Cisco Nexus 7000 Series NX-OS MPLS Configuration Guide

.i.MPLS:tree trace;

The MPLS LSP Multipath Tree Trace feature provides the means to discover all possible equal-cost multipath (ECMP) routing paths of a label switched path (LSP) between an egress and ingress router. Once discovered, these paths can be retested on a periodic basis using MPLS LSP ping or traceroute. This feature is an extension to the MPLS LSP traceroute functionality for the tracing of IPv4 LSPs.

You can use the MPLS LSP Multipath Tree Trace feature to discover all paths for an IPv4 LSP.

This implementation of the MPLS LSP Multipath Tree Trace feature is based on the IETF RFC 4379 Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures .

This section includes the following topics:

• Overview of MPLS LSP Multipath Tree Trace
• Discovery of IPv4 Load Balancing Paths by MPLS LSP Multipath Tree Trace
• Echo Reply Return Codes Sent by the Router Processing Multipath LSP Tree Trace

Overview of MPLS LSP Multipath Tree Trace

As the number of MPLS deployments increases, the number of traffic types that the MPLS networks carry could increase. In addition, load balancing on label switch routers (LSRs) in the MPLS network provides alternate paths for carrying MPLS traffic to a target router. The ability of service providers to monitor LSPs and quickly isolate MPLS forwarding problems is critical to their ability to offer services.

Before the release of the MPLS LSP Multipath Tree Trace feature, no automated way existed to discover all paths between provider edge (PE) routers, and troubleshooting forwarding problems between PEs was difficult.

The MPLS LSP Multipath Tree Trace feature provides an automated way to discover all paths from the ingress PE router to the egress PE router in multivendor networks that use IPv4 load balancing at the transit routers. Once the PE-to-PE paths are discovered, use MPLS LSP ping and MPLS LSP traceroute to periodically test them.

Discovery of IPv4 Load Balancing Paths by MPLS LSP Multipath Tree Trace

Echo Reply Return Codes Sent by the Router Processing Multipath LSP Tree Trace

Table 33-1 describes the codes that the router processing a multipath LSP tree trace packet returns to the sender about the failure or success of the request.

Customizing the Default Behavior of MPLS Echo Packets

MPLS Embedded Management Configuration

Before using the ping mpls , traceroute mpls , or traceroute mpls multipath command, you should ensure that the router is configured to encode and decode MPLS echo packets in a format that all receiving routers in the network can understand.

LSP ping drafts after Version 3 (draft-ietf-mpls-ping-03) have undergone numerous TLV format changes, but the implementations based on different drafts might not interoperate properly.

To allow later Cisco implementations to interoperate with draft Version 3 Cisco and non-Cisco implementations, a global configuration mode (MPLS OAM configuration) allows you to encode and decode echo packets in formats specified by draft Version 3 implementations.

Unless configured otherwise, a Cisco implementation encodes and decodes echo requests assuming the version on which the Internet Engineering Task Force (IETF) implementation is based.

To allow for seamless interoperability with earlier Revision 1 and 3 images, you can use MPLS Operation, Administration, and Maintenance (OAM) configuration mode parameters to force the default behavior of the Revision 4 images to be compliant or compatible in networks with Revision 1 or Revision 3 images.

To prevent failures reported by the replying router due to TLV version issues, you should configure all routers in the core. Encode and decode MPLS echo packets in the same draft version. For example, if the network is running RFC 4379 (Cisco Revision 4) implementations but one router can run only Version 3 (Cisco Revision 3), configure all routers in the network to operate in Revision 3 mode.

Cisco Revision 4 is the default version. The default version is the latest LSP ping version supported by the image on the router.

Prerequisites

The MPLS LSP Multipath Tree Trace feature requires RFC 4379 (Revision 4).

SUMMARY STEPS

1. configure terminal

2. mpls oam

3. echo re vision { 3 | 4 }

4. [ no ] echo vendor-extension

DETAILED STEPS

Configuring MPLS LSP Multipath Tree Trace

You can configure the MPLS multipath LSP tree trace traceroute. This task helps you to discover all LSPs from an egress router to an ingress router.

Prerequisites

Cisco LSP ping or traceroute implementations based on draft-ietf-mpls-lsp-ping-11 can in some cases detect the formatting of the sender of an MPLS echo request. However, in certain cases an echo request or echo reply might not contain the Cisco extension TLV. To avoid complications in which the echo packets are decoded assuming the wrong TLV formats, configure all routers in the network to operate in the same mode.

For an MPLS LSP multipath tree trace to be successful, the implementation in your routers must support RFC 4379 on all core routers.

If all routers in the network support FRC-4379 and another vendor’s implementation exists that is not capable of properly handling Cisco’s vendor TLV, the routers supporting the RFC-compliant or later configuration must include commands to disable the Cisco vendor TLV extensions.

SUMMARY STEPS

1. configure terminal

2. mpls oam

3. echo revision 4

4. (Optional) [ no ] echo vendor-extension

5. traceroute mpls multipath ipv4 destination-ip-address / destination-mask-length

DETAILED STEPS

Discovering IPv4 Load Balancing Paths Using MPLS LSP Multipath Tree Trace

You can discover IPv4 load balancing paths using the MPLS LSP Multipath Tree Trace feature.

MPLS Multipath LSP Traceroute Path Discovery

A Cisco router load balances MPLS packets based on the incoming label stack and the source and destination addresses in the IP header. The outgoing label stack and IP header source address remain constant for each path being traced. The router needs to find the set of IP header destination addresses to use all possible output paths. This might require exhaustive searching of the 127 .x.y.z /8 address space. Once you discover all paths from the source LSR to the target or destination LSR with the MPLS LSP Multipath Tree Trace feature, you can use MPLS LSP traceroute to monitor these paths.

Figure 33-1 shows how the MPLS LSP Multipath Tree Trace feature discovers LSP paths in a sample network. In Figure 33-1, the bitmap size is 16 and the numbers 0 to 15 represent the bitmapped addresses that the MPLS LSP Multipath Tree Trace feature uses to discover all the paths from the source LSR R-101 to the target LSR R-150. Figure 33-1 illustrates how the traceroute mpls multipath command discovers all LSP paths in the sample network.

Figure 33-1 MPLS LSP Multipath Tree Trace Path Discovery in a Sample Network

SUMMARY STEPS

1. configure terminal

2. mpls oam

3. echo revision 4

4. traceroute mpls multipath ipv4 destination-ip-address / destination - mask-length hashkey ipv4 bitmap bitmap-size

DETAILED STEPS

Monitoring LSP Paths Discovered by MPLS LSP Multipath Tree Trace Using MPLS LSP Traceroute

You can monitor LSP paths that are discovered by the MPLS LSP Multipath Tree Trace feature using the MPLS LSP traceroute. You can take output directly from the traceroute mpls multipath command and add it to a traceroute mpls command periodically to verify that the path is still operating.

Figure 33-2 shows the mapping of the output of a traceroute mpls multipath command to a traceroute mpls command.

Figure 33-2 Mapping of traceroute mpls multipath Command Output to a traceroute mpls Command

Each path that you discover with the MPLS LSP Multipath Tree Trace feature can be tested in this manner periodically to monitor the LSP paths in your network.

SUMMARY STEPS

1. traceroute mpls multipath ipv4 destination-address / destination-mask-length hashkey ipv4 bitmap bitmap-size

2. traceroute mpls ipv4 destination-address / destination-mask-length [ output interface tx-interface ] [ source source-address ] [ destination address-start ]

DETAILED STEPS

Step 1 Discover all MPLS LSPs from an egress router to an ingress router by entering the traceroute mpls multipath ipv4 destination-address / destination-mask-length hashkey ipv4 bitmap bitmap-size command.

This example shows how to discover all MPLS LSPs from an egress router to an ingress router:

The output of the traceroute mpls multipath command in the example shows the result of path discovery with the MPLS LSP Multipath Tree Trace feature. In this example, the command sets the bitmap size to 16. Path discovery starts by the MPLS LSP Multipath Tree Trace feature using 16 bitmapped addresses as it locates LSP paths from the source router to the target router with prefix and mask 10.1.1.150/32. MPLS LSP multipath tree trace starts using the 127 .x.y.z /8 address space with 127.0.0.1.

Step 2 Verify that the paths discovered when you entered a traceroute mpls multipath command are still operating by entering the traceroute mpls ipv4 destination-address / destination-mask-length [ output interface tx-interface ] [ source source-address ] [ destination address-start ] command.

For example, the output for Path 0 in the previous traceroute mpls multipath command in Step 1 is as follows:

If you put the output for path 0 in the traceroute mpls command, you see the following results:

You can take output directly from the traceroute mpls multipath command and add it to a traceroute mpls command periodically to verify that the path is still operating (see Figure 33-2).

Using DSCP to Request a Specific Class of Service in an Echo Reply

Use the reply differentiated services code point (DSCP) option to request a specific class of service (CoS) in an echo reply.

The reply DSCP option is supported in the experimental mode for IETF draft-ietf-mpls-lsp-ping-03.txt. Cisco implemented a vendor-specific extension for the reply DSCP option rather than using a Reply TOS TLV. A Reply TOS TLV serves the same purpose as the reply dscp command in IETF draft-ietf-mpls-lsp-ping-11.txt. This draft provides a standardized method of controlling the reply DSCP.

SUMMARY STEPS

1. traceroute mpls multipath ipv4 destination-address / destination-mask-length [ reply dscp dscp-value ]

DETAILED STEPS

Controlling How a Responding Router Replies to an MPLS Echo Request

This section describes how to control how a responding router replies to an MPLS echo request.

This section includes the following topic:

• Reply Modes for an MPLS LSP Multipath Tree Trace Echo Request Response

Reply Modes for an MPLS LSP Multipath Tree Trace Echo Request Response

The reply mode controls how a responding router replies to an MPLS echo request sent by a traceroute mpls multipath command. There are two reply modes for an echo request packet:

• ipv4—Reply with an IPv4 User Datagram Protocol (UDP) packet (default)
• router-alert—Reply with an IPv4 UDP packet with router alert

Note Use the ipv4 and router-alert reply modes with each other to prevent false negatives. If you do not receive a reply via the ipv4 mode, send a test with the router-alert reply mode. If both fail, something is wrong in the return path. The problem might be due to an incorrect ToS setting.

IPv4 UDP Reply Mode

The IPv4 UDP reply mode is the most common reply mode used with a traceroute mpls multipath command when you want to periodically poll the integrity of an LSP. With this option, you do not have explicit control over whether the packet traverses IP or MPLS hops to reach the originator of the MPLS echo request. If the originating (headend) router fails to receive a reply to an MPLS echo request when you use the reply mode ipv4 keywords, use the reply mode router-alert keywords.

Router-Alert Reply Mode

The router-alert reply mode adds the router alert option to the IP header. When an IP packet that contains an IP router alert option in its IP header or an MPLS packet with a router alert label as its outermost label arrives at a router, the router punts (redirects) the packet to the supervisor process level for handling, which forces the supervisor of each intermediate router to handle the packet at each intermediate hop as it moves back to the destination. Hardware and line card forwarding inconsistencies are thus bypassed. Router-alert reply mode is slower than IPv4 mode because the reply requires process-level supervisor handling at each hop.

Table 33-2 describes how an incoming IP packet with an IP router alert is handled by the router switching path processes when the outgoing packet is an IP packet or an MPLS packet. It also describes how an MPLS packet with a router alert option is handled by the router switching path processes when the outgoing packet is an IP packet or an MPLS packet.

SUMMARY STEPS

1. traceroute mpls multipath ipv4 destination-address / destination-mask-length reply mode { ipv4 | router-alert }

DETAILED STEPS

Specifying the Output Interface for Echo Packets Leaving a Router for MPLS LSP Multipath Tree Trace

You can specify the output interface for echo packets leaving a router for the MPLS LSP Multipath Tree Trace feature. You can use this task to test the LSPs that are reachable through a given interface.

You can control the interface through which packets leave a router. Path output information is used as input to LSP ping and traceroute.

The echo request output interface control feature allows you to force echo packets through the paths that perform detailed debugging or characterizing of the LSP. This feature is useful if a PE router connects to an MPLS cloud and there are broken links. You can direct traffic through a certain link. The feature also is helpful for troubleshooting network problems.

SUMMARY STEPS

1. traceroute mpls multipath ipv4 destination-address / destination-mask-length [ output interface tx-interface ]

DETAILED STEPS

Setting the Pace of MPLS Echo Request Packet Transmission for MPLS LSP Multipath Tree Trace

You can set the pace of MPLS echo request packet transmission for the MPLS LSP Multipath Tree Trace feature. Echo request traffic pacing allows you to set the pace of the transmission of packets so that the receiving router does not drop packets. If you have a large amount of traffic on your network you might increase the size of the interval to help ensure that the receiving router does not drop packets.

SUMMARY STEPS

1. traceroute mpls multipath ipv4 destination-address / destination-mask-length [ interval milliseconds ]

DETAILED STEPS

Enabling MPLS LSP Multipath Tree Trace to Detect LSP Breakages Caused by an Interface That Lacks an MPLS Configuration

You can enable the MPLS LSP Multipath Tree Trace feature to detect LSP breakages caused by an interface that lacks an MPLS configuration. If an interface is not configured for MPLS, then it cannot forward MPLS packets.

For an MPLS LSP Multipath Tree Trace of LSPs that carry IPv4 FECs, you can force an explicit null label to be added to the MPLS label stack even though the label was unsolicited. This process allows MPLS LSP multipath tree trace to detect LSP breakages that are caused by an interface that is not configured for MPLS. The MPLS LSP Multipath Tree Trace does not report that an LSP is functioning when it is unable to send MPLS traffic.

An explicit null label is added to an MPLS label stack if MPLS echo request packets are forwarded from an interface not configured for MPLS that is directly connected to the destination of the MPLS LSP Multipath Tree Trace or if the IP TTL value for the MPLS echo request packets is set to 1.

When you enter a traceroute mpls multipath command, you are looking for all MPLS LSP paths from an egress router to an ingress router. Failures at output interfaces that are not configured for MPLS at the penultimate hop are not detected. Explicit-null shimming allows you to test an LSP’s ability to carry MPLS traffic.

SUMMARY STEPS

1. traceroute mpls multipath ipv4 destination-address / destination-mask-length force-explicit-null

DETAILED STEP

Requesting That a Transit Router Validate the Target FEC Stack for MPLS LSP Multipath Tree Trace

You can request that a transit router validate the target Forwarding Equivalence Class (FEC) stack for the MPLS LSP Multipath Tree Trace feature.

An MPLS echo request tests a particular LSP. The LSP to be tested is identified by the FEC stack.

During an MPLS LSP Multipath Tree Trace, the echo packet validation rules do not require that a transit router validate the target FEC stack TLV. A downstream map TLV containing the correct received labels must be present in the echo request for target FEC stack checking to be performed.

To request that a transit router validate the target FEC stack, set the V flag from the source router by entering the flags fec keywords in the traceroute mpls multipath command. The default is that echo request packets are sent with the V flag set to 0.

SUMMARY STEPS

1. traceroute mpls multipath ipv4 destination-address / destination-mask-length [ flags fec ] [ ttl maximum-time-to-live ]

DETAILED STEPS

Setting the Number of Timeout Attempts for MPLS LSP Multipath Tree Trace

You can set the number of timeout attempts for the MPLS LSP Multipath Tree Trace feature.

A retry is tried if an outstanding echo request times out waiting for the corresponding echo reply.

SUMMARY STEPS

1. traceroute mpls multipath ipv4 destination-address / destination-mask-length [ retry-count retry-count-value ]

DETAILED STEPS

Example: Customizing the Default Behavior of MPLS Echo Packets

The following example shows how to customize the behavior of MPLS echo packets so that the MPLS LSP Multipath Tree Trace feature interoperates with a vendor implementation that does not interpret RFC 4379 as Cisco does:

The echo revision command is included in this example for completeness. The default echo revision number is 4, which corresponds to RFC 4379.

Example: Configuring MPLS LSP Multipath Tree Trace

The following example shows how to configure the MPLS LSP Multipath Tree Trace feature to interoperate with a vendor implementation that does not interpret RFC 4379 as Cisco does:

The echo revision command is included in this example for completeness. The default echo revision number is 4, which corresponds to the RFC 4379.

Example: Discovering IPv4 Load Balancing Paths Using MPLS LSP Multipath Tree Trace

The following example shows how to use the MPLS LSP Multipath Tree Trace feature to discover IPv4 load-balancing paths. The example is based on the sample network shown in Figure 33-3. In this example, the bitmap size is set to 16. Therefore, path discovery starts by the MPLS LSP Multipath Tree Trace feature using 16 bitmapped addresses as it locates LSP paths from the source router R-101 to the target router R-150 with prefix and mask 10.1.1.150/32. The MPLS LSP Multipath Tree Trace feature starts using the 127 .x.y.z /8 address space with 127.0.0.0.

The output of the traceroute mpls multipath command in the example shows the result of path discovery with the MPLS LSP Multipath Tree Trace feature as shown in Figure 33-3.

Figure 33-3 MPLS LSP Multipath Tree Trace Path Discovery in a Sample Network

Example: Using DSCP to Request a Specific Class of Service in an Echo Reply

The following example shows how to use DSCP to request a specific Class of Service (CoS) in an echo reply:

Example: Controlling How a Responding Router Replies to an MPLS Echo Request

The following example shows how to control how a responding router replies to an MPLS echo request:

Example: Specifying the Output Interface for Echo Packets Leaving a Router for MPLS LSP Multipath Tree Trace

The following example shows how to specify the output interface for echo packets leaving a router for the MPLS LSP Multipath Tree Trace feature:

Example: Setting the Pace of MPLS Echo Request Packet Transmission for MPLS LSP Multipath Tree Trace

The following examples show how set the pace of MPLS echo request packet transmission for the MPLS LSP Multipath Tree Trace feature. The time between successive MPLS echo requests is set to 300 milliseconds in the first example and 400 milliseconds in the second example:

Notice that the elapsed time increases as you increase the interval size.

Example: Enabling MPLS LSP Multipath Tree Trace to Detect LSP Breakages Caused by an Interface That Lacks an MPLS Configuration

The following examples shows how to enable the MPLS LSP Multipath Tree Trace feature to detect LSP breakages caused by an interface that lacks an MPLS configuration:

This example shows the additional information provided when you add the verbose keyword to the command:

Example: Requesting That a Transit Router Validate the Target FEC Stack for MPLS LSP Multipath Tree Trace

The following example shows how to request that a transit router validate the target FEC stack for the MPLS LSP Multipath Tree Trace feature:

Target FEC stack validation is always done at the egress router when the flags fec keywords are specified in the traceroute mpls multipath command.

Example: Setting the Number of Timeout Attempts for MPLS LSP Multipath Tree Trace

The following example sets the number of timeout attempts for the MPLS LSP Multipath Tree Trace feature to four:

The following output shows a traceroute mpls multipath command that found one unexplored path, one successful path, and one broken path:

Related Documents

MIBs