Inspecting An Mpls Lsp - 3Com MSR 50 Series Configuration Manual

1320 C 76: MPLS B
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Inspecting an MPLS LSP

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ASICS ONFIGURATION
With IP TTL propagation enabled at ingress, whenever a packet passes a hop
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along the LSP, its IP TTL gets decremented by 1. Therefore, the result of tracert
will reflect the path along which the packet has traveled.
With IP TTL propagation disabled at ingress, the IP TTL of a packet does not
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decrement when the packet passes a hop, and the result of tracert does not
show the hops within the MPLS backbone, as if the ingress and egress were
connected directly.
CAUTION:
Within an MPLS domain, TTL propagation always occurs between the
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multi-level labels.
The TTL value of a transmitted local packet is always copied regardless of
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whether IP TTL propagation is enabled or not. This ensures that the local
administrator can tracert for network test.
For network security, the structure of the MPLS backbone may need to be
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hidden in an MPLS VPN application. In this case, TTL propagation is not allowed
for private network packets at ingress.
ICMP response
On an MPLS VPN, P routers cannot route VPN packets carried by MPLS. When the
TTL of an MPLS packet expires, an ICMP response will be generated and
transported along the LSP until it reaches the destination router of the LSP, where
it is forwarded by IP routing. Such processing increases the network traffic and the
packet forwarding delay.
For description and configuration of P routers, refer to
Configuration" on page 1459
For an MPLS packet with only one level of label, the ICMP response message
travels along the IP route when the TTL expires.
In MPLS, the MPLS control plane is responsible for establishing an LSP. However, it
cannot detect the error when an LSP fails to forward data. This brings difficulty to
network maintenance.
MPLS LSP ping and traceroute provide a mechanism for detecting errors in LSP and
locating nodes with failure in time. Similar to IP ping and traceroute, MPLS LSP
ping and traceroute use MPLS echo requests and MPLS echo replies to check the
availability of LSPs. The MPLS echo request message carries FEC information to be
detected, and is sent along the LSP like other data packets of the same FEC. Thus,
the LSP can be checked.
MPLS LSP ping is a tool for checking the validity and availability of an LSP. It
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uses messages called MPLS echo requests. In a ping operation, an MPLS echo
request is forwarded along an LSP to the egress, where the control plane
determines whether the LSR itself is the egress of the FEC and responds with an
MPLS echo reply. When the ping initiator receives the reply, the LSP is
considered perfect for forwarding data.
MPLS LSP traceroute is a tool for locating LSP errors. By sending MPLS echo
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requests to the control plane of each transit LSR, it can determine whether the
LSR is really a transit node on the LSP.
and "MPLS L2VPN Configuration" on page
"MPLS L3VPN
1425.