Table Of Contents
MPLS Enhancements to Interfaces MIB
First Published: March 15, 2004Last Updated: April 18, 2008
This document describes the Multiprotocol Label Switching (MPLS) enhancements to the existing Interfaces MIB (RFC 2233) to support an MPLS layer. This layer provides counters and statistics specifically for MPLS.
History for MPLS Enhancements to Interfaces MIB Feature
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Prerequisites for MPLS Enhancements to Interfaces MIB
•SNMP must be installed and enabled on the label switching routers (LSRs)
•MPLS must be enabled on the LSRs
•MPLS IP must be enabled on an interface or an MPLS traffic engineering (TE) tunnel enabled on an interface
Restrictions for MPLS Enhancements to Interfaces MIB
•Link up and link down traps for the MPLS layer are not supported in this release. Link up and link down traps for the MPLS layer are supported in Cisco IOS Releases 12.2(33)SXH, 12.2(33)SB, 12.2(33)SRB and later releases.
•Write capability using the SNMP SET command is not supported for the MPLS layer in this release.
•Some counters, including discard and multicast, increment on the underlying physical layer; therefore, they equal 0 because they never reach the MPLS layer.
•Starting in Cisco IOS Release 12.4, the high-capacity counters for the MPLS layer interfaces of the Interfaces MIB contain 64 bits of counter data. In previous releases, the high capacity counters displayed 32 bits of counter data.
The following MIB objects are affected:
When the 64-bit values are less than the value of 232, the 32-bit and 64-bit values are identical.
After the counter increases to more than 232, the counters are different; the 64-bit value is computed by the following formula:
X * (232) + Y
–X is the number of times the 32-bit counter has rolled.
–Y is the residual value of the counter after the roll occurred. The Y value equals the 32-bit value.
When the high-capacity counter values are compared to their 32-bit values, there is a period of time that the counter values are not equal. The 64-bit values lag the 32-bit values when the counters poll the 32-bit hardware counters and computing the correct counter value. During the polling and computation interval, the following high-capacity counter values counters might be inconsistent:
The inconsistent values can occur if traffic is constantly flowing over an interface and a MIB walk is performed. The 32-bit value is correct at that moment. The 64-bit value lags slightly, because of the polling computations needed to generate it. Once traffic stops flowing over the interface, and a polling period has passed, the two counters are identical and correct.
The lag time depends on the following factors:
–The polling interval used by the Interfaces MIB. The less time the polling interval takes, the more accurate the value is.
–The size of the Interfaces MIB. A large MIB takes a long time to walk and might affect the values found at that instant.
–The number of computations needed to generate the 64-bit value. The number of MPLS-enabled interfaces increases the number of 64-bit counter values that need to be computed.
Information About MPLS Enhancements to Interfaces MIB
To configure the MPLS Enhancements to Interfaces MIB, you need to understand the following concepts:
Feature Design of the MPLS Enhancements to Interfaces MIB
The Interfaces MIB (IF MIB) provides an SNMP-based method for managing interfaces. Each entry in the IF MIB establishes indexing, statistics, and stacking relationships among underlying physical interfaces, subinterfaces, and Layer 2 protocols that exist within Cisco IOS software.
The enhancements add an MPLS layer to the IF MIB as a Layer 2 protocol to provide statistics for traffic encapsulated as MPLS on an interface. In this structure, MPLS-specific data such as MPLS-encapsulated traffic counters and the MPLS maximum transmission unit (MTU) resides on top of the underlying physical or virtual interface to allow separation from non-MPLS data.
The enhancements also allow you to display indexing, statistics, and stacking relationships using the ifStackTable. MPLS layer interfaces are stacked above the underlying physical or virtual interface that is actually forwarding the MPLS traffic. MPLS traffic engineering tunnels are then stacked above those MPLS layers.
The IF MIB supports several types of interfaces. A virtual interface that provides protocol statistics for MPLS-encapsulated traffic has been added. This interface is stacked above real Cisco IOS interfaces or subinterfaces, such as Ethernet (et0) or ATM (at1/1.1).
Cisco IOS software creates a corresponding MPLS layer above each interface capable of supporting MPLS when the MPLS encapsulation is enabled by issuing the mpls ip interface configuration command.
You can also create the interface layer if you enable MPLS TE by using the mpls traffic-eng tunnels command in interface configuration mode.
Note You must also issue these commands in global configuration mode for MPLS IP or MPLS TE to be enabled.
An IF MIB entry is created when you enable either MPLS IP or MPLS TE tunnels on an interface; the entry is removed when you disable both MPLS IP and MPLS TE.
Table 1 defines the ifStackTable objects.
Table 2 defines the ifRcvAddressTable objects.
Note Entries for the MPLS layer do not appear in the ifRcvAddressTable.
Interfaces MIB Scalar Objects
The IF MIB supports the following scalar objects:
•ifStackLastChange—The value of sysUpTime at the time of the last change of the entire interface stack. A change of the interface stack is defined to be any creation, deletion, or change in value of any instance of ifStackStatus. If the interface stack has been unchanged since the last reinitialization of the local network management subsystem, then this object contains a zero value.
•ifTableLastChange—The value of sysUpTime at the time of the last creation or deletion of an entry in the ifTable. If the number of entries has been unchanged since the last reinitialization of the local network management subsystem, then this object contains a zero value.
Stacking Relationships for MPLS Layer Interfaces
The ifStackTable within the IF MIB provides a conceptual stacking relationship between the interfaces and subinterfaces represented as entries in the ifTable.
The ifStackTable is indexed like a linked list. Each entry shows a relationship between two interfaces providing the ifIndexes of the upper and the lower interface. The entries chain together to show the entire stacking relationship. Each entry links with one another until the stack terminates with an ifIndex of 0 at the highest and lowest ends of the stack. For example, in Figure 1, the indexes .10.5 show that ifIndex 10 is stacked upon ifIndex 5. There are 0 entries at the highest and lowest ends of the stack; in Figure 1, the indexes .0.15 and .72.0 are the highest and lowest ends of the stack, respectively.
Figure 1 Sample ATM Stacking Relationship in the ifStackTable
Note The order of the entries in Table 3 may not be the same as that seen in the MIB walk, which has to follow SNMP ordering rules.
Stacking Relationships for Traffic Engineering Tunnels
MPLS TE tunnels are represented in Cisco IOS software and the IF MIB as virtual interfaces. When properly signaled, TE tunnels pass traffic through MPLS over a physical interface. This process dictates that a TE tunnel is to be stacked on an MPLS layer that is stacked on an underlying interface.
TE tunnels can also change paths in response to different error or network conditions. These changes are instigated by using the RSVP-TE signaling protocol. When a change occurs, a tunnel can switch to a different MPLS interface. If no signaling path exists, no paths will be chosen and thus no MPLS interface will be used.
Because a TE tunnel is represented as an IF MIB ifTable entry, the ifStackTable also contains an entry corresponding to the TE tunnel. If the TE tunnel is successfully signaled, the ifStackTable also contains a link between the tunnel interface and one MPLS interface. Note that because it is possible for a TE tunnel to not have a corresponding signaled path, it is thus possible for a TE tunnel's ifStackTable entry to not have a corresponding lower layer. In this case, the lower layer variable contains the value of 0.
Figure 2 shows a TE tunnel before (left) and after (right) being rerouted and the effect on the ifStackTable. When ifIndex 2 fails, the TE tunnel is rerouted through ifIndex1, the 15.2 entry is removed from the ifStackTable, and the 15.1 entry is added.
Sample TE Tunnel Stacking Relationship
MPLS Label Switching Router MIB Enhancements
All of the ifIndex references in the MPLS-LSR-MIB tables have changed from the ifIndex of the underlying physical or virtual interface to the ifIndex of the MPLS layer.
Table 4 shows the specific changes.
The following objects from the mplsInterfaceConfTable are affected:
•mplsInterfaceOutPackets—Count only MPLS-encapsulated out packets
•mplsInterfaceInPackets—Count only MPLS-encapsulated in packets
Benefits of the MPLS Enhancements to Interfaces MIB
Improved Accounting Capability
By viewing the MPLS layer, you get MPLS-encapsulated traffic counters that do not include non-MPLS encapsulated traffic (for example, IP packets). Therefore, the counters are more useful for MPLS-related statistics.
TE Tunnel Interfaces
For TE tunnel interfaces, the stacking relationship reflects the current underlying MPLS interface that is in use and dynamically changes as TE tunnels reoptimize and reroute.
The MPLS layer shows MPLS-specific information including the following:
•If MPLS is enabled
•MPLS operational status
How to Configure MPLS Enhancements to Interfaces MIB
This section contains the following procedures:
•Enabling the SNMP Agent (required)
Enabling the SNMP Agent
Perform the following task to enable the SNMP agent.
2. show running-config
3. configure terminal
4. snmp-server community string [view view-name] [ro] [number]
6. write memory
Configuration Examples for the MPLS Enhancements to Interfaces MIB
This section provides the following configuration examples:
MPLS Enhancements to Interfaces MIB: Examples
The following example shows how to enable an SNMP agent:Router# configure terminalRouter(config)# snmp-server community
In the following example, SNMPv1 and SNMPv2C are enabled. The configuration permits any SNMP manager to access all objects with read-only permissions using the community string public.Router(config)# snmp-server community public
In the following example, read-only access is allowed for all objects to members of access list 4 that specify the comaccess community string. No other SNMP managers have access to any objects.Router(config)# snmp-server community comaccess ro 4
The following sections provide references related to the MPLS Enhancements to Interfaces MIB feature.
Related Topic Document Title
•Cisco IOS Network Management Command Reference, Release 12.4T
•Cisco IOS Network Management Command Reference, Release 12.2SB
•Cisco IOS Network Management Command Reference, Release 12.2SR
"Configuring SNMP support" in the Cisco IOS Network Management Configuration Guide, Release 12.4
A description of SNMP agent support in Cisco IOS software for the MPLS Label Switching Router MIB (MPLS-LSR-MIB)
MPLS Label Switching Router MIB
A description of SNMP agent support in Cisco IOS for the MPLS Traffic Engineering MIB (MPLS TE MIB)
MPLS Traffic Engineering (TE) MIB
"Multiprotocol Label Switching (MPLS) Label Switch Router (LSR) Management Information Base," Internet draft, January 2002 [draft-ietf-mpls-lsr-mib-08.txt]; Srinivasan, C., Viswanathan, A., and Nadeau, T.D.
Note For information on using SNMP MIB features, see the appropriate documentation for your network management system.
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.
MIBs MIBs Link
Interfaces Group MIB (IF MIB)
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:
ATM—Asynchronous Transfer Mode. The international standard for cell relay in which multiple service types (such as voice, video, or data) are conveyed in fixed-length (53-byte) cells. Fixed-length cells allow cell processing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of high-speed transmission media, such as E3, SONET, and T3.
ATM-AAL5—ATM adaptation layer 5. One of four AALs recommended by the ITU-T. AAL5 supports connection-oriented variable bit rate (VBR) services and is used predominantly for the transfer of classical IP over ATM and LAN emulation (LANE) traffic. AAL5 uses simple and efficient AAL (SEAL) and is the least complex of the current AAL recommendations. It offers low bandwidth overhead and simpler processing requirements in exchange for reduced bandwidth capacity and error-recovery capability.
encapsulation—Wrapping of data in a particular protocol header. For example, Ethernet data is wrapped in a specific Ethernet header before network transit. Also, when bridging dissimilar networks, the entire frame from one network is simply placed in the header used by the data link layer protocol of the other network.
IETF—Internet Engineering Task Force. A task force (consisting of more than 80 working groups) that is developing standards for the Internet and the IP suite of protocols.
interface—The boundary between adjacent layers of the ISO model.
label—A short, fixed-length identifier that is used to determine the forwarding of a packet.
label switching—A term used to describe the forwarding of IP (or other network layer) packets using a label swapping algorithm based on network layer routing algorithms. The forwarding of these packets uses the exact match algorithm and rewrites the label.
LSR—label switching router. A device that forwards MPLS packets based on the value of a fixed-length label encapsulated in each packet.
MIB—Management Information Base. A database of network management information that is used and maintained by a network management protocol such as SNMP. The value of a MIB object can be changed or retrieved by means of SNMP commands, usually through a network management system. MIB objects are organized in a tree structure that includes public (standard) and private (proprietary) branches.
MPLS—Multiprotocol Label Switching. A method for forwarding packets (frames) through a network. It enables routers at the edge of a network to apply labels to packets (frames). ATM switches or existing routers in the network core can switch packets according to the labels with minimal lookup overhead.
MPLS interface—An interface on which MPLS traffic is enabled.
MTU—maximum transmission unit. Maximum packet size, in bytes, that a particular interface can handle.
NMS—network management system. System responsible for managing at least part of a network. An NMS is generally a reasonably powerful and well-equipped computer, such as an engineering workstation. NMSs communicate with agents to help keep track of network statistics and resources.
OID—object identifier. Values are defined in specific MIB modules. The Event MIB allows you or an NMS to watch over specified objects and to set event triggers based on existence, threshold, and Boolean tests. An event occurs when a trigger is fired; this means that a specified test on an object returns a value of true. To create a trigger, you or an NMS configures a trigger entry in the mteTriggerTable of the Event MIB. This trigger entry specifies the OID of the object to be watched. For each trigger entry type, corresponding tables (existence, threshold, and Boolean tables) are populated with the information required for carrying out the test. The MIB can be configured so that when triggers are activated (fired) either an SNMP Set is performed, a notification is sent out to the interested host, or both.
SNMP—Simple Network Management Protocol. A management protocol used almost exclusively in TCP/IP networks. SNMP provides a means for monitoring and controlling network devices, and for managing configurations, statistics collection, performance, and security.
traffic engineering tunnel—A label-switched tunnel that is used for traffic engineering. Such a tunnel is set up through means other than normal Layer 3 routing; it is used to direct traffic over a path different from the one that Layer 3 routing could cause the tunnel to take.
trap—A message sent by an SNMP agent to a network management station, console, or terminal, indicating that a significant event occurred. Traps are less reliable than notification requests, because the receiver does not send an acknowledgment when it receives a trap. The sender cannot determine if the trap was received.
tunnel—A secure communication path between two peers, such as routers.
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