Simple MPLS Packet Flow

 

I want to tell about simple MPLS packet Flow

All core Routers(PE1,PE2,P1,P2) run IGP protocol(ISIS or OSPF) and MPLS LDP.

IGP Phase(OSPF): PE2 advertise 20.20.20.0/30 network to OSPF and core routers learn that prefix.

LDP Phase: PE2 locate label 20.20.20.0/30 as FEC(forwarding Equivalent Class): 20.20.20.0/30:Label3 and advertised to P2(upstream device).

P2 locate label 20.20.20.0/30 as FEC: 20.20.20.0/30: Label300 and advertised to P1(upstream device).

P1 locate Label 20.20.20.0/30 as FEC: 20.20.20.0/30: Label200 and advertised to PE1(upstream device).

PE1 locate Label 20.20.20.0/30 as FEC:20.20.20.0/30: Label100 and advertised to upstream device(currently not have)

1. Host1 Operation:"ping 20.20.20.2

-Forwarding Lookup based on Destination IP and Out-Interface: IntH1

-TTL handling:TTL is 127 and reduce 126

-Ethernet-Frame: Ethernet Header: Src-MAC:Host1-mac, Dest-MAC:CE1-IntA2-MAC, Ether-Type(0x0800: IPv4)

IP Header: src-IP:10.10.10.2, Dest:20.20.20.2, TTL:126

-Encapsulate and send to CE1

-

2. CE1 Operation

-Decapsulate and check Ether-Type(0x0800:IPv4) and forwarding lookup(FIB) based on Destination IP and Out-Interface:IntA1

-TTL Handling:TTL will be 125

-Ethernet-Frame: Ethernet Header: Src-MAC:CE1:IntA1-MAC, Dest-MAC:PE1:IntB1-MAC, Ether-Type(0x0800: IPv4)

IP Header: src-IP:10.10.10.2, Dest:20.20.20.2, TTL:125

-Encapsulate and send to PE1

3. PE1 Operation

-Decapsulate and check Ether-Type(0x0800:IPv4) and forwarding lookup(FIB) based on Destination IP and Out-Interface:IntB2 and Push Label:200

-TTL Handling: Reduce TTL from 125 to 124 and copy that TTL value to MPLS Header TTL

-Ethernet-Frame: Ethernet Header: Src-MAC:PE1:IntB2-MAC, Dest-MAC:P1:IntC2-MAC, Ether-Type(0x8847: MPLS Unicast)

MPLS Header: Label:200, EXP:0, S:1(bottom label), TTL:124

IP Header: no change

-Encapsulate and send to P1

4. P1 Operation

-Decapsulate and check Ether-Type(0x8847: MPLS Unicast) and forwarding lookup(LFIB), Label:200 swapping to 300 and out-interface:IntC1

-TTL Handling Reduce TTL from 124 to 123

-Ethernet-Frame: Ethernet Header: Src-MAC:P1:IntC1-MAC, Dest-MAC:P2:-IntD1-MAC, Ether-Type(0x8847: MPLS Unicast)

MPLS Header: Label:300, EXP:0, S:1(bottom label), TTL:123

IP Header: no change

-Encapsulate and send to P2

5. P2 Operation

-Decapsulate and check Ether-Type(0x8847: MPLS Unicast) and forwarding lookup(LFIB), Label:300 to popping label and out-interface:IntD2

-TTL Handling Reduce TTL from 123 to 122 and copy that TTL to IP header TTL

-Ethernet-Frame: Ethernet Header: Src-MAC:P2:IntD1-MAC, Dest-MAC:PE2:-IntE2-MAC, Ether-Type(0x0800: IPv4)

IP Header: src-IP:10.10.10.2, Dest-IP:20.20.20.2, TTL:122

-Encapsulate and send to PE2

6. PE2 Operation

-Decapsulate and check Ether-Type(0x0800:IPv4) and forwarding lookup(FIB) based on Destination IP and Out-Interface:IntE1

-TTL Handling:TTL will be 121

-Ethernet-Frame: Ethernet Header: Src-MAC:PE2:IntE1-MAC, Dest-MAC:CE2:IntF1-MAC, Ether-Type(0x0800: IPv4)

IP Header: src-IP:10.10.10.2, Dest:20.20.20.2, TTL:121

-Encapsulate and send to CE1.

7. CE2 Operation

-Decapsulate and check Ether-Type(0x0800:IPv4) and forwarding lookup(FIB) based on Destination IP and Out-Interface:IntF2

-TTL Handling:TTL will be 120

-Ethernet-Frame: Ethernet Header: Src-MAC:CE2:IntF2-MAC, Dest-MAC:Host2:IntH2-MAC, Ether-Type(0x0800: IPv4)

IP Header: src-IP:10.10.10.2, Dest:20.20.20.2, TTL:120

-Encapsulate and send to Host2

8. Host2 Operation

--Decapsulate and check Ether-Type(0x0800:IPv4) and forwarding lookup(FIB) based on Destination IP and see its own IP and read Payload