A single SNMP agent sometimes needs to support multiple
instances of the same MIB module, and does so through the
use of multiple SNMP contexts. This typically occurs because
the technology has evolved to have extra dimension(s), i.e.,
one or more extra data and/or identifier values which are
different in the different contexts, but were not defined in
INDEX clause(s) of the original MIB module. In such cases,
network management applications need to know the specific
data/identifier values in each context, and this MIB module
provides mapping tables which contain that information.
Within a network there can be multiple Virtual Private
Networks (VPNs) configured using Virtual Routing and
Forwarding Instances (VRFs). Within a VPN there can be
multiple topologies when Multi-topology Routing (MTR) is
used. Also, Interior Gateway Protocols (IGPs) can have
multiple protocol instances running on the device.
A network can have multiple broadcast domains configured
using Bridge Domain Identifiers.
With MTR routing, VRFs, and Bridge domains, a router now
needs to support multiple instances of several existing
MIB modules, and this can be achieved if the router's SNMP
agent provides access to each instance of the same MIB module
via a different SNMP context (see Section 3.1.1 of RFC 3411).
For MTR routing, VRFs, and Bridge domains, a different SNMP
context is needed depending on one or more of the following:
the VRF, the topology-identifier, the routing protocol instance,
and the bridge domain identifier.
In other words, the router's management information can be
accessed through multiple SNMP contexts where each such
context represents a specific VRF, a specific
topology-identifier, a specific routing protocol instance
and/or a bridge domain identifier. This MIB module provides
a mapping of each such SNMP context to the corresponding VRF,
the corresponding topology, the corresponding routing protocol
instance, and the corresponding bridge domain identifier.
Some SNMP contexts are independent of VRFs, independent of
a topology, independent of a routing protocol instance, or
independent of a bridge domain and in such a case, the mapping
is to the zero length string.
With the Cisco package licensing strategy, the features
available in the image are grouped into multiple packages
and each packages can be managed to operate at different
feature levels based on the available license. This MIB
module provides option to associate an SNMP context to a
feature package group. This will allow manageability of
license MIB objects specific to a feature package group.
As technology evolves more we may need additional
identifiers to identify the context. Then we would need
to add those additional identifiers into the mapping.