Multiprotocol Lable Switching (Mpls): Label-Switched Paths (Lsps)

Label Switched Paths:

A predefined path that makes MPLS work are called Label Switched Paths. In a MPLS network router exchange MPLS information to exchange these paths. MPLS is often called a layer 2.5 technology because it shares both routing (layer 3) and switching (layer 2) characteristics.

a.LDP: MPLS LDP enables the peer LSR in an MPLS network to exchange label binding information for supporting hop-by-hop forwarding in an MPLS network. MPLS LDP provides the means for LSRs to request, distribute, and release label prefix binding information to peer routers in a network. LDP enables LSRs to discover potential peers and to establish LDP sessions with those peers for the purpose of exchanging label binding information.

MPLS LDP enables one LSR to inform another LSR of the label bindings it has made. Once a pair of routers communicates the LDP parameters, they establish a label-switched path (LSP). MPLS LDP enables LSRs to distribute labels along normally routed paths to support MPLS forwarding. This method of label distribution is also called hop-by-hop forwarding. With IP forwarding, when a packet arrives at a router the router looks at the destination address in the IP header, performs a route lookup, and forwards the packet to the next hop. With MPLS forwarding, when a packet arrives at a router the router looks at the incoming label, looks up the label in a table, and then forwards the packet to the next hop. MPLS LDP is useful for applications that require hop-by-hop forwarding, such as MPLS VPNs.

b. MPLS VPN: "MPLS" and "VPN" are two different technology types MPLS is a standards-based technology used to speed up the delivery of network packets over multiple protocols -such as the Internet Protocol, Asynchronous Transport Mode, and Frame-Relay network protocol. A virtual private network (VPN) uses shared public telecom infrastructure, such as the Internet, to provide secure access to remote offices and users in a cheaper way than an owned or leased line. VPNs are secure because they use tunneling protocols and procedures such as Layer Two Tunneling Protocol (L2TP). With those definitions understood, an MPLS VPN is a VPN that is built on top of an MPLS network, usually from a service provider, to deliver connectivity between enterprise office locations.

There are three kinds of MPLS-based VPN:

Layer 3 VPNs: With L3 VPNs the service provider participates in the customer's Layer 3 routing. The customer's CE router at each site speaks a routing protocol such as BGP or OSPF to the provider's PE router, and the IP prefixes advertised at each customer site are carried across the provider network. L3 VPNs are attractive to customers who want to leverage the service provider's technical expertise to insure efficient site-to-site routing.
Layer 2 VPNs: The provider interconnects the customer sites via the Layer 2 technology - usually ATM, Frame Relay, or Ethernet - of the customer's choosing. The customer implements whatever Layer 3 protocol he wants to run, with no participation by the service provider at that level. L2 VPNs are attractive to customers who want complete control of their own routing; they are attractive to service providers because they can serve up whatever connectivity the customer wants simply by adding the appropriate interface in the PE router.
Virtual Private LAN Service: VPLS makes the service provider's network look like a single Ethernet switch from the customer's viewpoint. The attraction of VPLS to customers is that they can make their WAN look just like their local campus- or building-scope networks, using a single technology (Ethernet) that is cheap and well understood. Unlike traditional Metro Ethernet services built around actual Ethernet switches, service providers can connect VPLS customers from regional all the way up to global scales. So a customer with sites in London, Dubai, Bangalore, Hong Kong, Los Angeles, and New York can connect all his sites with what appears to be a single Ethernet switch.

c. MPLS on RIP:RIPv2 is one of the protocols that can be used between customer's edge router and provider's edge router. CE to PE support RIP as one of the dynamic routing protocols. Version 2 is supported. There are few features in MP-BGP that preserve protocol specific information that would be lost in redistribution, mainly the RIP metric. One feature is to preserve the RIP metric across the MPLS network by putting it in the MED attribute and during redistribution use that as the RIP metric. This helps in topologies when there are backdoor links (directly connected RIP and MPLS connected).MPLS VPN hop won't be considered as a RIP hop, so a metric of 0 will be maintained. While a backbone connection will add one hop. Metric transparent- Causes RIP to use the routing table metric for redistributed routes as the RIP metric. IOS-XR will automatically redistribute metric transparently without any commends. This could be confusing at first, trying to find a command that doesn't exist. IOS-XR can have an issue if the directly connected interface is redistributed to BGP-MP. It will redistribute it with metric 0 and not accept it.

R1 (config)#router rip
R1(config-router)# address-family ipv4 vrf (Vrf)
R1(confif-router)# redistribute bgp 100 metric transparent