tracert vc

Function

The tracert vc command detects the status of a PW or locates a faulty node on a PW in Down state.

Format

tracert vc pw-type pw-id [ -exp exp-value | -f first-ttl | -m max-ttl | -r reply-mode | -t timeout-value ] ^* label-alert [ no-control-word ] [ remote remote-ip-address ] [ full-lsp-path ] [ draft6 ] [ pipe | uniform ]

tracert vc pw-type pw-id [ -exp exp-value | -f first-ttl | -m max-ttl | -r reply-mode | -t timeout-value ] ^* normal [ no-control-word ] [ remote remote-ip-address ] [ full-lsp-path ] [ draft6 ] [ pipe | uniform ]

Parameters

Parameter	Description	Value
pw-type	Specifies the PW type.	Currently, PWs of the following types are supported: ethernet, vlan, and ip-interworking.
pw-id	Specifies the local PW ID.	The value is an integer that ranges from 1 to 4294967295.
-exp exp-value	Specifies the EXP value in the outer label of an MPLS Echo Request packet. The default value is 0. NOTE: If DSCP priority has been configured by running the set priority command, the exp-value parameter does not take effect.	The value is an integer that ranges from 0 to 7.
-f first-ttl	Specifies the initial Time-to-Live (TTL).	The value is an integer that ranges from 1 to 255, and must be smaller than the value of max-ttl. The default value is 1.
-m max-ttl	Specifies the maximum TTL.	The value is an integer that ranges from 1 to 255, and must be larger than the value of first-ttl. The default value is 30.
-r reply-mode	Specifies the mode in which the peer returns MPLS Echo Reply packets. 1: No MPLS Echo Reply packet is returned. 2: The MPLS Echo Reply packet is encapsulated in IPv4/IPv6 UDP packets. 3: MPLS Echo Reply packets are encapsulated in IPv4/IPv6 /IPv6 UDP packets carrying the Router Alert option. 4: MPLS Echo Reply packets are returned through the control channel of the application plane.	The value is an integer that ranges from 1 to 4.
-t timeout-value	Specifies the timeout interval of an MPLS Echo Reply packet.	The value is an integer that ranges from 0 to 65535, in milliseconds. The default value is 5.
control-word	Indicates that the control word is encapsulated in the MPLS Echo Request packet.	-
label-alert	Indicates that the router alert label is encapsulated in the MPLS Echo Request packet.	-
no-control-word	Indicates that the control word is not encapsulated in the MPLS Echo Request packet.	-
normal	Indicates the normal mode where the router alert label and control word are not encapsulated in the MPLS Echo Request packet.	-
remote	Specifies information about the PW on the remote PE.	-
remote-ip-address	Specifies the remote IP address. By default, the system searches for the IP address of the next hop based on the PW on the local PE. If label-alert is configured, you can specify the IP address of any switching node or the termination node.	-
remote-pw-id	Specifies the ID of the remote PW. By default, the ID of the local PW is used. If the tracert operation is performed in control word mode for a multi-segment PW, the IP address of the termination node must be specified.	-
ptn-mode	Specifies the PTN mode. In a multi-segment PW scenario, this parameter is indicated that trace VC packets are replied. You need to run the lspv pw reply ptn-mode command on both the SPE and TPE.	-
full-lsp-path	Displays the responses from all nodes along the LSP that the MPLS Echo Request packets pass through. If this parameter is not specified, only the responses from the PW nodes along the LSP are displayed.	-
pipe	Specifies the pipe mode. When a probe packet passes through the MPLS domain, the entire domain is regarded as one hop and the IP TTL of the probe packet is reduced by one on both the ingress and egress.	-
uniform	Specifies the uniform mode. The IP TTL of the probe packet is reduced by one each time it passes through one hop in the MPLS domain.	-
draft6	Specifies the version of the tracert vc command. If this parameter is specified, the tracert operation is performed according to "draft-ietf-mpls-lsp-ping-06". By default, the tracert operation is performed according to RFC 4379. NOTE: Tracert VC based on draft6 is applied only to VLL over LDP scenarios.	-

Views

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Default Level

0: Visit level

Usage Guidelines

Usage Scenario

If a fault occurs on the PW, the ping vc command can be used to check the connectivity of the network, and the tracert vc command can be used to locate the fault and provide reference for fault diagnosis. If a PW is Up, the ping vc command can be used to locate the fault on the PW. For example, a forwarding entry is abnormally lost or incorrect. If a PW is down, the tracert vc command must be used to locate the faulty node on the PW.

The tracert vc command applies to the following networking scenarios:

PWE3 VLL PW tracert

In PWE3 VLL networking, PW tracert can help you obtain information about SPEs and Ps along the path that the message travels from the source to the destination, check the connectivity of the PW, and locate the fault of the PW.

A PWE3 VLL PW tracert can be performed in control word mode, label alert mode, or TTL mode. The default mode is label alert. The TTL mode and control word mode are mutually exclusive.

To detect faults on a VLL network with control word enabled, run the tracert vc pw-type pw-id control-word command.

To encapsulate packets with the router alert label and detect faults on a VLL network, run the tracert vc pw-type pw-id label-alert command.

If control word is not enabled and packets are not encapsulated with the router alert label, to detect faults on a VLL network, run the tracert vc pw-type pw-id command.

The TTL value of the PW Tracert Request message is incremented by 1 each time. Each time the transit node (P) receives an Echo Request message after the TTL value of the message expires, it sends the Echo Request message to the LSPV module. Then the transit node returns an Echo Reply message carrying the next hop information.

Prerequisite

The UDP module of each node works properly; otherwise, the tracert operation will fail.
The MPLS module has been enabled on each node and works properly.
The ICMP module of each node works properly; otherwise, " * * * " is displayed.

Procedure

The execution process of the tracert vc command is as follows:

The source sends an MPLS Echo Request packet with the TTL being 1. After the TTL times out, the first hop sends an MPLS Echo Reply packet to the source.
The source sends an MPLS Echo Request packet with the TTL being 2. After the TTL times out, the second hop sends an MPLS Echo Reply packet to the source.
The source sends an MPLS Echo Request packet with the TTL being 3. After the TTL times out, the third hop sends an MPLS Echo Reply packet to the source.
The preceding steps continue until the MPLS Echo Request packet reaches the destination.

When the device on each hop receives the MPLS Echo Request packet, it will respond with an MPLS Echo Reply packet, indicating that the tracert test ends. In the command output information of the source device, you can view the path that the packet passes through.

Configuration Impact

In control word mode, if a transit node receives an MPLS Echo Request packet whose TTL does not time out, it does not send the packet to the CPU. In this mode, the source obtains only a little PW information and cannot obtain information about the downstream devices of the transit node. This mode is recommended when the traffic volume is heavy.

In router alert mode, a transit node sends the received MPLS Echo Request packets to the CPU. In this mode, the source obtains a lot of PW information; therefore, device performance is affected when the traffic volume is heavy. This mode is recommended when the traffic volume is light.

Information specified by remote is encapsulated in MPLS Echo Request packets. The PW can be searched on the remote PE based on the specified information. By default, information contained in the MPLS Echo Request packets is about the PW on the local PE. This applies to single-segment PWs.

Precautions

When the probe packet reaches the egress or the TTL reaches the upper threshold, the PW tracert is terminated.
You can press Ctrl + C to terminate the execution of the tracert vc command.

When a PE is single-homed to an SPE and two multi-segment PWs are deployed for PW redundancy, end-to-end detection cannot be performed for the secondary PW if services are transmitted over the primary PW. If services are transmitted over the secondary PW, the primary PW can only be detected segment by segment.

Example

# Trace a multi-segment PW. The encapsulation type, local PW ID, and remote PW ID of the PW is ethernet, 100, and 200.

<HUAWEI> tracert vc ethernet 100 control-word remote 200 draft6 full-lsp-path
  TTL   Replier            Time    Type      Downstream
  0                                Ingress   10.1.1.2/[1025 ]
  1     10.1.1.2           230 ms  Transit   10.2.1.2/[3 ]
  2     10.2.1.2           230 ms  Transit   10.3.1.2/[3 ]
  3     10.3.1.2           100 ms  Transit   10.4.1.2/[3 ]
  4     10.4.1.2           150 ms  Egress

**Table 1** Description of the **tracert vc** command output
Item	Description
TTL	TTL value in an Echo Request packet. It represents the number of hops along the path through which an Echo Request packet passes.
Replier	IP address of the node sending MPLS Echo Reply packets.
Time	Time when the packet is processed.
Type	Node type. The value can be: Ingress: indicates an ingress node. Transit: indicates a transit node. Egress: indicates an egress node.
Downstream	IP address and label of the downstream node.