MPLS VPN BGP Role

• BGP well suited to carry the traffic of  hundreds of thousands of routes.
• It is flexible & extended policies to be implemented.
• So it is used well in MPLS VPN.
• In MP BGP 4 address families will be supported.
• IPv4
• IPv6
• VPNv4
• VPNv6
• Remaining are unicast, multicast & VRF
• BGP Extended Community: RT
• It tells to the PE routers if the route imported into a VRF add at least one RT to that IPv4 route

Order

• IP IGP routing protocols build the ip tables.
• LSR assign a local label for each route learned(but not bgp learned routes)
• LSRs share their labels with other LSRs using LDP
• LSRs build their own LIB(Label Information Base), LFIB(Label Forward Information Base) & FIB(Forward Information Base) based on what they have learned from their LDP neighbor.

LDP Neighbor:

• Hellow Messages
• LDP link hello uses destination UDP port 646 & is sent to 224.0.0.2 every 5 sec.
• Session is TCP based on destination port 646.
• Router with highest LDP router ID(Active LSR) will initiate TCP session.
• Keepalives are sent for every 60 sec.
•  

MPLS VPN Routes Updating

• IGP or eBGP are advertises the CE routes in PE routing table
• At PE router IPv4 routes learned from the CE router is inserted into VRF routing table
• PE routers are fully meshed with MP-BGP.
• To this updated VRF routes RD is added & make them VPNv4 route & then RTs are added.
• Then these VPNv4 routes are redistribute into MP-BGP.
• The iBGP between PEs advertises the VPNv4 route with MPLS label & RTs
•  RTs tells that which vrf can import which route.
• After that RD is removed from VPNv4 route.
•  Then IPv4 route is inserted into VRF routing table.
• PE advertises these routes towards the customer routers.

• The communication between sites is controlled by RTs
• An RT is a BGP extended community that indicates which route should be imported from MP-BGP into the VRF.
• 
  

• An RT is a BGP extended community.
• that indicates which routes should be imported from MP-BGP into the VRF.
• Exporting an RT means,
• that the exported vpnv4 route receives an additional BGP extended community, this is the RT, when the route is redistributed from the vrf routing table into MP-bgp.
• Importing an RT means,
• received vpnv4 route from mp-bgp is checked for a matching extended community, this is the rt,.
• If the result is a match, the prefix is put into the vrf routing table.

• Each vrf on the PE router must have on RD.
• RD format:
• ASN:NN
• If customer(VPN) connected to different PE routers then 2 separate RDs must be used.

• VPN prefixes are propagate across the MPLS VPN network my MP-BGP.
• RDs used to these VPN prefixes make unique.
• A prefix derived from the combination of the IPv4 prefix & the RD is calld VPNv4 prefix.
• MP-BGP will carry the these VPNv4 prefixes between the PE routers

• In cisco ios, cef is the only switching method supported for forwarding ip packets from the VRF interface.
• So CEF must be enabled globally on all PE routers & all VRF interfaces
• 
  

• Routing should be separate & private for each customer(VPN) on a PE router.
• so each vpn has its own routing table.
• Thi private routing table is called VRF routing table.
• 
  

• VRF is combination of
• VPN routing table
• VRF CEF table
• ip routing protocols
• For every VPN attached to the PE, there is one VRF

MPLS VPN components:

• VRF:
• Allows multiple tables on the same routers.
• Each vrf have separate:
• RIB
• FIB
• LFIB
• VRF is locally significant to router.
• The traffic entered into the VRF enabled interfaces is belong to that vrf.
• Only one vrf can be assigned to each VRF but one VRF contain any number of interfaces.
• Route Distinguishers:
• VPN routes are propagated across a MPLS VPN network by MP-iGMP
• To make these routes unique RDs are used.
• RD is locally significant & globally relevance.
•  Routing Timers:
• Export RTs
• Attached to a route when it is converted into VPNv4 route
• Import RTs
• RTs are used to select VPNv4 routes to insert into matching VRF tables.
• The matched route is only added to vrf table only when RT is attached to the matched route on PE router.
• Routing Protocols:
• IGP 1: 
• Between CEs & PEs
• used to advertise routes in the VRF routing table
• IGP will be any of the IGP protocol/static route/ebgp
• IGP 2:
• This is core MPLS IGP
• Support the LDP
• LDP:
• Between MPLS enabled routers
• MP-BGP:
• Only between PE routers
• 
  
• 2 Types of labels in label stack:
• Outer/Top/LDP label:
• Used for switching the label in the mpls core network
• Inner/Bottom/VPN label:
• Used for switching towards the egress pe router & identify the outgoing interface.
• MPLS VPN Label Operations:
• Every PE router assign a VPN label to every local VRF route.
• This vrf routes with VPN labels are advertised to remaining PE routers in MP-iBGP updates.
• After converging on PE routers,
• For every non-local VRF route will be labeled with VPN/inner label along with inner/LDP label for every BGP next-hop.
• 
  

MPLS TABLES

Control plane:

• Collecting & propagating the information that is used to forward traffic.
•  Build RIB
• From routing protocols
• From routing protocols
• Build LIB
• Using label exchange protocol
•  Using label exchange protocol
•  From these 2 give the information to the forwarding plane

Forwarding Plane:

•  Decides how a packet will be forwarded
•  Build 2 tables FIB & LFIB
•  Responsible for forwarding the packet based ip add or label

RIB:

• Nothing but IP routing table
• Sh ip route
• Table columns Protocol, prefix, next hop

LEP:

• A LEP
•   Bind locally significant labels to routes in the RIB
•  Then exchange these label bindings with neighbor LSRs
• Stores the local & received label binding in LIB table
• LEPs are:
• LDP/TDP: in this labels are assigned to only non-bgp routes in Routing Tables
• MP-BGP: distribute label bindings for bgp routes in routing table

LIB:

• Contains the Local Label binding & label bindings learned from neighbors.
• LIB table will seen using “sh mpls ldp binding” or “sh mpls ip bindings"
• Table columns are prefix, lsr/local, label

FIB:

• Made by CEF
• Stores all labels information
• Contain each prefix, next-hop & outgoing interfaces
• See by using “sh ip cef detail"
• Column’s are prefix, Next-hop, label.

LFIB:

• It contains labels used to forward packets but not all labels bindings in RIB
• See by using “sh mpls forwarding table"
• Table column’s are inlabeL, outlabel, next-hop

MPLS VPN services

• connection less service:
• VPN connection less network don't need the tunnels & encryption for network privacy.
• Centralized services:
• VPNs in layer 3 allows the targeted services to  a group of users which are represented by VPN.
•  Scalability:
• Security:
• MPLS VPN offer same level security as connection-oriented VPNs.
• Easy to create:
• MPLS VPNs are connection less, no need of specific point-to-point connection maps or topologies are required.
• So it is easy for customers to create new VPNs & user community.
• Flexible Addressing:
• Most of customers use private address spaces.
• MPLS VPNs allow customers to continue to use their present address space without NAT.
• A NAT is required only if  2 vpns with overlapping address spaces want to communicate.
• this enable customer use their own private address in freely in public ip network.
• Integrated Class of service (COS) support:
• cos provides performance & policy implementation

VRF contain :

• ip routing table
• CEF table
• set of interfaces that use the cef forwarding table
• set of rules & routing protocol parameters to control the information in the routing tables

config b/n PE to CE

• create vrf & apply to interfaces
• create ip vrf by command
•  ip vrf A  in global mode
• Route Distinguishers
• goal is to make the prefix unique in entire mpls network
• formate of RD changed based on service provider
• AS followed by locally significant number
• router-id followed by locally significant number
•  rd config
• rd 200:1  /** config under ip vrf A
• rd 200:2
• apply vrfs to interfaces 
• int e0/0
•    ip vrf forwarding A
•    ip address 1.1.1.1 255.255.255.0    /** reenter ip add because enabling of vrf on interface remove the ip add of the interface
• sh ip route    /*** global routing table shows the separate tables for the each customer
• sh route | in interface | in ip address
• under the igp process enable seperate address family
• router eigrp    /**under given igp route process
•   address-family ipv4 vrf A
• sh ip vrf detail
• sh ip route vrf *    /** sh all vrf routing tables
• address-family ip v4 vrf / vpv4

MPLS configuration

• MPLS also called Dynamic Label Switching
• Before configuring first enable the CEF
• command used is ip cef in global mode
• Verify by show command show ip cef
• It increases the packet switching speed.
• main
• Enable MPLS forwarding of ipv4 packets along the routed paths( also called Dynamic Label Switching
• must be enable on interface & device
• command: mpls ip
• 
  
• unique router-id important in MPLS
• As a router-id loopback address is more advantage than interface address 
• command used to router-id as loopback is
• mpls ldp router-id loopback0 force
• In sometimes loopback ip address used as router-id cannot be reachable
• at that time an interface is used as router-id, to this use the following command under interface
•  mpls ldp discovery transport-address interface
• enable mpls on routing protocol enabled interfaces
• mpls ldp autoconfig under routing process.
• enable mpls authentication globally.
• mpls ldp password required  in global mode
• mpls ldp neighbour 150.1.5.55 password CISCO
• to show mpls neighbours
• show mpls ldp neighbors
• to show mpls enabled interfaces
• show mpls interfaces
• to show mpls authentication
• show mpls ldp neighbor password
• to show LFIB table
• show mpls forwarding-table
• to check the packets are forwarding by MPLS
• traceroute 150.1.5.5
• normally LDP will generate & adverties labels for every prefix found in the local routing table
• to avoid this & enable only on some prefixes uses the access control list
• exampls:

                                access-list 10 permit 150.1.0.0 0.0.255.255

                                 no mpls ldp advertise-labels

                                 mpls ldp advertise-labels for 10

• PE configuration
• in mpls network full mesh of PEs was created using ibgp peerings
• for example

router bgp 100

    neighbor 155.1.5.5 remote-as 1oopback0

    neighbor 155.1.5.5 update-source loopback0

    address-family vpv4 unicast         /** activating vpv4 address family

    neighbor 150.1.5.5 activate 

    neighbor 150.1.5.5 send-community extended

   neighbor 150.1.5.5 route-reflector-client

•   to define VRF use command
• ip vrf vrf-name
• 
  

MP-BGP VPNv4

• VRF lite is the USING VRF without MPLS.
• VRF lite main problem is scalability issue.
• this scalability problem will be overcomed by "dynamic tunneling"
• For dynamic tunneling MPLS technology is used.
• there are remote customers.
• they were connected via cloud.
• Remote customers are connected to cloud via Provider edge routers.
• Provider edger routers are connected with full mesh of label switching routers.
• These Label switching packets are used for tunneling VPN packets.
• When packet switching via tunnel between & to reach the customer uses the 2 types of Labels
• one for switch between 2 provider edge routers(outer label)
• 2nd one for selecting the correct vrf on outgoing provider edge(inner label)
• This label is also known as VPN label
• MPLS label switching routers are unidirectional.
• MPLS LSR are not used normal IGP protocols.
• send from source udp port number 646  to destination tcp add 224.0.0.2
• 
  

MPLS VPN Data Plane

• To support the forwarding of packets, 
• ingress PEs need appropriate FIB entries, 
• Ps & PEs needing appropriate LFIB entries
• The outer label identifies the segments of the LSP between between the ingress PE & the egress PE,
• but it doesn't identify how the egress PE should forward the packet.
• The inner label identifies the egress PE's forwarding details, in particular the outgoing interface for the unlabeled packet.
• Building the Inner (VPN) label:
• The inner label called VPN label
• VPN label must be allocated for each route added to each customer VRF.
• More specifically, a CE will advertise routes to the PE, 
• PE stores these routes in the corresponding customer's VRF
• In order to prepare to forward packets to those customer subnets, 
• the PE needs to allocate a new local label 
• That local label contain the prefix & the route's next-hop ip address & outgoing interface  & stores this information in LFIB.
• Steps in LSRs fill the FIB & LFIB when using MPLS VPNs
• An unlabeled packet arrives on an interface assigned to VRF, 
• which will cause ingress PE to use VRF's FB to make a forwarding decision.
• At ingress PEs VRF, FIB shows the outgoing interface for destination ip &
• Add a label stack with 2 labes
• an inner label(having original destination IP address)
• an outer label
• Then ingress PE forwards the packet to next Ps
• P uses the LFIB entry for incoming  label (outer label), swap this label.

MPLS VPN Configuration

• Main steps in configuring MPLS VPN configuration:
• Creating each VRF, RD, & RT, plus associating the customer-facing PE interfaces with the correct VRF
• Configuring the IGP between PE & CE
• Configuring mutual redistribution between the IGP & BGP
• Configuring MP-BGP between PEs

• VPNs are configured only on PE routers only.
• The customer routers no need to know about VPNs
• P routers no need to know about the MPLS VPN features
• VRFs allow PEs to store routes learned from various CEs, even if the prefixes overlap.
• RD allows PEs to store routes as unique prefixes.
• RT tells the PEs which routes should be added to each VRF
• which provides greater control & ability to allow sites to be reachable from multiple VPNs.
• VRF configuration on PE use the following commands:
• Configure the VRF using command:
• ip vrf <vrf-name>
• Configure the RD under VRF sub-command using
• rd <rd-value>
• Configure the RT under VRF sub-command using
• rt {import|export} <rt-value>
• Associating an interface with the VRF under interface sub-command using
• ip vrf forwarding <vrf-name>
• Each VRF has:
• One RD
• At least one import & export routing tag.
• If we give unique RD to every VRF, overlapping of prefixes will be overcomes.
• Configuring the IGP between PE & CE:
• Configure a routing protocol between PE & CE.
• This allows the PE router to learn the customer routes & CE to learn the other customer routes learned by PE from other PE in the MPLS cloud.
• Any IGP or even BGP can be used as the routing protocol.
• Show Commands:
• sh ip route vrf cust-A
• shows connected route on PE router & router learned from CE.
• Configuring Redistribution between PE-CE IGP & MP-BGP
• PE have no ability to advertise these routes across the MPLS VPN cloud.
• Then redistribute the IGP learned routes from CE into BGP table contain other CE routes learned from remaining PEs & vice-versa.
• 2 methods to add new routes to BGP table are
• Using network command
• Redistribution
• The BGP network command works well when adding small number of predictable prefixes.
• The Redistribution process works best when 
• the prefixes are not predictable
• there may be many no.of prefixes,... etc.
• So MPLS VPN BGP configurations uses the Redistribution process for adding new routes.
• MPLS VPN mutual redistribution configuration requires specific VRF told by both IGP & BGP.
• Redistribution command under the IGP & BGP process is
•  address-family ipv4 vrf  <vrf-name>
• Configuring MP-BGP between PFs
• To configure each peer, commands used are in normal BGP in non-MPLS configurations & others occur inside a new VPNv4 address family.
• Compare MPLS VPN BGP  & traditional BGP configuration. 
• The PE neighbors are defined under the main BGP process, not for particular address family.
• In MPLS VPN designs loopback is used as update source on the PE routers.
• In that case, the neighbor update-source command is also under the mail BGP process.
• The PE neighbors are then activated, using the neighbor activate command, under the VPNv4 address family process (address-family vpnv4).
• BGP must be told to send the community PA (neighbor send-community) command, under the address-family vpnv4 command.
• The VPNv4 address family does not refer to any particular VRF.
• Thre is no need of iBGP neighbor per VRF on each remote VRF.

Loop Back Address in MPLS

• Enable Loopback interfaces on all P & PE routers.
• These loopback addresses must be in  the core IGP.
• Establish MP-BGP sessions with these loopback addresses on all PE routers.
• These loopback interfaces will be used & referred as BGP next-hop address which carries MPLS VPN traffic.
• A BGP next-hop address must be an IGP route.
• 
  

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.
• 
  

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.
• 
  

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.
• 
  

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.

• 
  

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.
• 
  

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.
• 
  

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.
•  

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)
• 
  

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.

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
• 
  

•  

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

BENEFITS

• Highly Scalable
• In VPN(Virtual Private Networks) MPLS support any-to-any communication among VPN sites
• for this no requirement of full mesh of PVCs or sub optimal routing.
• Explicit Routing capabilities. 
• It will be possible due to the MPLS Traffic Engineering.
• MPLS enables an ATM switch to perform virtually all of the functions of an IP router.
• Eliminates the dependence on particular OSI layer technology.
• Eliminate the need for multiple layer-2 networks to satisfy different types of traffic.

• Before using the MPLS you must enable the CEF (Cisco Express Forwarding) on the router.
• MPLS terminology
• LSR: Label Switch Routher
• A router forward IP Packets based on the Labels.
• Edge-LSR:
• A router at the end of MPLS network.
• which forward both labeled & unlabeled packets
• Ingress E-LSR:
• A router at an end of MPLS network(Edge-LSR or E-LSR), which add labels to the unlabeled & labeled ip packets.
• Egress E-LSR:
• A router removes the labels of received labeled ip packet & forward as unlabeled.
• CE Router:
• 
  

• Generally in ip routing, packets are forward based on the ip address
• But in MPLS, ip packets are assigned with labels.
• Gnerally when forwarding ip packet, whole packet will be read. then forward
• But in MPLS, read only the top label, based on thins info ip packet will be forwarded 
• Today MPLS is, for the most part, a standardized version of Cisco's proprietary "tag switching".

MPLS

MPLS: (Multip Protocol Label Switching)

Introduction: