TTL Behavior in the Case of IPto Label or LabeltoIP

In MPLS, the usage of the TTL field in the label is the same as the TTL in the IP header. When an IP packet enters the MPLS cloud—such as on the ingress LSR—the IP TTL value is copied (after being decremented by 1) to the MPLS TTL values of the pushed label(s). At the egress LSR, the label is removed, and the IP header is exposed again. The IP TTL value is copied from the MPLS TTL value in the received top label after decrementing it by 1. In Cisco IOS, however, a safeguard guards against possible routing loops by not copying the MPLS TTL to the IP TTL if the MPLS TTL is greater than the IP TTL of the received labeled packet. If the MPLS TTL would be copied to the IP header, the smaller IP TLL value would be overwritten by a newer but higher value. If the IP packet would be injected into the MPLS cloud again—such as the result of a routing loop—the packet could live forever because the TTL would never reach 0. Figure 3-4 shows the default behavior of copying or propagating the TTL between the IP header and the MPLS labels and vice versa.

Figure 3-4 Propagation Behavior of TTL Between IP Header and MPLS Labels

| | MPLS Label IPv4 Packet

| | MPLS Label IPv4 Packet

TTL = 248

Ii ig ii rsi

Ingress LSR LSR LSR Egress

LSR LSR

Label Switched Path

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