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Lecture 13

Lecture 13. MP_REACH_NLRI Attribute. The MP_REACH_NLRI attribute is encoded as shown below: +---------------------------------------------------------+ | Address Family Identifier (2 octets) | +---------------------------------------------------------+

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Lecture 13

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  1. Lecture 13

  2. MP_REACH_NLRI Attribute The MP_REACH_NLRI attribute is encoded as shown below: +---------------------------------------------------------+ | Address Family Identifier (2 octets) | +---------------------------------------------------------+ | Subsequent Address Family Identifier (1 octet) | +---------------------------------------------------------+ | Length of Next Hop Network Address (1 octet) | +---------------------------------------------------------+ | Network Address of Next Hop (variable) | +---------------------------------------------------------+ | Number of SNPAs (1 octet) | +---------------------------------------------------------+ | Length of first SNPA(1 octet) | +---------------------------------------------------------+ | First SNPA (variable) | +---------------------------------------------------------+ | Length of second SNPA (1 octet) | +---------------------------------------------------------+ | Second SNPA (variable) | +---------------------------------------------------------+ | ... | +---------------------------------------------------------+ | Length of Last SNPA (1 octet) | +---------------------------------------------------------+ | Last SNPA (variable) | +---------------------------------------------------------+ | Network Layer Reachability Information (variable) | +---------------------------------------------------------+

  3. AFI/SAFI/NHOP Fields Address Family Identifier (AFI): This field carries the identity of the Network Layer protocol associated with the Network Address that follows. For example, AFI=1 for IPv4, AFI=2 for IPv6. Subsequent Address Family Identifier (SAFI): This field provides additional information about the type ofthe NLRI carried in theattribute. For example, SAFI = 4 means NLRI with MPLS label. Network Address of Next Hop: The next hop information carried in the MP_REACH_NLRI path attribute defines the Network Layer address of the border router that should beused as the next hop to the destinations listed in the MP_NLRIattribute in the UPDATE message.

  4. MP_UNREACH_NLRI The MP_UNREACH_NLRI attribute is encoded as shown below: +---------------------------------------------------------+ | Address Family Identifier (2 octets) | +---------------------------------------------------------+ | Subsequent Address Family Identifier (1 octet) | +---------------------------------------------------------+ | Withdrawn Routes (variable) | +---------------------------------------------------------+

  5. NLRI with Label The NLRI is encoded as one or moretriples of the form <length, label, prefix>:  +---------------------------+ | Length (1 octet) | +---------------------------+ | Label (3 octets) | +---------------------------+ ............................. +---------------------------+ | Prefix (variable) | +---------------------------+ RFC 3107 Label: The Label field carries one or more labels (that corresponds to the stack of labels.   Prefix: The Prefix field contains address prefixes followed by enough trailing bits to make the end of the field fall on an octet boundary.

  6. Label Advertisement/Withdraw between Directly Connected Speakers • The advertise a label for a route, BGP speaker includes the label in the NLRI and sets the SAFI field appropriately in the Update message. • The Next Hop attribute in the Update message identifies the speaker assigning the label and adverting the route. • To withdraw a route and the associated label, two options are available: • Include the NLRI of the previously advertised route in the Withdrawn Routes field and set the label field to 0x800000 • Alternatively, advertise a new route to label binding with the same NLRI

  7. Label Advertisement/Withdraw between Non-Directly Connected Speakers • In MPLS VPN application (more on this topic later), border BGP routers are interconnected through an arbitrary number of intermediate routers. • In order not to burden intermediate routers with external BGP routes, only border routers exchange routing information via iBGP. • To transport transit traffic across intermediate routers without them knowing anything about external routes, LSPs are established via signaling protocols such as LDP or RSVP-TE. • To select the outgoing interface on the border router, another label is used. This label is exchanged via iBGP between border routers which are non-directly connected.

  8. MPLS VPN

  9. How are multiple sites interconnected? • The interconnectivity between multiple sites of VPN can be provided through a number of ways: • Circuit-switched network – interconnect routers via point-to-point leased lines (e.g., DS1, DS3). The DS1/DS3 are circuit switched over SONET/SDH infrastructure (e.g., SONET ADM, DCS) • ATM/FR network – interconnect enterprise routers via point-to-point ATM/FR VCs (e.g., ATM/FR Switches) • IP network – interconnect enterprise routers via point-to-point IP tunnels (e.g., GRE tunnel, IP SEC tunnel). • All of the above options belong to what is commonly termed as overlay model.

  10. Layer 2 Overlay Model • In this model, customer edge (CE) routers are interconnected by a full mesh of point-to-point links emulated by ATM VCs, FR DLCIs or GRE Tunnels. CE-CE routers in different sites are routing peers. • Pros • Natural traffic isolation and security due to point-to-point VC connectivity. • QoS (e.g., ATM VCs an be used to guarantee requested QoS) • Cons • Full-mesh VCs are needed to form CE-CE routing adjacency. • If not fully meshed, traffic must traverse extra hops which causes extra delay and may waste backbone BW. • Provider has to provision a larger number of VCs.

  11. L2 Virtual Circuits Enterprise B Shared Backbone Enterprise B Customer Edge (CE) Device Provider Edge (PE) Device Enterprise A Enterprise A CE-CE interconnected via L2 VCs are routing peers. Enterprise B Layer 2 Overlay Model

  12. Layer 3 Peer Model • In this model, customer sites (CE) exchange routing information only with the directly connected provider edge (PE) router. CE-PE are routing peers. • Pros • CE routers peer with PE routers. • No need for full-mesh VC connectivity. • Cons • Routing isolation and security

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