MPLS Header & Label

• The MPLS header is 4-byte header,
• located immediately before the IP header
• also referred as MPLS shim header 
• MPLS label is actually a 20-bit field in the MPLS header.
• MPLS Label or MPLS Label Stack (specifically)
• Fields in MPLS Header are:
• Label:
• length is 20-bits,with 
• identifies the potion of a LSP
• EXP(Experiment):
• 3-bits in length
• Used to map the standard IP packet Type Of Service (TOS) into the Experimental field fro MPLS Class Of Service(COS)
• only used for experimental purpose only
• S(Stack bit):
• MPLS labels are stacked one on other label.
• to indicate last MPLS header before ip header
• TTL(Time To Live)
• The TTl field from the IP TTL is decremented by 1 & then copied into the MPLS label TTL field.
• When exiting from the MPLS network, MPLS label TTL value is copied back to the IP TTL field
• If this field is set to 0, the packet will be discarded
• this field length is 8-bits
• MPLS label stack Placement:
• It is placed between Layer 2 header & Layer 3 header.
• For this some times MPLS labels stack referred as shim header
• Router forward packets based on the MPLS label header because it comes before the Layer 3 header.
• In MPLS, ip packets are switched instead of routed.
• "Labels are bound to routes in the routing table"
• In label stack, the outer label is used to forward the packet along the LSP, inner label is used to identify the VPN site. 
• This beneath label called as the VPN label

TERMS

• Overlay Model:
• In which the routers are connected in a full mesh through virtual circuits.
• Forward Equivalence Class (FEC):
• FEC is group of IP packets that are treated in same way(based on a number of criteria, like ip protocol id, port numbers, etc.
• CE: Customer Edge device
• Router that connect to the customer network & a service provider
• CE devices are not LSRs & can handle regular unlabeled IP packets
• PE: Provider Edge device
• This is a service provider equipment
• It connects to a customer & into the Provider(P) Network.
• P: Provider Device:
• Service provider equipment
• It exist in Provider network & connect to another service provider device not the customer
• LSR: Label Switch Router
• A router/switch that is capable of forwarding packets based on labels
• Edge-LSR: 
• More specific term for the PE router
• Also an LSR
• Push/Pop the label  to/from the ip packet and forward to next hop.
• A PE device is an Edge-LS in MPLS based networks.
• RIB: Routing Information Base:
• A router's unicast ip forwarding control plane uses routing protocols, static routes and connected routes to create a Routing Information Base.
• FIB: Forward Information Base:
• adding a FIB entry for each destination IP prefix in the routing table
• it will be possible after enabling the CEF.
• FIB entry has detailed information needed for forwarding:
• next-hop router
• outgoing interface
• Used for incoming unlabeled packets
• LFIB:

MPLS LDP

• LSRs uses LDP to send messages to their neighbors.
• By advertising an IP prefix & label in the update, the LSR says:
• if you want to send packets to this prefix, send them(packets) to me"
• Stand for RFC 3036 "LDP specification"
• Neighbor discovery:
• send via UDP port 646 to 224.0.0.5
• Neighbor adjacency
• uses tcp port 646 to remote LDP router-id
• Label advertisement 
• Advertise FEC for
• connected IGP interfaces
• IGP learned routes
• For MPLS unicast ip routing:
• LDP simply advertises labels for each prefix listed in the IP routing table.
• New ip route in the unicast ip routing table triggers the LDP advertisement.
• To learn the new route LSR allocates a label called a local label
• Local Label:
• which represent the ip prefix just added to the routing table.
• 
  

MPLS Commands

• ip cef
• mpls label protocol [LDP/TDP]
• LDP is default for new version ios
• TDP is default for old version ios
• mpls ip
• sh mpls ldp inerface
• sh mpls ldp neighbour
• sh mpls ldb bindings (local/remote)
• sh mpls forwarding-table
• sh ip cef
• config)# mpls ldp advertise-labels for 20 to 30
• advertise labels only 20 to 30 to its neighbors.
• sh control-plane host open-ports
• sh ip cef a.b.c.d  255.255.255.0
• sh mpls ldp parameters
• sh mpls ldp discovery
• mpls ldp router-id <interface> force
• mpls ldp discovery transport-address interface
• if some reasons loopback ip address is not reachable, tcp connection will not establish.
• then ldp to establish a tcp connection using physical interface ip address use above command under the interface level.
• mpls ldp neighbor <ip> password <password>
• ip-neighbor's ldp router-id
• mpls ldp password required
• to make use of password mandatory use the above command globally.
• 
  

MPLS Traffic Engineering

• Traffic Engineering: Manipulating traffic to fit to the available network resources. 
• In Traffic Engineering, simply tweaking the IP metrics on interfaces.
• Traffic engineering with MPLS is the best of connection-oriented traffic engineering techniques (such as ATM PVC placement) & merge them with IP routing.
• MPLS is an integration of Layer2 & Layer 3 technologies.
• MPLS enables Traffic Engineering, by making Layer 2 feature available to Layer 3.

MPLS & Routing

• A label represent a set of packets but not the particular path in network.
• Routing path is choosen by the existing layer 3 routing protocols

Distribution of LABEL BINDING

• Each LSR in network have independent & local  decision when forwarding ip packet.
• Label Binding:
• Each LSR in the network makes an independent, local decision as to which label value to use to represent a Forwarding Equivalence Class (different or same ip packets with same forwarding function).
• Each LSR informs its neighbors of the label bindings it has made.
• For this following protocols are used:
• TDP: Tag Distribution Protocol
• MPLS forwarding along normally routed path
• Cisco proprietorial and legacy 
• RSVP: Resource Reservation Protocol
• To support MPLS traffic engineering.
• BGP: Border Gateway Protocol
• Used to support MPLS VPNs
• Label value changes as the ip packet traverse the network.

Label Switching Functions

• In label switching, analysis of the layer 3 header is done only once.
• After this analysis, add a fixed length, unstructured value called a label.
• Many different header add to the same label (those headers are have same next hop)
• i.e. a label represents a forwarding equivalence class
• Means a set of packets which are different but they are indistinguishably by the forwarding function.
• The initial choice of label may not depend upon the content of layer 3 packet header
• Ex: Forwarding decisions at subsequent hops can also be based on routing policies.
• The packet header need not be reanalysis  during packet transit through the network.
• Because the label is fixed length and unstructured.
• So the MPLS forwarding table lookup process is straight forward & fast