MPLS Tunnel

• MPLS tunnels are known as LSP(label switching path)
• MPLS tunnels(LSP) are unidirectional.
• MPLS main advantages No need to know about source & destination IP address.
• No need to run BGP in MPLS core. 
• Router outside the sp network can be label switched based on the BGP next-hop
• MPLS tunnel label, transports MPLS labeled VPN packets b/n Provider Edge routers along the LSP.
• MPLS VPN label remains the same between PEs.
• MPLS tunneling is most widely supported, particularly for manually configured, point to point tunnels.
• MPLS tunnel problems:
• BGP next-hop values must be loopback interface of remote PE.
•  BGP next-hop determine what label value should be used.
• Incorrect next-hop vlalue can result in traffic black hole in MPLS network
• label is PHPed one shop to soon
• MPLS tunnels are similar to Frame-Relay or ATM PVCs.
• Frame-Relay packets are switched based on the DLCI value found in the header.
• This DLCI value is purely local
• These DLCI value on packet header is rewritten every time the packet switched out.
• similar principle is employed in MPLS.
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MPLS Troubleshooting

• LDP Neighborship failed
• MPLS not enabled,
• LDP TCP-646/711 ports filtered
• No L3 route to LDP neigh
• Router ID
• Label not assigned
• CEF not enabled
• Label not shared
• LDP/TCP comaptible problems between neighbor.
• Slow convergence
• Don't use RIP(slow protocol) as IGP
• IGP is main reason for delay in convergense
• Large packets dropped
• Multiple labels may be present, pushing the MTU to a size not supported by the infrastructure.
• MPU not supported by switches

Config MPLS

• Requirements:
• CEF enabled:
• ip cef globally.
• IGP routing with full connectivity.
• Enable MPLS ip globally & on interfaces.
• Optional :
• Specify TDP/LDP/both as protocol
• Specify LDP router ID
• Specify transport IP address 
• If there are so many interfaces to enable MPLS
• use MPLS LDP autoconfig under the routing process(OSPF or EIGRP or etc)
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Loop prevention in MPLS

• LDP learns best routes from IGP.
• IGP will give best loop free paths.
• If the IGP have loops, MPLS TTL stops the forwarding of packet
• by TTL run from 255 to 0.
• for every switching of packet TTL will be decremented by 1.
• The initial TTL MPLS use in the label is copied from original IP packet TTL.

Unsolicited & Lieberal

• Without asking, labels can advertise towards downstream is called downstream Unsolicited label advertising.
• Liberal Label retention:
• LSR learn the both best & 2nd best path from all received advertisements.