IP routers make forwarding decision based on IP packet header, and local CEF and FIB table.
MPLS routers make forwarding decision based on the MPLS label and the LFIB .
MPLS is great technology these days not because it forwards the packets faster , but because of applications and solutions we can provide for our customers like MPLS VPN’s .
MPLS uses the IP routing information to determine the direction and next hop to forward a labeled packet .
Does this before the first packet even arrives.
Common MPLS Terminologies
- LSP: Label Switch Path , Data Plane for a labeled Packet through the LSRs – LSP is Unidirectional
- LSR: Label Switch Router
- Ingress LSR: PE (Provider Edge First hop)
- Intermediate LSR: P (Provider)
- Egress LSR: PE (Provider Edge last hop)
- UpStream: Beginning at the network that is being advertised , this is the Control Plane for LDP
- DownStream: Transit path is downstream towards the network packets are trying to reach , Also called data plane .
MPLS Control Plane:
LDP : Label Distribution Protocol
LIB : Label Information Base
MPLS Data Plane:
LFIB: MPLS Label forwarding Information base
#show mpls ldp bindings #show mpls forwarding-table #show mpls ldp neighbor
- Labels live between L2 and L3 of the OSI
- A single Label corresponds to a single route ,and is locally significant to the router
- Labels are 4 bytes long
- Each router generates a local label for each route and then shares them via LDP with LDP neighbors
- Labels identify a Forward equivalence Class ( FEC ) . RFC 3031
- Implicit Null : Reserved Label #3
- PHP : Penultimate Hop Pop : Next to last LSR , Removes top label , so that egress LSR ( PE ) doesn’t have to .
In Frame Mode:
The label inserted between L2 and L3
L2 protocol identifier ( PID ) is changed to indicate that the packet has an MPLS label
- Unlabeled IP Unicast PID : 0x0800
- Labeled IP Unicast PID : 0x8847
- Labeled IP Multicast PID : 0x8848
In Cell-Mode (ATM Switches as LSRs) The fields of the ATM header are used as the labels.
- LDP: uses 646 TCP
- TDP: uses 711 TCP
- RSVP is used only for TE