Cisco Express Forwarding (CEF) is required by Multiprotocol Label Switching (MPLS). MPLS relies on CEF to forward labeled packets through the network. If CEF is not enabled, an MPLS router cannot switch labeled packets and MPLS functionality is lost. By default, CEF is enabled on Cisco routers; however, if CEF is disabled, you should enable it by issuing the ip cef command in global configuration mode. CEF depends on the IP routing functionality of the router and cannot be enabled unless IP routing is enabled. By default, IP routing is enabled on Cisco routers; however, if IP routing is disabled, you should enable it by issuing the ip routing command in global configuration mode.
The CEF Forwarding Information Base (FIB) is built from information contained in the IP routing table. When the routing table is updated, the nexthop information in the FIB is also updated. A routing protocol, such as Border Gateway Protocol (BGP) or Intermediate SystemtoIntermediate System (ISIS), can be used to populate the routing table, and, therefore, the FIB. However, neither BGP nor ISIS is required by MPLS; another internal gateway protocol (IGP), such as Open Shortest Path First (OSPF) or Enhanced Interior Gateway Routing Protocol (EIGRP), can be used instead.
A label exchange protocol, such as Tag Distribution Protocol (TDP), is used by MPLS to exchange label information. However, TDP itself is not required by MPLS; another label exchange protocol, such as Label Distribution Protocol (LDP) or Resource Reservation Protocol (RSVP), can be used instead.
Cisco Discovery Protocol (CDP) is used to collect information about neighboring Cisco devices, such as the host name, network address, port information, device type, and IOS version. CDP is enabled by default on Cisco devices. However, CDP is not required by MPLS.