MPLS ( Multi-Protocol Label Switching )

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MPLS ( Multi-Protocol Label Switching )

• ?????????,?????IP??????????,? IETF ( Internet
  Engineering Task Force,?????????? ) ???,?
  Cisco?3Com ???????????MPLS ?????????? IP
  ??????,???????????????,? MPLS ??????,???????????,?
  ??????,?????????IP??????,????????????,???? QoS (
  Quality of Service ) ??????????????,??????????????
  ??????????

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MPLS Related Protocols

• Data forwarding
• Label encapsulation
• Label operations PUSH, SWAP and POP
• Label distribution protocols (RFC 3036)
• Provide procedures by which one LSR informs
  another of the label/FEC binding
• Extensions to routing protocols
• Existing routing protocols can be extended to
  distribute traffic engineering information

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Key Terminology in MPLS

• FEC (Forwarding Equivalence Class)
• A group of IP packets which are forwarded in the
  same manner (??????,???FEC????,???????????????????
  )
• Label
• A short fixed length identifier which is used to
  identify a FEC
• Label Swapping
• Looking up the incoming label to determine the
  outgoing label, encapsulation and port
• Label Switched Path (LSP)
• Path through one or more LSRs for a particular
  FEC
• Label Switching Router (LSR)
• An MPLS capable router

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What is a Label

• The label can be carried in a layer 2 header
  (e.g., ATM and frame relay) or in a shim that
  sits between the layer 2 header and IP (e.g., LAN
  and PPP)

Payload
IP
shim
Layer 2
Label value (20 bits)
Exp
S
TTL
4 Octets
Exp Experimental (3 bits) S Bottom of label
stack (1 bit) TTL Time-To-Live (8 bits)
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MPLS Header

• Label locally significant 20-bit label
• Experimental 3-bits that are often used for
  priority
• Stack 1-bit bottom of stack indicator
• TTL brought from the IP header to support TTL
  even though MPLS does not examine IP header

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Data Forwarding
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Ingress and Transit Operation
Input Output port 2 label 40 port 3 SWAP label
45
FEC Output 10.60.0.0/16 port 4 PUSH
label 40
Label 40
Label 40
Label 45
To 10.60.30.4
Port 1
Port 4
Port 2
Port 3
Ingress LSR
LSR
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Egress Operation

• The egress router has to do two table lookups
• There is a concern that this might cause a
  performance penalty on the egress router

To 10.60.30.4
Label 45 To 10.60.30.4
FEC Output Next Hop 10.60.0.0/16 Port 4
10.1.2.6
Input Output port 1 label 45 POP
Port 1
Port 4
Egress LSR
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