3com S7906E Instruccion De Instalación
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With IP TTL propagation enabled at ingress, whenever a packet passes a hop 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.
TTL gets decremented by 1. Therefore, the result of tracert will reflect the path along which the
packet has traveled.
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With IP TTL propagation disabled at ingress, the IP TTL of a packet does not decrement when the
packet passes a hop along the LSP, and the result of tracert does not show the hops within the
MPLS backbone, as if the ingress and egress were connected directly.
packet passes a hop along the LSP, and the result of tracert does not show the hops within the
MPLS backbone, as if the ingress and egress were connected directly.
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Within an MPLS domain, TTL propagation always occurs between the multi-level labels.
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The TTL value of a transmitted local packet is always copied regardless of whether IP TTL
propagation is enabled or not. This ensures that the local administrator can tracert for network test.
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 hidden in an MPLS VPN
application. In this case, TTL propagation is not allowed for private network packets at ingress.
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.
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 MPLS L3VPN Configuration and MPLS L2VPN
Configuration
in the MPLS Volume.
For an MPLS packet with only one level of label, the ICMP response message travels along the IP route
when the TTL expires.
when the TTL expires.
Inspecting an MPLS LSP
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.
error when an LSP fails to forward data. This brings difficulty to network maintenance.
MPLS LSP ping and traceroute can be used to detect errors in LSPs and locate nodes with failures 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 the FEC
information of the LSP to be detected, and is sent along the LSP like other data packets of the FEC.
Thus, the LSP can be checked.
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 the FEC
information of the LSP to be detected, and is sent along the LSP like other data packets of the FEC.
Thus, the LSP can be checked.
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MPLS LSP ping is a tool for checking the validity and availability of an LSP. It uses messages
called MPLS echo requests. In a ping operation, MPLS echo requests are forwarded along an LSP
to the egress, where the control plane confirms that the LSR is the egress of the FEC and responds
called MPLS echo requests. In a ping operation, MPLS echo requests are forwarded along an LSP
to the egress, where the control plane confirms that the LSR is the egress of the FEC and responds