Configuring Ttl Processing Mode At Ingress - HP 12500 Series Configuration Manual

NOTE:
MPLS packets carrying L2VPN or IPv6 packets are always successfully forwarded, even if they are larger
•
than the MPLS MTU.
If the MPLS MTU of an interface is greater than the MTU of the interface, data forwarding may fail on the
•
interface.
• If you do not configure the MPLS MTU of an interface, fragmentation of packets will be based on the
MTU of the interface, and the length of fragments does not take the MPLS labels into account. Thus, after
an MPLS label is inserted into a fragment, the length of the MPLS fragment may be larger than the
interface MTU.

Configuring TTL processing mode at ingress

At the ingress of an LSP, a label stack encapsulated with a TTL field is added to each packet. Whether the
label TTL takes the IP TTL or not depends on whether IP TTL propagation is enabled:
• With IP TTL propagation enabled: When the ingress labels a packet, it copies the TTL value of the
original IP packet to the TTL field of the label. When an LSR forwards the labeled packet, it
decrements the TTL value of the label at the stack top by 1. When an LSR pops a label, it copies the
TTL value of the label at the stack top back to the TTL field of the IP packet. In this case, the TTL value
of a packet is decreased hop by hop when forwarded along the LSP. Therefore, the result of tracert
will reflect the real path along which the packet has traveled.
Figure 10 Label TTL processing when IP TTL propagation is enabled
TTL 253
Ingress
• With IP TTL propagation disabled: When the ingress labels a packet, it does not copy the TTL value
of the original IP packet to the TTL field of the label, and the label TTL is set to 255. When an LSR
forwards the labeled packet, it decrements the TTL value of the label at the stack top by 1. When an
LSR pops a label, it compares the IP TTL and the label TTL and uses the smaller value as the TTL of
the IP packet. In this case, the result of tracert does not show the hops within the MPLS backbone,
as if the ingress and egress were connected directly.
TTL 252
TTL 252
LSR A LSR B
TTL IP TTL
Copy TTL value
TTL 251 TTL 250
TTL 252 TTL 252
LSR C
TTL Label TTL
The penultimate hop copies
the label TTL to the IP packet
and pops the label.
21
TTL 249
LSR D LSR E
Egress
TTL 249
TTL 249
TTL 248