Enabling Sending Mpls Ttl-Expired Messages - HP 10500 Series Configuration Manual

When TTL propagation is disabled, the ingress node sets the label TTL to 255. Each LSR on the LSP
decreases the label TTL value by 1. The LSR that pops the label does not change the IP TTL value when
popping the label. Therefore, the MPLS backbone nodes are invisible to user networks, and the IP tracert
facility cannot show the real path in the MPLS network.
Figure 13 Without TTL propagation
Follow these guidelines when you configure TTL propagation:
HP recommends that you set the same TTL processing mode on all LSRs of an LSP.
•
To enable TTL propagation for a VPN, you must enable it on all PE devices in the VPN, so that you
•
can get the same traceroute result (hop count) from those PEs.
To enable TTL propagation:
Step
1. Enter system view.
2. Enable TTL propagation.
After you enable or disable TTL propagation, execute the reset mpls ldp command to make the
configuration take effect. For more information about the reset mpls ldp command, see MPLS Command
Reference.

Enabling sending MPLS TTL-expired messages

This feature enables an LSR to generate an ICMP TTL-expired message upon receiving an MPLS packet
with a TTL of 1. If the MPLS packet has only one label, the LSR sends the ICMP TTL-expired message back
to the source through IP routing. If the MPLS packet has multiple labels, the LSR sends it along the LSP to
the egress, which then sends the message back to the source.
To enable sending MPLS TTL-expired messages:
Step
Enter system view.
1.
2. Enable sending MPLS TTL-expired
messages.
Command
system-view
mpls ttl propagate { public |
vpn }
Command
system-view
mpls ttl expiration enable
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Remarks
N/A
By default, TTL propagation is enabled only
for public-network packets.
This command affects only the propagation
between IP TTL and label TTL. Within an
MPLS network, TTL is always copied between
the labels of an MPLS packet.
Remarks
N/A
By default, this function is enabled.