). Multiplication must precede division to avoid distortion.