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CCIE Service Provider OEQ s MPLS Describe Unicast IP Forwarding in Traditional IP Networks (Non-MPLS)? In traditional IP networks, the next-hop address is determined using the packet s destination IP address along with the information from the local routing table. Describe Forwarding in MPLS Networks? In MPLS networks, packets are forwarded (switched) based on labels. Labels are generated per router and communicated to peer routers. Routers assign labels to define paths called Label Switched Paths (LSP) between endpoints. What are the two architectural components of MPLS? Control plane contains all the Layer 3 routing information and the processes within to exchange reachability information for a specific Layer 3 IP prefix in addition to label assignment. Contains RIB, LIB, IGP s, BGP, and LDP. Data plane forwarding engine for packets processed by the router. Forwarding is based on labels or destination IP Addresses. Contains FIB and LFIB Describe FEC? Forwarding Equivalence Class (FEC) is a group of packets forwarded in the same manner. Usually, this occurs over the same path with the same forwarding treatment. An example would be a set of routes in the same VPN. Describe the role of an LSR in an MPLS enabled network? Label Switch Router (LSR) - Performs the function of label switching; the LSR receives a labeled packet and swaps the label with an outgoing label and forwards the new labeled packet from the appropriate interface. The LSR, depending on its location in the MPLS domain, can either perform label disposition (removal, also called pop), label imposition (addition, also called push) or label swapping (replacing the top label in a label stack with a new outgoing label value). Describe an MPLS LSP? MPLS Label Switched Path (LSP) - The label path from source to destination for a packet through an MPLS-enabled network. LSPs are unidirectional in nature and usually derived from IGP routing information. Describe the FIB? Forwarding Information Base (FIB) is a lookup table used to make destination-based switching decisions during CEF operation. The FIB maintains next-hop address information based on the information in the IP routing table. To display entries in the FIB, issue the show ip cef or show ip cef detail command. Because there is a one-to-one correlation between FIB entries and routing table entries, the FIB contains all known routes. The FIB resides in the data plane, which is the forwarding engine for packets processed by the router. Describe the LIB? Label Information Base (LIB) functions in the control plane and maintains a table of local and remote labels bindings which are mapped to local and remote IP prefixes. Once label bindings are learned, the MPLS LDP control plane is ready to setup MPLS forwarding. This is done in the LFIB where outgoing labels found in the LIB are mapped to outgoing interfaces found in the FIB. The label distribution protocol (LDP/TDP/RSVP) used between adjacent MPLS neighbors is responsible for the creation of label entries in the LIB and LFIB. LIB is used by the label distribution protocol s to map IP destination prefixes in the routing table to remote (outgoing) labels received from downstream neighbors, as well as local labels. To display entries in the LIB, issue the show mpls ip binding or show mpls ldp bindings <prefix> command. Describe the LFIB? Label Forwarding Information Base (LFIB) functions in the data plane and combines the LIB mapping s (local label to outgoing label) along with the respective outgoing interface. Labeled packets that enter the router are switched according to the LFIB. IP Packets that enter the router are switched according to the CEF table.
Network Layer Reachability Information (NLRI) learned from routing protocols is used to populate the Routing Information Base (RIB) or routing table. The routing table, in turn, provides prefix to next-hop mapping information for the FIB. The LIB is populated using information from the label distribution protocol (LDP/TDP/RSVP/BGP). This information from the FIB and LIB are then used to populate the LFIB. The Label binding of a specific prefix to a local label and a nexthop label (received from downstream LSR) is stored in the LIB and LFIB structures. To display entries in the LFIB, issue the show mpls forwarding-table command. What are the four initial steps of MPLS operation? The implementation of MPLS for data forwarding involves the following four steps: 1. MPLS LDP or TDP session establishment (between LSR s) 2. MPLS label assignment (per LSR) 3. MPLS label distribution (using a label distribution protocol) 4. MPLS label retention MPLS operation typically involves adjacent LSR s forming an LDP session, assigning local labels to destination prefixes and exchanging these labels over established LDP sessions. Upon completion of label exchange between adjacent LSRs, the control and data structures of MPLS, namely FIB, LIB, and LFIB, are populated, and the router is ready to forward data plane information based on label values. Which two protocols are commonly used to perform MPLS session establishment? Label Distribution Protocol (LDP) and Tag Distribution Protocol (TDP) are protocols in which two Label Switch Routers (LSR) exchange label mapping information. The two LSR s are called LDP/TDP peers and the exchange of information is bi-directional. LDP/TDP is used to build and maintain LSR databases that are used to forward traffic through MPLS networks. TDP sessions are built using TCP port 711 and LDP uses TCP port 646. What are the four main functions of LDP? 1. Discovery of LDP enabled LSR s 2. Session establishment and maintenance 3. Advertisement of label mappings 4. Housekeeping by means of error notification Where is an MPLS label inserted onto network traffic? The label is inserted after the Layer 2 Frame Header and before the Layer 3 Header.
What is the role of CEF in an MPLS enabled network? Cisco Express Forwarding (CEF) is the foundation on which MPLS and its services operate. CEF is a proprietary switching mechanism which enhances IPv4 forwarding performance. When CEF is used, the router maintains a FIB, which mirrors the IP routing table and contains a mapping of destination networks to appropriate next-hop adjacencies. By building a FIB table, CEF eliminates the need for route cache maintenance that is associated with earlier switching paths such as fast switching and optimum switching. When an IP Packet enters a router and the packet is to be forwarded out of the router labeled, CEF needs to label the packet with a label stack. CEF is the only switching method in Cisco IOS that can switch an incoming IP packet and forward it as a labeled packet. LDP is primarily used where? internal networks of MPLS service providers What is the role of FEC in MPLS? Forwarding Equivalence Class (FEC) is a term used in MPLS to describe a set of packets with similar and / or identical characteristics which may be forwarded the same way; that is, they may be bound to the same MPLS label. Based on the output below, which MPLS operation will be performed on prefix 10.1.1.12/32? R1#show mpls forwarding-table Local Outgoing Prefix Bytes tag Outgoing Next Hop tag tag or VC or Tunnel Id switched interface 16 Pop tag 10.1.1.12/30 636 Se3/0 point2point 17 Untagged 10.10.10.1/32 0 Se4/0 point2point 10.1.1.12/30 will have the topmost label removed and then will be forwarded out Se3/0. Describe MPLS Label Assignment? An MPLS label is assigned to IP prefixes (networks) reachable by a router as learned from an IGP and then imposed on packets forwarded to those IP networks.