3Com 3C13636 Configuration Manual page 849

3Com Router 3000 Ethernet Family
Configuration Guide
forwards a labeled packet, it decrements the TTL value in the top label by one. When
the LSR pops the stack, it copies the TTL value in the top label back to the IP packet or
the lower label.
If an LSP has a non-TTL LSP segment which comprises a sequence of ATM-LSRs or
FR-LSRs unable to handle the TTL field, the TTL value must be decremented by the
value that reflects the length of the non-TTL LSP segment before a packet is forwarded
into this segment.
In MPLS VPN networking, you may hide the MPLS backbone topology for security sake.
You cannot however, apply on the ingress TTL duplication to VPN packets.
Perform the following configuration in MPLS view.
Table 2-5 Configure IP TTL duplication of MPLS
Enable IP TTL duplication of MPLS.
Disable IP TTL duplication of MPLS.
By default, IP TTL duplication is enabled for public-network packets and disabled for
VPN packets.
If IP TTL duplication is enabled at the ingress, the TTL value of packets is decremented
at each LSR hop that it passes. This allows the tracert to reflect the path that a packet
travels.
If IP TTL duplication is disabled at the ingress, the TTL value is not decremented at the
LSR hops that packets travel and as such, those hops in the MPLS backbone are
excluded from the path shown after you run a tracert, just as if the ingress and the
egress are directly connected.
Note that:
Inside an MPLS domain, if an MPLS packet has a label stack, the TTL value in a
label is always copied from other labels in the stack.
The TTL value of the transmitted local packets is copied regardless whether IP
TTL duplication is enabled or not. This ensures that the local administrator can
execute the tracert command to test the network.
At the egress, if IP TTL duplication is enabled, the TTL value in the MPLS label is
copied to the TTL field in the IP header and decremented by one.
Configure IP duplication for VPN packets in the same way at the ingress and the
egress: if the ttl propagate vpn command is enabled at the ingress, enable it at
the egress; if the command is disabled at the ingress, disable it also at the egress.
This ensures that the results of traceroutes can reflect the real network conditions.
You are recommended to enable this function on the involved PEs to ensure
consistency of the results gotten by tracerting on different PEs.
You need not to configure the ttl propagate command on any P routers.
Operation
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2-4
Chapter 2 MPLS Basic Capability Configuration
Command
ttl propagate { public | vpn }
undo ttl propagate { public | vpn }