IP Forwarding Table MIB
obsoleted by
- rfc-2096 — IP Forwarding Table MIB
Extracted elements (24)
The ipForwardEntry index uses four fields (destination, protocol, policy, next-hop) rather than just destination, allowing a management station to represent and manage multiple multipath routes to the same destination and distinguish routes selected under different policy conditions.
The ipForwardPolicy field uses the IP TOS field encoding defined by Router Requirements rather than Host Requirements because the two documents disagree on TOS field width. The mapping leaves room to widen the TOS field without impact to deployed systems.
The ipForwardTable obsoletes and replaces the ipRouteTable from MIB-I and MIB-II. The principal additions are: next hop autonomous system number, multiple next hop (multipath) support, and policy routing support via IP TOS field indexing.
The next hop AS number (ipForwardNextHopAS) was added primarily to assist administrators of regional networks who need visibility into the AS of the next hop when managing inter-domain routing.
The inclusion of dynamic routing protocol values in ipForwardProto (EGP, OSPF, BGP, IS-IS, etc.) is not intended to imply that hosts should support those protocols; the field is descriptive of how the route was learned.
The ipForwardTable is proposed to deprecate and replace the ipRouteTable defined in MIB-II (RFC 1213). Implementations of the older table should migrate to ipForwardTable to gain multipath and policy routing support.
Any assignment of ipForwardDest to value x MUST be rejected if the bitwise logical-AND of x with the corresponding ipForwardMask is not equal to x, ensuring the destination address is consistent with its mask.
Any assignment of ipForwardMask to value x MUST be rejected if the bitwise logical-AND of x with the corresponding ipForwardDest is not equal to ipForwardDest.
Setting ipForwardType to invalid(2) invalidates the corresponding entry in ipForwardTable. Whether the agent removes the entry is implementation-specific; management stations MUST be prepared to receive entries not currently in use and MUST examine ipForwardType to interpret them correctly.
ipForwardAge (INTEGER, read-only, default 0) is the number of seconds since the route was last updated or determined correct. No 'too old' semantics are implied without knowledge of the routing protocol.
ipForwardDest (IpAddress, read-only) is the destination IP address of the route. A value of 0.0.0.0 represents a default route. Multicast (Class D) addresses are not permitted.
ipForwardEntry represents a particular route to a particular destination under a particular policy. It is indexed by the four-tuple: ipForwardDest, ipForwardProto, ipForwardPolicy, and ipForwardNextHop, enabling multipath routes to the same destination.
ipForwardIfIndex (INTEGER, read-write, default 0) identifies the local interface (ifIndex) through which the next hop of this route should be reached.
ipForwardInfo (OBJECT IDENTIFIER, read-write, default {0 0}) is a reference to MIB definitions specific to the routing protocol responsible for this route. If absent, it MUST be set to { 0 0 }, a syntactically valid OID that all ASN.1/BER implementations must support.
ipForwardMask (IpAddress, read-write, default 0.0.0.0) is the subnet mask logical-ANDed with the destination address before comparison to ipForwardDest. Agents without arbitrary subnet mask support derive the mask from the IP Address Class.
ipForwardMetric1 through ipForwardMetric5 (INTEGER, read-write, default -1) provide the primary and four alternate routing metrics for the route. Semantics are determined by ipForwardProto; unused metrics MUST be set to -1.
ipForwardNextHop (IpAddress, read-only) is the address of the next system en route for remote routes; 0.0.0.0 for local routes.
ipForwardNextHopAS (INTEGER, read-write, default 0) is the Autonomous System Number of the next hop router. Zero when unknown or not relevant to the protocol indicated by ipForwardProto.
ipForwardNumber is a read-only Gauge indicating the number of current ipForwardTable entries that are not invalid.
ipForwardPolicy (INTEGER, read-only) encodes the set of conditions for selecting among multipath routes. By default it represents the IP TOS field, mapped so that the 8-bit TOS byte (excluding the low-order reserved bit) yields an integer policy code from 0 to 30. Zero indicates the default path.
ipForwardProto (INTEGER, read-only) identifies the routing mechanism by which the route was learned. Defined values include: other(1), local(2), netmgmt/static(3), icmp(4), egp(5), ggp(6), hello(7), rip(8), is-is(9), es-is(10), ciscoIgrp(11), bbnSpfIgp(12), ospf(13), bgp(14), idpr(15).
ipForwardTable is the entity's IP routing table, defined as a SEQUENCE OF IpForwardEntry. It obsoletes the ipRouteTable defined in RFC 1213.
ipForwardType (INTEGER, read-write) classifies the route as other(1), invalid(2), local(3), or remote(4). local(3) means the next hop is the final destination; remote(4) means it is not. Default value is invalid(2).
Security issues are not discussed in this memo. The document contains no analysis of threats related to unauthorized modification of routing table entries via SNMP SET operations on read-write objects.