No collected IPv6 prefixes on Cisco Nexus routers

[Morten Brekkevold]

This is related to bug 778422 and bug 777821.

A customer reports no IPv6 prefixes are being collected from their Nexus 7000 series router after an NX-OS software update. It appears that the NX-OS update has updated its support for RFC 4293 (an updated IP-MIB), and is now reporting IPv6 router addresses in ENTITY-MIB::ipAddressTable, instead of in the proprietary CISCO-IETF-IP-MIB.

The column of the ipAddressTable that ipdevpoll uses for prefix information here is ipAddressPrefix. According to the RFC, this should be a row pointer to a row in the ipAddressPrefixTable.

However, every RFC 4293-compliant device ipdevpoll has been tested against in the past report a pointer to the IP-MIB::ipAddressPrefixOrigin column here, while the new NX-OS update points to an IP-MIB::ipAddressPrefixEntry row. The difference is an extra item in the OID sequence, which ipdevpoll happily chops off. In the NX-OS case, this item isn't there, so the wrong item is chopped off, and the remaining suffix cannot be parsed by ipdevpoll.

[Morten Brekkevold (mbrekkevold)]

An SNMP RowPointer is defined as follows:

"Represents a pointer to a conceptual row. The value is the
name of the instance of the first accessible columnar object
in the conceptual row.

For example, ifIndex.3 would point to the 3rd row in the
ifTable (note that if ifIndex were not-accessible, then
ifDescr.3 would be used instead)."

IMHO, NX-OS gets this _WRONG_, and a bug should be reported towards that.

[Morten Brekkevold (mbrekkevold)]

committed a work-around for buggy NX-OS here: https://nav.uninett.no/hg/stable/rev/72d9fbaf71fb

If NX-OS reports IP-addresses on ifIndexes within the range 1-9, which are the valid columns of the ipAddressPrefixTable, the work-around will still fail. Someone should still report this bug to Cisco.