Ttl Processing In Mpls - 3Com MSR 50 Series Configuration Manual

PHP

TTL Processing in MPLS

Conservative: In this mode, an LSR keeps only label to FEC bindings that are
■
from its next hops for the FECs.
In liberal mode, an LSR can adapt to route changes quickly; while in conservative
mode, there are less label to FEC bindings for an LSR to advertise and keep.
The conservative label retention mode is usually used together with the DoD mode
on LSRs with limited label space.
Basic concepts for label switching
Next hop label forwarding entry (NHLFE): Operation to be performed on the
■
label, which can be Push or Swap.
FEC to NHLFE map (FTN): Mapping of a FEC to an NHLFE at the ingress node.
■
Incoming label map (ILM): Mapping of each incoming label to a set of NHLFEs.
■
The operations performed for each incoming label includes Null and Pop.
Label switching process
Each packet is classified into a certain FEC at the ingress LER. Packets of the same
FEC travel along the same path in the MPLS domain, that is, the same LSP. For
each incoming packet, an LSR examines the label, uses the ILM to map the label to
an NHLFE, replaces the old label with a new label, and then forwards the labeled
packet to the next hop.
As described in "Architecture of MPLS" on page
MPLS network forwards an incoming packet based on the label of the packet,
while the egress LER removes the label from the packet and forwards the packet
based on the network layer destination address.
In fact, on a relatively simple MPLS application network, the label of a packet is
useless for the egress, which only needs to forward the packet based on the
network layer destination address. In this case, the penultimate hop popping (PHP)
feature can pop the label at the penultimate node, relieving the egress of the label
operation burden and improving the packet processing capability of the MPLS
network.
MPLS TTL processing involves two aspects: TTL propagation and ICMP response
path.
IP TTL propagation
An MPLS label contains an 8-bit long TTL field, which has the same meaning as
that of an IP packet.
According to RFC 3031 "Multiprotocol Label Switching Architecture", when an
LSR labels a packet, it copies the TTL value of the original IP packet or the upper
level label to the TTL field of the newly added label. When an LSR forwards a
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 or lower level label.
TTL can be used not only to prevent routing loops, but to implement the tracert
function:
MPLS Configuration Basics
1314, each transit LSR on an
1319