- Forwards based on labels instead of L3 destinations.
- MPLS not much faster than today’s traditional IP forwarding, but MPLS does create less overhead.
- Can forward any L3 protocols, not just IP.
- Label Switching Routers
- Routers that run MPLS and can receive/transmit labeled packets.
- Label Switched Path
- Entire labeled path through an MPLS Domain.
- Sequence of routers.
- Source to destination and vice versa can be completely different paths.
- Often return is similar due to routing protocols.
- Shim between layer 2 frame and layer 3 packet – hence MPLS L2.5
- 4 bytes/32 bits.
- 1st 20 bits – label number
- 2nd 3 bits – experimental, used for QoS, similar to DSCP.
- 3rd 1 bit – S field, used to show whether last label in stack.
- 4th 8 bits – TTL, similar to IP TTL used to discard if reaches 0.
- Label Distribution Protocol
- Uses Multicast to send hellos to other LDP routers, create neighbor adjacency.
- UDP to discover neighbors. Adjacency built via TCP.
- Transport Address is what’s used to build the TCP connection for adjacency.
- Will choose an IP from neighbor router, need to make sure its actually routable in the IGP.
- LDP generates a label locally for each prefix found in the RIB. Label information is then added to the Label Information Base (LIB).
- LIB is used to generate info in the LFIB.
- show mpls ldp bindings
- shows LIB of MPLS router
- show mpls forwarding-table
- shows LFIB of MPLS router.
- The Label Switched Path (LSP) can be seen through a traceroute on MPLS router.