How MPLS Works

In the previous lesson, the label appeared at R1 and disappeared at R5.
This lesson follows a single packet across the same five routers to show the exact operation each router performs.

Push at the Ingress (R1)

A packet from 192.168.1.0/24 arrives at R1, heading for 192.168.2.10.

As the ingress LSR, R1 performs an IP lookup using its FIB (Forwarding Information Base), the forwarding table built from the routing table.

Figure 1 – Ingress label push operation (R1)

Look at R1's FIB entry for destination 192.168.2.0/24.
It provides two essential instructions:

• The next hop: R2

• The label to push: 58

R1 inserts label 58 between the frame header and the IP header, then forwards the packet.
This operation is a push.

On Cisco routers, Cisco Express Forwarding (CEF) builds the FIB, which is why you query it with show ip cef:

R1# show ip cef 192.168.2.0 255.255.255.0 detail
192.168.2.0/24, epoch 0, per-destination sharing
  local label info: global/24
  nexthop 10.0.12.2 GigabitEthernet0/1 label 58

The nexthop line indicates both the destination IP address of the next hop (10.0.12.2) and the label (58) to be pushed.

Swap at the Intermediate LSR (R2)

R2 receives a labeled packet, so it does not consult the IP routing table at all.
Instead, it uses a dedicated table: the LFIB (Label Forwarding Information Base), indexed directly by the incoming label.

Figure 2 – Intermediate label swap operation (R2)

Think of the LFIB as the label lookup table: incoming label in, outgoing label out, next hop out.
The LFIB entry for incoming label 58 provides two instructions:

• The outgoing label: 31

• The next hop: R3

R2 strips label 58, writes label 31, and forwards the packet.
This operation is a swap.

Here is the LFIB on R2:

R2# show mpls forwarding-table 192.168.2.0 255.255.255.0
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop
Label      Label      or Tunnel Id     Switched      interface
58         31         192.168.2.0/24   9834          Gi0/1      10.0.23.3

You can clearly see both the local (incoming) label 58 and the outgoing label 31 for destination network 192.168.2.0/24.

Swaps Across the Core (R3 and R4)

Notice that the label value changed: labels are locally significant, meaning each router manages its own label space independently.

Figure 3 – Intermediate label swap operation (R3)

R3 repeats the exact same lookup process using its own LFIB table.

• Label in: 31

• Label out: 67

• Next hop: R4

Here is the LFIB on R3:

R3# show mpls forwarding-table 192.168.2.0 255.255.255.0
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop
Label      Label      or Tunnel Id     Switched      interface
31         67         192.168.2.0/24   7215          Gi0/1      10.0.34.4

Same structure as R2, just different values: incoming label 31 maps to outgoing label 67 toward R4.

One intermediate router remains before the egress edge: R4.

Figure 4 – Intermediate label swap operation (R4)

R4 swaps incoming label 67 for outgoing label 42 and forwards the packet to R5.