That’s as simple on how to enable MPLS on a router. An other way to do this is using the command
“mpls ldp autoconfig” under the OSPF process.

We can also quickly check the LDP neighbour using the command “show mpls ldp neighbor”

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Few things to take note on the output above. I will focus on R2 as it has both R1 & R3 as LDP neighbours.

• Peer LDP identifier  is the loopback of both R1 and R3 while the Local LDP identifier is Loopback of R2
• MPLS works differently as other routing protocols such as OSPF. What i mean here is that MPLS needs to discover it neighbour using UDP Multicast Hello Packet (Multicast IP is 224.0.0.2) before becoming an LDP neighbours.
• Once the neighbour is established, the LDP routers will be using a TCP connection using the Transport IP address which is usually the highest loopback address

MPLS is using a label for every prefix learned by a MPLS routers. Unlike other routing protocol such as BGP or OSPF, MPLS is using a MPLS label to switch packet across the MPLS networks. So what are the components of a MPLS header. MPLS headers consist of 4 fields including Label Value ( 20 bits ), EXP Bits ( 3 bits) , S field ( 1 bit) and TTL field ( 8 bits). The Label value is local to the router generating the labels which has both the “Local” and “Outgoing” label values. The experimental bits are use for setting the QoS value or the IP precedence value of a packet. The S field will indicate whether there are more MPLS headers to arrived on the router ( 0 = means more MPLS headers are coming, whilst 1 = that is the last MPLS headers). The TTL field is similar to the IP packet in which every hop it is being decremented by 1.

MPLS network consist of the following routers:

•  Provider Edge Routers ( PE) – the router that connects to the customer edge routers.
• P or Transit Routers – This is also called the Label Switch routers
• Customer Edge routers

So how does the label works in the MPLS network?

Key thing to understand is the 3 operational process on how a MPLS label is switch.

• Push – This simply means “adding” a MPLS label to the IP packet
• Swap – This means MPLS labels will need to be “swap” with another MPLS value. This is due to the fact that labels are only local to the router.
• Pop – MPLS labels are being “removed” or “pop”. This is usually a function that is being done by the Transit router or LSR before the IP packet is handover to the PE router.

Its better to explained this MPLS label operations with my topology above. First, I need to show the local MPLS bindings on each router and explained a bit of the information provided. So what does the “show mpls ldp bindings” will provide to us?  This is equivalent to the Forwarding Information Base (FIB table) for a routing protocol and in MPLS it is called ” Label Information Based (LIB). For any dynamic or stating routing protocols,  The FIB table shows the Prefix learned,  the next hop IP address and the interface to reach the prefix and is programmed in Data Plane. Basically the FIB table is derived from the Routing Table from the Control Plane and its been programmed into the ASIC(Hardware) for faster processing.

To check the FIB table, we can run the command “show ip cef”.

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Going back to MPLS, to check the Label Information Base (LIB), we can run the command
“show mpls ldp bindings”.

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I’m more interested to check the Label Forwarding Information Based as this shows the actual label that is Push, Swap, Pop. Let’s check them now on the three routers.

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Let me share how the push, swap and pop labels are happening. R1 is advertising the prefix 10.10.10.10/32. So locally R1 has created an IP packet containing this Prefix and is adding a label on it. As check on the MPLS forwarding table of R1, the local label that is being added is “Implicit-Null”.  Meaning when R2 sees that the label is Implicit-Null, it will pop (removed) the label.

Finally, lets check the traceroute.

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