Erroneous interpretation of LLDP-MIB port numbers may cause wrong topology to be detected

[Morten Brekkevold]

Local port numbers, as referenced by the LLDP-MIB, can be either dot1dBasePort values or ifIndex values. The definition of the textual convention LldpPortNumber is a bit vague:

LldpPortNumber ::= TEXTUAL-CONVENTION
    DISPLAY-HINT "d"
    STATUS current
    DESCRIPTION
            "Each port contained in the chassis (that is known to the
            LLDP agent) is uniquely identified by a port number.

            A port number has no mandatory relationship to an
            InterfaceIndex object (of the interfaces MIB, IETF RFC 2863).
            If the LLDP agent is a IEEE 802.1D, IEEE 802.1Q bridge, the
            LldpPortNumber will have the same value as the dot1dBasePort
            object (defined in IETF RFC 1493) associated corresponding
            bridge port. If the system hosting LLDP agent is not an
            IEEE 802.1D or an IEEE 802.1Q bridge, the LldpPortNumber
            will have the same value as the corresponding interface's
            InterfaceIndex object.

            Port numbers should be in the range of 1 and 4096 since a
            particular port is also represented by the corresponding
            port number bit in LldpPortList."
    SYNTAX Integer32(1..4096)

This definition, has in turn, been used by NAV to justify interpreting local port numbers from the LLDP-MIB as BRIDGE-MIB::dot1dBasePort references if there are matching entries in the BRDIGE-MIB::dot1dBasePortTable.

However, it seems multiple vendors still use ifIndex references, despite this definition. It may not be noticable on devices where dot1dBasePort values happen to coincide with ifIndex values, but it has become very obvious that this is not the case on Juniper EX switches. There can actually be overlaps, but not matches, between ranges of dot1dBasePort numbers and ifIndex numbers, causing NAV's LLDP-based topology information to contain false information about neighbors.

The LLDP-MIB::lldpLocPortTable may hold clues as to how to really interpret references to local port numbers, and should be employed by NAV. Whenever NAV cannot safely determine that dot1dBasePort numbers are being used, it should default to interpreting port numbers as ifIndexes.

[Morten Brekkevold (mbrekkevold)]

fix committed here: https://nav.uninett.no/hg/nav/rev/7972e6302eed