3com S7906E Instruccion De Instalación
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The conservative label retention mode is usually used together with the DoD mode on LSRs with limited
label spaces.
label spaces.
Currently, the S7900E series supports only the liberal mode.
Basic concepts for label switching
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Next hop label forwarding entry (NHLFE): Operation to be performed on the label, which can be
Push or Swap.
Push or Swap.
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FEC to NHLFE mapping (FTN): Mapping of a FEC to an NHLFE at the ingress node.
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Incoming label mapping (ILM): Mapping of each incoming label to a set of NHLFEs. The operations
performed for each incoming label can be Null or Pop.
performed for each incoming label can be Null or 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.
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.
PHP
, each transit LSR on an MPLS network forwards an incoming
packet based on the label of the packet, while the egress removes the label from the packet and
forwards the packet based on the network layer destination address.
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.
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.
TTL Processing in MPLS
MPLS TTL processing involves two aspects: IP 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 lower 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 the lower level label.
copies the TTL value of the original IP packet or the lower 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 the lower level label.
TTL can be used not only to prevent routing loops, but to implement the tracert function: