Label switching is a high-performance packet forwarding technology that integrates the performance and traffic management capabilities of data link layer (Layer 2) switching with the scalability, flexibility, and performance of network layer (Layer 3) routing.

In conventional Layer 3 forwarding mechanisms, as a packet traverses the network, each switch extracts all the information relevant to forwarding the packet from the Layer 3 header. This information is then used as an index for a routing table lookup to determine the next hop for the packet.

In the most common case, the only relevant field in the header is the destination address field, but in some cases, other header fields might also be relevant. As a result, the header analysis must be done independently at each switch through which the packet passes. In addition, a complicated table lookup must also be done at each switch.

In label switching, the analysis of the Layer 3 header is done only once. The Layer 3 header is then mapped into a fixed length, unstructured value called a label .

Many different headers can map to the same label, as long as those headers always result in the same choice of next hop. In effect, a label represents a forwarding equivalence class --that is, a set of packets which, however different they may be, are indistinguishable by the forwarding function.

The initial choice of a label need not be based exclusively on the contents of the Layer 3 packet header; for example, forwarding decisions at subsequent hops can also be based on routing policy.

Once a label is assigned, a short label header is added at the front of the Layer 3 packet. This header is carried across the network as part of the packet. At subsequent hops through each MPLS switch in the network, labels are swapped and forwarding decisions are made by means of MPLS forwarding table lookup for the label carried in the packet header. Hence, the packet header does not need to be reevaluated during packet transit through the network. Because the label is of fixed length and unstructured, the MPLS forwarding table lookup process is both straightforward and fast.

Each label switching router (LSR) in the network makes an independent, local decision as to which label value to use to represent a forwarding equivalence class. This association is known as a label binding. Each LSR informs its neighbors of the label bindings it has made. This awareness of label bindings by neighboring switches is facilitated by the following protocols:

• Label Distribution Protocol (LDP)--enables peer LSRs in an MPLS network to exchange label binding information for supporting hop-by-hop forwarding in an MPLS network

• Border Gateway Protocol (BGP)--Used to support MPLS virtual private networks (VPNs)

When a labeled packet is being sent from LSR A to the neighboring LSR B, the label value carried by the IP packet is the label value that LSR B assigned to represent the forwarding equivalence class of the packet. Thus, the label value changes as the IP packet traverses the network.

For more information about LDP configuration, see the see MPLS: LDP Configuration Guide at http:/​/​www.cisco.com/​c/​en/​us/​td/​docs/​ios-xml/​ios/​mpls/​config_library/​xe-3s/​mp-xe-3s-library.html

Note	As the scale of label entries is limited in, especially with ECMP, it is recommended to enable LDP label filtering. LDP labels shall be allocated only for well known prefixes like loopback interfaces of routers and any prefix that needs to be reachable in the global routing table.

A Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN) consists of a set of sites that are interconnected by means of an MPLS provider core network. At each customer site, one or more customer edge (CE) routers attach to one or more provider edge (PE) routers.

Before configuring MPLS Layer 3 VPNs, you should have MPLS, Label Distribution Protocol (LDP), and Cisco Express Forwarding (CEF) installed in your network. All routers in the core, including the PE routers, must be able to support CEF and MPLS forwarding.

The QoS EXP Matching feature allows you to classify and mark network traffic by modifying the Multiprotocol Label Switching (MPLS) experimental bits (EXP) field in IP packets.

The QoS EXP Matching feature allows you to organize network traffic by setting values for the MPLS EXP field in MPLS packets. By choosing different values for the MPLS EXP field, you can mark packets so that packets have the priority that they require during periods of congestion. Setting the MPLS EXP value allows you to:

• Classify traffic: The classification process selects the traffic to be marked. Classification accomplishes this by partitioning traffic into multiple priority levels, or classes of service. Traffic classification is the primary component of class-based QoS provisioning.

• Police and mark traffic: Policing causes traffic that exceeds the configured rate to be discarded or marked to a different drop level. Marking traffic is a way to identify packet flows to differentiate them. Packet marking allows you to partition your network into multiple priority levels or classes of service.

1.    enable


2.    configure terminal


3.    ip cef distributed


4.    mpls label range minimum-value maximum-value


5.    mpls label protocol ldp

Device> enable

Device# configure terminal

Device(config)# ip cef distributed

Device(config)# mpls label range 16 4096

Device(config)# mpls label protocol ldp

1.    enable


2.    configure terminal


3.    interface type slot/subslot /port


4.    mpls ip


5.    mpls label protocol ldp


6.    end

Device> enable

Device# configure terminal

Device(config)# interface gigabitethernet 1/0/0

Device(config)# interface vlan 1000

Device(config-if)# mpls ip

Device(config-if)# mpls label protocol ldp

Device(config-if)# end

1.    show ip cef summary

Switch# show ip cef summary

IPv4 CEF is enabled for distributed and running
VRF Default
 150 prefixes (149/1 fwd/non-fwd)
 Table id 0x0
 Database epoch:        4 (150 entries at this epoch)
Switch#

1.    show mpls interfaces detail


2.    show running-config interface


3.    show mpls forwarding

For physical (Gigabit Ethernet) interface:
Switch# show mpls interfaces detail interface GigabitEthernet 1/0/0

        Type Unknown
        IP labeling enabled
        LSP Tunnel labeling not enabled
        IP FRR labeling not enabled
        BGP labeling not enabled
        MPLS not operational
        MTU = 1500

For Switch Virtual Interface (SVI):
Switch# show mpls interfaces detail interface Vlan1000

        Type Unknown
        IP labeling enabled (ldp) :
          Interface config
        LSP Tunnel labeling not enabled
        IP FRR labeling not enabled
        BGP labeling not enabled
        MPLS operational
        MTU = 1500

For physical (Gigabit Ethernet) interface:
Switch# show running-config interface interface GigabitEthernet 1/0/0

Building configuration...


Current configuration : 307 bytes
!
interface TenGigabitEthernet1/0/0
no switchport
ip address xx.xx.x.x xxx.xxx.xxx.x
mpls ip
mpls label protocol ldp
end

For Switch Virtual Interface (SVI):
Switch# show running-config interface interface Vlan1000

Building configuration...


Current configuration : 187 bytes
!
interface Vlan1000
ip address xx.xx.x.x xxx.xxx.xxx.x
mpls ip
mpls label protocol ldp
end

For physical (Gigabit Ethernet) interface:
Switch#show mpls forwarding-table 
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
500        No Label   l2ckt(3)         0             Gi3/0/22   point2point 
501        No Label   l2ckt(1)         12310411816789 none       point2point 
502        No Label   l2ckt(2)         0             none       point2point 
503        566        15.15.15.15/32   0             Po5        192.1.1.2   
504        530        7.7.7.7/32       538728528     Po5        192.1.1.2   
505        573        6.6.6.10/32      0             Po5        192.1.1.2   
506        606        6.6.6.6/32       0             Po5        192.1.1.2   
507        explicit-n 1.1.1.1/32       0             Po5        192.1.1.2   
556        543        19.10.1.0/24     0             Po5        192.1.1.2   
567        568        20.1.1.0/24      0             Po5        192.1.1.2   
568        574        21.1.1.0/24      0             Po5        192.1.1.2   
574        No Label   213.1.1.0/24[V]  0             aggregate/vpn113 
575        No Label   213.1.2.0/24[V]  0             aggregate/vpn114 
576        No Label   213.1.3.0/24[V]  0             aggregate/vpn115 
577        No Label   213:1:1::/64     0             aggregate  
594        502        103.1.1.0/24     0             Po5        192.1.1.2   
595        509        31.1.1.0/24      0             Po5        192.1.1.2   
596        539        15.15.1.0/24     0             Po5        192.1.1.2   
597        550        14.14.1.0/24     0             Po5        192.1.1.2   
633        614        2.2.2.0/24       0             Po5        192.1.1.2   
634        577        90.90.90.90/32   873684        Po5        192.1.1.2   
635        608        154.1.1.0/24     0             Po5        192.1.1.2   
636        609        153.1.1.0/24     0             Po5        192.1.1.2   
Switch# 
end