R3303-HP HSR6800 Routers MPLS Configuration Guide

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Ste
p
Command Remarks
3. Enable MPLS TTL propagation.
ttl propagate { public | vpn }
Optional.
Enabled only for public network
packets by default.
Sending back ICMP TTL exceeded messages for MPLS TTL
expired packets
After you enable an LSR to send back ICMP TTL exceeded messages for MPLS TTL expired packets, when
the LSR receives an MPLS packet that carries a label with TTL being 1, it generates an ICMP TTL exceeded
message, and send the message to the packet sender in one of the following ways:
• If the LSR has a route to the packet sender, it sends the ICMP TTL exceeded message to the packet
sender directly through the IP route.
• If the LSR has no route to the packet sender, it forwards the ICMP TTL exceeded message along the
LSP to the egress, which sends the message to the packet sender.
Usually, for an MPLS packet carrying only one level of label, the first method is used. For an MPLS packet
carrying a multi-level label stack, the second method is used. However, because ASBRs, SPEs in HoVPN
applications, and carrier backbone PEs in nested VPNs might receive MPLS VPN packets that carry only
one level of labels but these devices have no IP routes to the packet senders, the first method is not
applicable. In this case, you can configure the undo ttl expiration pop command on these devices so the
devices use the second method.
For more information about HoVPN and nested VPN, see "Configuring MPLS L3VPN."
To configure the device to send back an ICMP TTL exceeded message for a received MPLS TTL expired
packet:
Ste
p
Command Remarks
1. Enter system view.
system-view N/A
2. Enter MPLS view.
mpls N/A
3. Enable the device to send
back an ICMP TTL exceeded
message when it receives an
MPLS TTL expired packet.
ttl expiration enable
Optional.
Enabled by default.
4. Configure the device to use IP
routes or LSPs to send back the
ICMP TTL exceeded messages
for TTL-expired MPLS packets
that have only one level of
label.
• (Method 1) Use IP routes:
ttl expiration pop
• (Method 1) Use LSPs:
undo ttl expiration pop
Optional.
Use either method as required.
By default, an ICMP TTL exceeded
message is sent back along an IP
route when the TTL of an MPLS
packet with a one-level label stack
expires.
This configuration does not take
effect when the MPLS packets carry
multiple levels of labels. ICMP TTL
exceeded messages for such MPLS
packets always travel along the
LSPs.