MPLS ( Multi-Protocol Label Switching )

屬於第三代網路架構，是新一代的 IP 高速骨幹網路交換標準，由 IETF ( Internet Engineering Task Force ，網際網路工程專案小組 ) 所提出，由 Cisco 、 3Com 等網路設備大廠所主

導。 MPLS 的運作原理是提供每個 IP 封包一個標籤，由此決定封包的路徑以及優先順序，與 MPLS 相容的路由器，會將封包轉送到其

路徑前，僅讀取封包標籤，無須讀取每個封包的 IP 位址以及標頭，因此網路速度便會加快許多，同時藉由 QoS ( Quality of Service ) 的機制對所傳送的封包加以分級，進而大幅提升網

路服務品質並且提供更多樣化的服務。

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

Key Terminology in MPLS FEC (Forwarding Equivalence Class)

A group of IP packets which are forwarded in the same manner ( 同類轉送等級，每一類 FEC 內的封包，都將以同樣的服務品質傳送到同樣的目的地 )

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

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)

PayloadIP“shim”Layer 2

Label value (20 bits) ExpS TTL

Exp: Experimental (3 bits)S: Bottom of label stack (1 bit)TTL: Time-To-Live (8 bits)

4 Octets

MPLS Header

Label – locally significant 20-bit label Experimental – 3-bits that are often us

ed for priority Stack – 1-bit bottom of stack indicator TTL – brought from the IP header to s

upport TTL even though MPLS does not examine IP header

Data Forwarding

Edge LSR(Ingress)

Edge LSR(Egress)

LSR

Label

IP

LSR(Penultimate)

PUSH POPSWAP SWAP

L2 header

Ingress and Transit Operation

Port 1 Port 4

Ingress LSR LSR

Port 2 Port 3

FEC Output10.60.0.0/16 port 4

PUSH label 40

To: 10.60.30.4

Input Outputport 2 label 40 port 3

SWAP label 45

Label: 40 Label: 40 Label: 45

Egress Operation

Port 1 Port 4

Egress LSR

Input Outputport 1 label 45 POP

Label: 45To: 10.60.30.4

To: 10.60.30.4

FEC Output Next Hop10.60.0.0/16 Port 4 10.1.2.6

The egress router has to do two table lookups

There is a concern that this might cause a performance penalty on the egress router