MPLS Forwarding Table

Outgoing label or VC

• no label means MPLS is not enable on that interface
• Pop label means MPLS is enabled and MPLS was enabled on directly connected interface.
• Digit indicates the remote interfaces on which MPLS is enabled

MPLS Applications

• MPLS change network design 
• by eliminating the need for an Overlay (full mesh of routers).
• Performance is improved 
• because packets are switched instead of routed.
• QoS can be implemented end to end
• by having an PE router classify packets & map a value to the Experimental (EXP) field of the MPLS label stack.
• Traffic Engineering is made possible through label stacking & traffic-engineered tunnels.

MPLS OPERATION

• Unlabeled packet enter into the service provider network via PE router.
• PE router add label impose a label to the unlabeled packet & then forward to the P router(also known as LSR) along the Label Switch Path(LSP) in the core network of service provider.
• In the core network of service provider each P routers forward the packet by swapping the labels along the LSR learned by protocol LDP.
• At other end when leaving service provider network PE router (also known as Edge-LSR) pops the label by mechanism called Penultimate Hop Popping.
• Penultimate meaning is "next to last"
• last hop in the service provider network must
• look up MPLS label
• POP MPLS label
• Look for IPv4 destination
• PHP avoids extra look up for MPLS label on last hop
• For this implict NULL label was advertised

MPLS Architecture

• Labels are bound to routes in the routing table
• MPLS architecture components:
• Control plane
• Forwarding plane
• CONTROL:
• Responsible for 
• binding a label to network routes
• for this we need routing table
• to get routing table we need a routing protocol
• and distribute those bindings among other MPLS enabled routers
• for this 2 protocols are used
• TDP
• LDP
• Tag Distribution Protocol(TDP):
• Cisco proprietary protocol
• used to bind tags to network routes in the routing table.
• FORWARDING:
• The routing table is built in the control plane & cached in forwarding plane.
• Forward Information Base is built by CEF.
• FIB is a cached version of the ip routing table that eliminates the need for a lookup of routing table.
• Router compares the packet's destination ip address to the CEF FIB, ignore the ip routing table.
• CEF optimizes the organization of FIB, so that router easily find the correct FIB entry,
• resulting in a smaller forwarding delay & high volume of packets per second through a router.
• For each packet, the router finds the matching FIB entry,
• then finds the adjacency table entry referenced by the matching FIB entry, 
• and forward the packet
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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