Propagation of IP TTL

There are two modes of operation for routed IP packets: pipe TTL mode and uniform TTL mode.

Currently, switches that run Extreme OS support only the pipe TTL mode. In pipe TTL mode, the LSP is viewed as a point-to-point link between the ingress LSR and the egress LSR; intermediate LSRs in the MPLS network are not viewed as router hops from an IP TTL perspective. Thus, the IP TTL is decremented once by the ingress LSR, and once by the egress LSR.

In this mode, the MPLS TTL is independent of the IP TTL. The MPLS TTL is set to 255 on all packets originated by the switch and on all packets that enter a pseudowire.