protocols.
With MPLS unicast IP forwarding, the MPLS forwarding logic forwards packets based on labels. However, when choosing the interfaces out which to forward the packets, MPLS considers only the routes in the unicast IP routing table.
Many of the more helpful MPLS applications, such as MPLS Virtual Private Networks (VPN) and MPLS traffic engineering (TE), use MPLS unicast IP forwarding as one part of the MPLS network.
CEF Review
The FIB entry details the information needed for forwarding: the next-hop router and the outgoing interface. Additionally, the CEF adjacency table lists the new data-link header that the router will then copy in front of the packet before forwarding. For the data plane, a CEF router compares the packet’s destination IP address to the CEF FIB, ignoring the IP routing table. CEF optimizes the organization of the FIB so that the router spends very little time to find the correct FIB entry, resulting in a smaller forwarding delay and a higher volume of packets per second through a router. For each packet, the router finds the matching FIB entry, then finds the adjacency table entry referenced by the matching FIB entry, and forwards the packet.
Overview of MPLS Unicast IP Forwarding
The term Label Switch Router (LSR) refers to any router that has awareness of MPLS labels, for example, Routers PE1, P1, and PE2.
FIB: Used for incoming unlabeled packets. Cisco IOS matches the packet’s destination IP address to the best prefix in the FIB and forwards the packet based on that entry.
LFIB: Used for incoming labeled packets. Cisco IOS compares the label in the incoming packet to the LFIB’s list of labels and forwards the packet based on that LFIB entry.
The MPLS Header and Label
The MPLS header is a 4-byte header, located immediately before the IP header. The MPLS EXP bits allow for QoS marking, which can be done using CB Marking. The MPLS EXP bits allow for QoS marking, which can be done using CB Marking. The LSRs will decrement the MPLS TTL field, and not the IP TTL field, as the packet passes through the MPLS network.
MPLS TTL propagation refers to the MPLS routers propagate the same TTL value across the MPLS network—the same TTL values that would have occurred if MPLS was not used at all.
Cisco routers can be configured to disable MPLS TTL propagation. When disabled, the ingress ELSR sets the MPLS header’s TTL field to 255, and the egress E-LSR leaves the original IP header’s TTL field unchanged. As a result, the entire MPLS network appears to be a single router hop from a TTL perspective, and the routers inside the MPLS network are not seen from the customer’s traceroute command.
PE1 can be configured to use TTL propagation for locally created packets, which allows the traceroute command issued from PE1 to list all the routers in the MPLS cloud. At the same time, PE1 can be configured to disable TTL propagation for “forwarded” packets (packets received from customers), preventing the customer from learning router IP addresses inside the MPLS network. (The command is no mpls ip propagatettl.)
MPLS IP Forwarding: Control Plane
MPLS supports many different control plane protocols. For example, MPLS VPNs use two control plane protocols: LDP and multiprotocol BGP (MP-BGP). While multiple control plane protocols can be used for some MPLS applications, MPLS unicast IP forwarding uses an IGP and one MPLS-specific control plane protocol: LDP.
MPLS LDP Basics
For unicast IP routing, LDP simply advertises labels for each prefix listed in the IP routing table. To do so, LSRs use LDP to send messages to their neighbors, with the messages listing an IP prefix and corresponding label. By advertising an IP prefix and label, the LSR is essentially saying, “If you want to send packets to this IP prefix, send them to me with the MPLS label listed in the LDP update.”
The LDP advertisement is triggered by a new IP route appearing in the unicast IP routing table. Upon learning a new route, the LSR allocates a label called a local label. The local label is the label that, on this one LSR, is used to represent the IP prefix just added to the routing table
The routers in the MPLS cloud must use some IP routing protocol to learn IP routes to trigger the LDP process of advertising labels. Typically, for MPLS unicast IP routing, you would use an interior gateway protocol (IGP) to learn all the IP routes, triggering the process of advertising the corresponding labels.
The MPLS Label Information Base Feeding the FIB and LFIB
LSRs store labels and related information inside a data structure called LIB. The FIB and LFIB contain labels only for the currently used best LSP segment, while the LIB contains all labels known to the LSR, whether the label is currently used for forwarding or not. To make a decision about the best label to use, LSRs rely on the routing protocol’s decision about the best route.
To enable MPLS for simple unicast IP forwarding, an LSR simply needs to enable CEF, globally enable MPLS, and enable MPLS on each desired interface. Also, IOS uses LDP by default.
The term remote binding refers to a label-prefix binding learned through LDP from some LDP neighbor.
The FIB is used to forward packets that arrived unlabeled, and the LFIB is used to forward packets that arrived already labeled.
show mpls forwarding-table 10.3.3.0 24
show mpls ldp bindings 10.3.3.0 24
Label Distribution Protocol Reference
LDP uses a Hello feature to discover LDP neighbors and to determine to what IP address the ensuing TCP connection should be made. LDP multicasts the Hellos to IP address 224.0.0.2, using UDP port number 646 for LDP.
After discovering neighbors through an LDP Hello message, LDP neighbors form a TCP connection to each neighbor, again using port 646.table. After the TCP connection is up, each router advertises all its bindings of local labels and prefixes
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