CS Summer 2003 Lecture 13

CS Summer 2003 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) |

CS Summer 2003 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 of the NLRI carried in the attribute. 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 be used as the next hop to the destinations listed in the MP_NLRI attribute in the UPDATE message.

CS Summer 2003 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encoded as shown below: | Address Family Identifier (2 octets) | | Subsequent Address Family Identifier (1 octet) | | Withdrawn Routes (variable) |

CS Summer 2003 NLRI with Label The NLRI is encoded as one or more triples of the form : | Length (1 octet) | | Label (3 octets) | | Prefix (variable) | 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. RFC 3107

CS Summer 2003 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a route and the associated label, two options are available: Include the NLRI of the previously advertised route in the Withdrawn Routes field and set the label field to 0x Alternatively, advertise a new route to label binding with the same NLRI

CS Summer 2003 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.

CS Summer 2003 MPLS VPN

CS Summer 2003 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.

CS Summer 2003 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 1. Natural traffic isolation and security due to point-to-point VC connectivity. 2.QoS (e.g., ATM VCs an be used to guarantee requested QoS) Cons 1.Full-mesh VCs are needed to form CE-CE routing adjacency. 2.If not fully meshed, traffic must traverse extra hops which causes extra delay and may waste backbone BW. 3.Provider has to provision a larger number of VCs.

CS Summer 2003 Layer 2 Overlay Model Enterprise A Shared Backbone Enterprise A Enterprise B L2 Virtual Circuits Provider Edge (PE) Device CE-CE interconnected via L2 VCs are routing peers. Customer Edge (CE) Device

CS Summer 2003 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 1.CE routers peer with PE routers. 2.No need for full-mesh VC connectivity. Cons 1.Routing isolation and security