MPLS TTL field & It propagation

• MPLS TTL is similar to IP header's TTL
• IP header's TTL used for:
• identifying loops
• traceroute command to find the ip address of each router in a particular end to end route.
• MPLS TTL used for same above ip TTL functions.
• From this we confirmed that, presence or absence of MPLS in a network has no effect on the TTL related processes.
• When switching LSR will decrement the MPLS TTL but not the IP TTL.
• TTL in MPLS network:
• At Ingress E-LSR:
• It decrements the IP TTL field in unlabeled packet
• then push a label in unlabeled packet
• & copy the decremented IP TTL into the new MPLS TTL.
• At LSR:
• When LSR swaps a label, MPLS TTL will be decremented 
• & doesn't effect the IP TTL
• At Egress E-LSR:
• After an egress E-LSR decrements the MPLS TTL field, it pops the MPLS label (header)
• & then copies the MPLS TTL to the IP TTL.
• A looping packet would decrements to TTL 0 and discarded.

MPLS Label Filtering

• By default LDP will generate & advertise labels for every prefix in the local routing table.
• To filter & generate labels only for required prefixes
• we can use access control lists to select the required prefixes eligible for label generation.
• example:
• create access list:
• """access-list 10 permit 150.1.0.0 0.0.255.255"""
• Stop automatic assigning of labels to prefixes.
• """no mpls ldp advertise-labels"""
• use of access list to filter the label generation
• """mpls ldp advertise-labels for 10"""
• Before MPLS label filtering:

• After MPLS label filtering:

MPLS forwarding using FIB & LFIB

• To forward packets LSR uses:
• CEF FIB
• MPLS LFIB
• Both the FIB & LFIB hold 
• necessary label information
• outgoing interface
• next-hop 
• CEF FIB: Forward Information Base
• Used for incoming unlabeled packets.
• Router matches the packet's destination IP address to the best prefix in the FIB
• And forward the packet based on that entry.
• MPLS LFIB: Labeled Forward Information Base:
• Used for labeled packets.
• Router compares the label in the incoming packet to the LFIB's list of label 
• and forward the packet based on that LFIB entry.

• Above image taken from Cisco press: ccie R&S certification guide, 4th edition

• MPLS enable forwarding process based on something other than the destination ip address such as:
• VPN from which the packet originated
• forwarding to balance traffic with traffic engineering
• & forwarding over different links based on QoS goals.

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VRF: Virtual Routing & Forwarding

• VRF:
• VRF tables are the fundamental building block for virtualizing a router, it turn into multiple virtual routers.
• Technically VRF is a separate RIB(Routing Information Base) & FIB (Forward Information Base)
• Any interface on the router could be assigned to a VRF.
• using command "ip vrf forwarding <name>"
• this command will erase all existing ip address config on the interface (to avoid duplication)
• After this configuration, all packets recevied on the interface are routed & forwarded using the associated VRF table.
• VRF enabled interfaces are not showed in global routing table
• i.e show ip route
• Each VRF has its own routing table
• to see this routing table use "show ip vrf "
• Interfaces showed in global routing table are not in any vrf.
• i.e. VRF & global routes are separate.
• VRFs without MPLS is considered as "VRF Lite"
• If 2 VRFs have same ip prefix but they cannot route to each other.
• Because they are separately labeled.
• We cannot manually leak the traffic between VRFs by creating static routes.
• i.e. interfaces are route with other interfaces which are in same VRF.
• BGP is enhanced to handle VRF specific routes.
• A new sepcial MP-BGP address family named "VPN IPv4" has been added to bgp along with new NLRI format.
• To support multiple customers in MPLS VPN, VRF tables were used.
• VRF tables are used to store routes separately for different customer VPNs.
• The use of separate tables solves some problems:
• Leakage packets from one customer to another due to overlapping prefixes
• VRF has 3 main components:
• An IP routing table (RIB)
• A CEF FIB, populated based on that VRF's RIB
• A separate process of the routing protocol used to exchange routes with the CE's.
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MPLS Laeyer 3 VPNs

• VPNs:
• Customers can connect geographically divers sites across the provider's network
• Traditionally VPN were based on IPsec(layer-3) or TLS(laery-2)
• These 2 were slow & having less features.
• By using MPLS we will overcome these problems.
•  With Layer-3 VPNs the service provider participate in the customer's Layer-3 routing.
• Service provider's PE router  connect with CE router with L3 protocols
• Layer 2 VPNs: Provider connect the customer site with layer 2 technologies like ATM, Frame-relay or ethernet.
• MPLS Layer 3 VPNs:
• Combines the logic of MPLS tunnels with layer 3 routing information
•  PE routers learn customer routes from Customer Edge(CE) routers.
• PE routers advertise customer routes to other PEs via multi-protocol BGP.
• No need to know about the customer route in the middle of the SP network.
• BGP next-hops point to MPLS tunnels
• ex: loopbacks of PE routers
• MPLS L3 VPNs have 2 basic components
• Seperation of customer routing information
• to do this VRF (Virtual Routing & Forwarding) used.
• VRF used on PE routers to keep track on customer routes on per interface basis.
• Exchange of customer routing information.
• to do this MP-BGP is used over the MPLS network.
• Traffice is label switched towards the BGP next-hops.
• The idea of MPLS VPN is 
• establishing a full-mesh of dynamic MPLS LSRs between PE routers.
• using these PE routers for tunneling VPN packets across the network core.
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