Definitions of Managed Objects for Bridges
obsoleted by
- rfc-4188 — Definitions of Managed Objects for Bridges
obsoletes
- rfc-1286 — Definitions of Managed Objects for Bridges
Extracted elements (31)
dot1dTpPortMaxInfo uses a separate object rather than reusing ifMtu because ifMtu is defined as the size available to the network layer, which can differ from the actual MAC INFO field size — especially when multiple layers of encapsulation are present.
Several IEEE 802.1d objects were excluded as redundant with MIB-II: BridgeName ≡ sysDescr, BridgeUpTime ≡ sysUpTime, PortAddresses ≡ ifPhysAddress, PortName ≡ ifDescr, PortType ≡ ifType. The BridgeIdentifier is derivable by combining dot1dStpPriority and dot1dBaseBridgeAddress. Transitory state flags (TopologyChange, TopologyChangeAcknowledged) were also excluded as not useful.
The MIB was kept minimal by applying six criteria: start with a small essential set; require objects to be needed for fault or configuration management; consider evidence of actual use; limit total object count; exclude objects derivable from other MIBs; and instrument critical sections with at most one counter per layer.
Port numbers have no mandatory relationship to interface numbers; in simple cases they match, but multiple ports may share one ifIndex (e.g., multiple X.25 virtual circuits on one interface). The dot1dBasePortCircuit object disambiguates such cases with a unique OID per port.
A bridge implementing this MIB is assumed to also implement at least the 'system' group and the 'interfaces' group from MIB-II. Bridge MIB counters apply only to the subset of interface data for protocols being bridged, not all traffic on the interface.
Implementation of the dot1dBase group is mandatory for all bridges. This group contains objects applicable to all bridge types regardless of bridging mode.
Implementation of the dot1dStp group is optional; it is implemented only by bridges that support the Spanning Tree Protocol. Bridges not implementing STP omit this group entirely.
Implementation of the dot1dTp group is optional; it is implemented only by bridges that support the transparent bridging mode. Both transparent-only and SRT bridges implement this group.
Configurable STP root timer objects — dot1dStpBridgeMaxAge (range 600..4000), dot1dStpBridgeHelloTime (100..1000), dot1dStpBridgeForwardDelay (400..3000), all in hundredths of a second — specify values this bridge would advertise if elected root. Agents may return badValue for non-whole-second values.
dot1dBaseBridgeAddress (dot1dBase 1) is a read-only MacAddress that uniquely identifies the bridge. It is recommended to be the numerically smallest MAC address of all ports, but is only required to be unique. Concatenated with dot1dStpPriority it forms the BridgeIdentifier used in STP.
dot1dBasePortCircuit (dot1dBasePortEntry 3) is an OBJECT IDENTIFIER that disambiguates ports sharing the same dot1dBasePortIfIndex, for example multiple X.25 virtual circuits on one interface. Ports with a unique ifIndex may set this to { 0 0 }.
dot1dBasePortDelayExceededDiscards and dot1dBasePortMtuExceededDiscards are per-port Counters tracking frames discarded due to excessive transit delay and excessive frame size, respectively. Both are incremented by transparent and source route bridges.
dot1dBaseType (dot1dBase 3) is a read-only INTEGER indicating bridge capability: unknown(1), transparent-only(2), sourceroute-only(3), or srt(4). Whether a specific bridging mode is actually active is confirmed by the presence of entries in mode-specific port tables.
dot1dStaticAllowedToGoTo is a read-write OCTET STRING bitmask specifying which ports may receive a frame matching the static filter entry. Each octet covers eight ports; within each octet, MSB represents the lowest-numbered port. The default is a string of all ones (forward to all ports).
dot1dStaticStatus defines five lifecycle states for static filter entries: other(1), invalid(2) which removes the entry when written, permanent(3) which survives bridge resets, deleteOnReset(4), and deleteOnTimeout(5). The default is permanent(3).
dot1dStpForwardDelay (dot1dStp 11) controls how long a port stays in each of the Listening and Learning states before transitioning to Forwarding, in hundredths of a second. It is also used to age all dynamic forwarding database entries when a topology change is underway.
dot1dStpPortForwardTransitions (dot1dStpPortEntry 10) is a Counter tracking the number of times this port has transitioned from the Learning state to the Forwarding state, useful for diagnosing STP instability.
dot1dStpPortPathCost (dot1dStpPortEntry 5) is a read-write INTEGER (1..65535) representing this port's contribution to the path cost toward the STP root. IEEE 802.1D-1990 recommends the default be inversely proportional to the attached LAN speed.
dot1dStpPriority (dot1dStp 2) is the read-write two-octet writable portion of the Bridge ID, range 0..65535. Combined with dot1dBaseBridgeAddress it forms the full 8-octet BridgeIdentifier used in STP Configuration BPDUs.
dot1dTpAgingTime (dot1dTp 2) is a read-write INTEGER (10..1000000 seconds) specifying the timeout for aging out dynamically learned forwarding entries. IEEE 802.1D-1990 recommends a default of 300 seconds.
dot1dTpFdbStatus defines five forwarding database entry states: other(1), invalid(2) for aged-out entries not yet flushed, learned(3) for dynamically learned active entries, self(4) for the bridge's own addresses, and mgmt(5) for entries also present in the static table.
dot1dTpFdbTable is the Forwarding Database for transparent bridges, indexed by unicast MAC address (dot1dTpFdbAddress). Each entry also records dot1dTpFdbPort (port number or 0 if unknown) and dot1dTpFdbStatus.
dot1dTpLearnedEntryDiscards (dot1dTp 1) counts forwarding database entries discarded due to insufficient storage space. A persistently increasing value indicates the FDB is regularly full, which has 'unpleasant performance effects on the subnetwork.'
The newRoot SNMP trap (TRAP-TYPE ::= 1, enterprise dot1dBridge) is sent by a bridge shortly after it is elected the new STP root, e.g., upon expiration of the Topology Change Timer. Implementation is optional.
The topologyChange SNMP trap (TRAP-TYPE ::= 2, enterprise dot1dBridge) is sent when any configured port transitions from Learning to Forwarding or from Forwarding to Blocking. It is suppressed when a newRoot trap is sent for the same transition. Implementation is optional.
The Bridge MIB is rooted at OID { mib-2 17 } (dot1dBridge), with five subgroups: dot1dBase (1), dot1dStp (2), dot1dSr (3, separately documented), dot1dTp (4), and dot1dStatic (5). This document obsoletes RFC 1286.
The RFC explicitly states that 'Security issues are not discussed in this memo.' No authentication, access control, or integrity mechanisms for MIB object access are specified in this document.
STP port state (dot1dStpPortState) defines six states: disabled(1), blocking(2), listening(3), learning(4), forwarding(5), broken(6). A port progresses from blocking through listening and learning toward forwarding; the bridge places a malfunctioning port into broken(6). Disabled ports report disabled(1).
BridgeId is an 8-octet OCTET STRING uniquely identifying a bridge for STP. The first 2 octets (network byte order) contain the priority value; the last 6 octets contain the bridge's MAC address (typically the numerically smallest port MAC address).
MacAddress is a 6-octet OCTET STRING encoded in 'canonical' order (least significant bit first, as defined by IEEE 802.1a), even for 802.5 which transmits MAC addresses MSB first. 16-bit addresses are represented by setting the upper 4 octets to zero (e.g., AAFF → 00000000AAFF).
Timeout is an INTEGER representing STP timer values in units of 1/100 seconds in the MIB, while BPDUs store them in 1/256 seconds. The conversion is: BPDU = floor((n*256)/100); MIB = ceiling((b*100)/256). Arithmetic must multiply before dividing to avoid distortion from multiple conversions.