MPLS%20Architecture

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MPLS Architecture
2
MPLS Network Model
MPLS
LSR Label Switched Router LER Label Edge
Router
3
MPLS Benefits

• Comparing MPLS with existing IP core and IP/ATM
  technologies, MPLS has many advantages and
  benefits
• The performance characteristics of layer 2
  networks
• The connectivity and network services of layer 3
  networks
• Improves the price/performance of network layer
  routing
• Improved scalability

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MPLS Benefits (contd)

• Improves the possibilities for traffic
  engineering
• Supports the delivery of services with QoS
  guarantees
• Avoids need for coordination of IP and ATM
  address allocation and routing information

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Necessity of L3 Forwarding

• For security
• To allow packet filtering at firewalls
• Requires examination of packet contents,
  including the IP header
• For forwarding at the initial router - used when
  hosts dont support MPLS
• For Scaling
• Forward on a finer granularity than the labels
  can provide

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MPLS Architecture

• Down stream label assignment for unicast traffic
• On demand
• Unsolicited
• Path selection
• Hop by hop
• Explicit
• Ordered vs. independent control
• Loop detection and prevention mechanisms

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Label Distribution Protocol (LDP)

• Set of procedures used by LSRs to establish LSPs
• Mapping between network-layer routing information
  directly to data-link layer switched paths
• LDP peers
• Two LSRs which use LDP to exchange label/stream
  mapping
• Information exchange known as LDP Session

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LDP Messages

• Discovery messages
• Used to announce and maintain the presence of an
  LSR
• Session/Adjacency messages
• Used to establish, maintain and terminate
  sessions between LDP peers
• Advertisement messages
• Used to create, change, and delete label mappings
• Notification messages
• Used to provide advisory information and to
  signal error information

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Forwarding Equivalence Class (FEC)

• Introduced to denote packet forwarding classes
• Comprises traffic
• To a particular destination
• To destination with distinct service requirements

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LSP - FEC Mapping

• FEC specified as a set of two elements
• IP Address Prefix - any length from 0 32
• Host Address - 32 bit IP address
• A given packet matches a particular LSP if and
  only if IP Address Prefix FEC element matches
  packets IP destination address

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Label Spaces

• Useful for assignment and distribution of labels
• Two types of label spaces
• Per interface label space Interface-specific
  labels used for interfaces that use interface
  resources for labels
• Per platform label space Platform-wide incoming
  labels used for interfaces that can share the
  same label space

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LDP Discovery

• A mechanism that enables an LSR to discover
  potential LDP peers
• Avoids unnecessary explicit configuration of LSR
  label switching peers
• Two variants of the discovery mechanism
• Basic discovery mechanism used to discover LSR
  neighbors that are directly connected at the link
  level
• Extended discovery mechanism used to locate LSRs
  that are not directly connected at the link level

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LDP Discovery (Contd)

• Basic discovery mechanism
• To engage - send LDP Hellos periodically
• LDP Hellos sent as UDP packets for all routers on
  that subnet
• Extended discovery mechanism
• To engage - send LDP targeted Hellos periodically
• Targeted Hellos are sent to a specific address
• Targeted LSR decides whether to respond or to
  ignore the targeted Hello

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Session Establishment

• Exchange of LDP discovery Hellos triggers session
  establishment
• Two step process
• Transport connection establishment
• If LSR1 does not already have a LDP session for
  the exchange of label spaces LSR1a and LSR2b,
  it attempts to open a TCP connection with LSR2
• LSR1 determines the transport addresses at its
  end (A1) and LSR2s end (A2) of the TCP
  connection
• If A1gtA2, LSR1 plays the active role otherwise
  it is passive
• Session initialization
• Negotiate session parameters by exchanging LDP
  initialization messages

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Label Distribution and Management

• Two label distribution techniques
• Downstream on demand label distribution
• An LSR can distribute a FEC label binding in
  response to an explicit request
• Downstream Unsolicited label distribution
• Allows an LSR to distribute label bindings to
  LSRs that have not explicitly requested them
• Both can be used in the same network at the same
  time however, each LSR must be aware of the
  distribution method used by its peer

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Label Distribution Control Mode

• Independent Label Distribution Control
• Each LSR may advertise label mappings to its
  neighbors at any time
• Independent Downstream on Demand mode - LSR
  answers without waiting for a label mapping from
  next hop
• Independent Downstream Unsolicited mode - LSR
  advertises label mapping for a FEC whenever it is
  prepared
• Consequence upstream label can be advertised
  before a downstream label is received

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Distribution Control Mode (contd)

• Ordered Label Distribution Control
• Initiates transmission of label mapping for a FEC
  only if it has next FEC next hop or is the egress
• If not, the LSR waits till it gets a label from
  downstream LSR
• LSR acts as an egress for a particular FEC, if
• Next hop router for FEC is outside of label
  switching network
• FEC elements are reachable by crossing a domain
  boundary

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Label Retention Mode

• Conservative Label Retention Mode
• Advertised label mappings are retained only if
  they are used for forwarding packets
• Downstream on Demand Mode typically used with
  Conservative Label Retention Mode
• Advantage only labels required are maintained
• Disadvantage a change in routing causes delay
• Liberal Retention Mode
• All label mappings are retained regardless of
  whether LSR is next hop or not
• Faster reaction to routing changes

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Label Information Base

• LSR maintains learned labels in Label Information
  Base (LIB)
• Each entry of LIB associates an FEC with an (LDP
  Identifier, label) pair
• When next hop changes for a FEC, LSR will
  retrieve the label for the new next hop from the
  LIB

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Hierarchical Routing in MPLS

• External Routers A,B,C,D,E,F - Talk BGP
• Internal Routers 1,2,3,4,5,6 - Talk OSPF

Domain 2
C
D
1
Domain 3
Domain 1
6
2
3
4
5
B
F
E
A
Note Internal routers in domains 1 and 3 not
shown
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Hierarchical Routing (contd)

• When IP packet traverses domain 2, it will
  contain two labels, encoded as a label stack
• Higher level label used between routers C and D,
  which is encapsulated inside a lower level label
  used within Domain 2
• Operation at C
• C needs to swap BGP label to put label that D
  expects
• C also needs to add an OSPF label that 1 expects
• C therefore pushes down the BGP label and adds a
  lower level label

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Explicit Routing in MPLS

• Two options for route selection
• Hop by hop routing
• Explicit routing
• Explicit Routing (Source Routing) is a very
  powerful technique
• With pure datagram routing, overhead of carrying
  complete explicit route is prohibitive
• MPLS allows explicit route to be carried only at
  the time the LSP is setup, and not with each
  packet
• MPLS makes explicit routing practical

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Explicit Routing (Contd)

• In an explicitly routed LSP
• LSP next hop is not chosen by the local node
• Selected by a single node, usually the ingress
• The sequence of LSRs may be chosen by
• Configuration (e.g., by an operator or by a
  centralized server)

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Loops and Loop Handling

• Routing protocols used in conjunction with MPLS
  are based on distributed computation which may
  contain loops
• Loops handling - 3 categories
• Loop Mitigation/Survival
• Loop Detection
• Loop Prevention

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Loop Mitigation

• Minimizes the impact of loops by limiting the
  amount of resources consumed by the loop
• Method
• Based on use of TTL field which is decremented at
  each hop
• Use of dynamic routing protocol converging
  rapidly to non-looping paths

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Loop Detection

• Loops may be setup but they are subsequently
  detected
• The detected loop is then broken by dropping
  label relationship
• Broken loops now necessitates packets to be
  forwarded using L3 forwarding

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Loop Detection (Contd)

• Method is based on transmitting a Loop Detection
  Control Packet (LDCP) whenever a route changes
• LDCP is forwarded towards the destination until
• Last MPLS node along the path is reached
• TTL of the LDCP expires
• It returns to the node which originated it

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Loop Prevention

• Ensures that loops are never set up
• Labels are not used until it is sure to be loop
  free
• Methods
• Labels are propagated starting at the egress
  switch
• Use source routing to set up label bindings from
  the egress switch to each ingress switch