TTL Behavior in the Case of Labelto Label

If the operation that is performed on the labeled packet is a swap, the TTL of incoming label -1 is copied to the swapped label. If the operation that is performed on the labeled packet is to push one or more labels, the received MPLS TTL of the top label -1 is copied to the swapped label and all pushed labels. If the operation is pop, the TTL of the incoming label -1 is copied to the newly exposed label unless that value is greater than the TTL of the newly exposed label, in which case the copy does not happen. Figure 3-5 shows examples of TTL propagation in the case of Label-to-Label operation for a swap, push, and pop operation.

Figure 3-5 TTL Propagation in Label-to-Label Operation in the Case of a Swap, Push, and Pop Operation

MPLS Label IPv4 Packet

SWAP

PUSH

SWAP

PUSH

LSR

TTL = 251

TTL = 251

TTL = 249

TTL = 248

TTL = 252

TTL = 248

TTL = 252

TTL = 252

TTL = 252

TTL = 252

TTL = 252

TTL = 252

TTL = 253

TTL = 253

TTL = 253

TTL = 253

TTL = 253

TTL = 253

->■

->-

-►

->-

->-

->-

The intermediate LSR does not change the TTL field in underlying labels or the TTL field in the IP header. An LSR only looks at or only changes the top label in the label stack of a packet.

NOTE The TTL behavior of labeled packets described here refers to the TTL operation in Cisco IOS.

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