MPLS Route Targets

• MPLS uses Route Targets to determine in which VRFs, a PE places IBGP-learned routes.
• It is 64-bit extended BGP community.
• It is attached to a VPNv4 BGP route to indicate its VPN membership
• Any number of RTs attached to a single route up to the BGP update packet size of 4096 bits.
• Export RTs
• Attached to a route when it is converted into a VPN4 route.
• Identify the VPN membership by associating routes to a VRF
• Import RTs
• Used to select VPNv4 routes for insertion into matching VRF tables.
• On the receiving PE router, a route is imported into a vrf only if at least one RT attached to the route matches at least one import RT configured in that VRF(route map condition must be met if configured).
• An import or export map allows route control on a per-route basis.
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MP-BGP & Routing Distinguishers

• Routes learned from the CE router are advertised to other PE routers uses the IBGP from all the routes, from all the different VRFs.
• If use normal BGP is used, may overlapping of prefixes will be occurred.
• MPLS deals this problem by
• Add another number in front of the original BGP NLRI.
• Each different number can represent a different customer.
• To do this MPLS uses the MultiProtocol BGP.
• MP BGP  allows re-define the NLRI filed in BGP updates.
• This re-defination allows for an additional variable-length umber, called Address family
• This address family added at, in front of the prefix. 
• MPLS RFC 4363, "BGP/MPLS IP Virtual Private Networks(VPNs)," defines a specific new address family to support IPv4 MPLS VPNs--named as an MP-BGP address family called  Route Distinguishers (RDs)
• RDs allow BGp to advertise & distinguish between duplicate IPv4 prefixes.
• The concept is simple:
• Advertise each NLRI as the traditional IPv4 prefix, but add another number (the RD)
• RD uniquely identifies the route.
• In the new NLRI format, called VPN-V4, has 2 parts:
• 64-bit RD
• 32-bit IPv4 prefix
• example: 1:111:10.2.2.0/24
• Every VRF must be configured with an RD.
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PE Role in MPLS

• PE router:
• An LSR that shares a link with at least one Customer Edge router, 
• edge of MPLS VPN, IBGP & VRF tables
• PE & P routers can together label switch packets from the ingress PE to the egress PE router.
• PE .have several other duties:
• Learn customer routes
• & keep track of which routes belong to which customer.
• Exchange routes with connected CE routers from various customers.
• To keep the track of the possibly overlapping prefixes.
• PE routers do not put the routers in normal IP routing table
• instead , PEs store routes in separate per-customer routing tables, called VRFs
• To exchange these customer routes with other PEs use IBGP.
• never advertise these routes to P routers.
• PEs advertise Route Targets in BGP updates as BGP Extended Community Path Attributes (PAs)

Feeding the FIB & LFIB

• LIB: Label Information Base
• Each LSR store all labels & their related information in Label Information Base.
• Each LSR must choose the best label & outgoing interface & then populate that information into the FIB & LFIB
• As a result, the FIB & LFIB having the best currently used LSP.
• Best route in IP routing table become the best LSP in LIB.
• LSR makes the following decision:
• for each route in the routing table
• find the corresponding label information in LIB
• based on the outgoing interface & next hop router.
• Add the corresponding label information to the FIB & LFIB.
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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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