R2511-HP MSR Router Series MPLS Configuration Guide(V5)

Figure 11 TTL processing when TTL propagation is disabled

Configuration guidelines

HP recommends configuring the same TTL processing mode on all LSRs along an LSP.

To enable IP 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. For more information about PEs, see

"Configuring MPLS L3VPN."

Configuration procedure

To configure TTL propagation of MPLS:

Ste

Command

Remarks

1. Enter system view.

system-view N/A

2. Enter MPLS view.

mpls N/A

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."