Figure 5-8 Networking of PWE3 SH Tracert
CE1 CE2
CE3
CE4
PE1
PE2
PE3
PE4
P
VPN1
VPN1
VPN2
VPN2
LSP1
LSP2
On PE1, you can start PWE3 tracert of VPN 1 by running the related command. This
PWE3 tracert is the same as the LSP tracert on the public network, except that a PW
label is added to packets and the remote PE checks whether the PW label is consistent
with the VC ID.
The source PE of the PWE3 tracert continues to send MPLS echo request packets with
the Time-to-Live (TTL) of the out-label from one to a certain value and the TTL of the
in-label as one. Each Label Switching Router (LSR) does not forward the received
packet with the TTL of the out-label as one. Based on the contents of the packet, each
LSR checks the correctness of specific services and labels, and then sends an MPLS
echo reply packet to the source PE. In this way, the source PE can collect information
about each LSR that a PW passes through and information about the egress PE. At
present, the MPLS echo reply packet is an IP packet that does not carry any label.
The following takes the LSP between PE1 and PE4 for example to explain the
mechanism adopted by PWE3 tracert to collect node information.
By starting PWE3 tracert, PE1 can collect information about nodes that the LSP from
PE1 to PE4 passes through. By comparing paths obtained by the PWE3 tracert and PW
paths generated by the protocol, you can analyze whether there is an error.
If the PWE3 tracert obtains only information about PE4 (TTL=2) instead of information
about P (TTL=1), it indicates that P does not support MPLS ping.
If the PWE3 tracert obtains only information about P (TTL=1) instead of information
about PE4 (TTL=2), it indicates that PE4 or the link between P and PE4 is faulty.
If the PWE3 tracert obtains information about PE1, PE2, and PE4, it indicates that P
may be faulty. A new path is generated by the protocol.
– PWE3 MH Tracert
Figure 5-9 shows the networking diagram of PWE3 MH Tracert. An MH-PW is set up
between CE1 and CE2. The IDs of PW segments are different. The LSP is UPE1 - P1
- SPE1 - SPE2 - P2 - UPE2.
Quidway S9300 Terabit Routing Switch
Configuration Guide - VPN 5 PWE3 Configuration
Issue 03 (2009-08-20) Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
5-9
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