What is the difference between MPLS and IP?

MPLS VS IP

(1) IP forwarding principle:

The router checks the destination IP address of the data packet and forwards the data according to the routing table. IP networks forward data according to the IP packet header.

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(2) MPLS forwarding principle:

The MPLS router (LER LSR) receives the MPLS datagram and forwards the MPLS datagram according to the label forwarding. MPLS Multi-Protocol Label Switching can carry multiple routing protocols.

(3) The most basic IP packet header:

MPLS packet header structure. Usually, the MPLS header has 32 bits, including:

• 20 bits for label
• 3 bits of EXP, not specified in the protocol, usually used as COS
• The 1-bit S is used to indicate whether it is the bottom of the stack, indicating that MPLS labels can be nested.
• 8-bit TTL

(4) MPLS Terminology

• Label: equivalent to the IP address in the IP network, which is meaningful for local routing.
• FEC: Equivalent to the network prefix in the IP network, one routing entry corresponds to one FEC. Each FEC generates a corresponding label. For example: 192.168.1.0/24 network prefix, then 192.168.1.1~192.168.1.254 belong to the same FEC.
• LSP: Label Switching Channel. The path that data flows along is LSP.
• LSR: Label Switching Router, a router in an MPLS network
• LER: Label switching edge router, which belongs to the edge router of MPLS network.

MPLS forwarding works

(1) How to generate label forwarding entries?

Note: The label forwarding table is similar to the routing table in the IPv4 network.

The router generates a corresponding label for each routing entry and puts the label into the label forwarding table.

A mapping relationship (FEC) needs to exist between routers and labels.

(2) How to insert the MPLS label header into the IP packet on LER?

When a data packet enters the MPLS domain from the IP domain, the LER inserts an MPLS packet header, and the specific label paper is generated according to the label forwarding table.

(3) How do routers in an MPLS domain send packets to their destinations?

The LSR device exchanges the label in the MPLS packet header according to the label forwarding table.

When an IP message enters, the LER device searches the label forwarding table and performs a label operation (PUSH) on the IP message. When the IP message leaves, the labeled message is popped out (POP) and forwarded according to the IP route.

• IP network forwarding principle: IP hop-by-hop forwarding requires a longest match search in the routing table (possibly multiple times) at each hop, which is slow.
• MPLS forwarding principle: MPLS label forwarding establishes a label forwarding channel (LSP) for the message through the pre-assigned labels. At each device through which the channel passes, only fast label exchange (one-time search) is required.

IP forwarding vs MPLS forwarding

(1) MPLS forwarding advantages:

• The packet header has very few fields, and the router can process this type of packet header efficiently.
• The forwarding process is simple, check the label
• MPLA forwarding still requires checking the label forwarding table

(2) MPLS forwarding defects: label survival depends on the IGP protocol and the routing table.