Manual browser: mpls(4)

MPLS(4) Kernel Interfaces Manual MPLS(4)


mplsMultiprotocol Label Switching


options MPLS
pseudo-device ifmpls
#include <sys/types.h>
#include <netmpls/mpls.h>


MultiProtocol Label Switching represents a mechanism which directs and carries data in high-performance networks, its techniques being applicable to any network layer protocol.

In an MPLS domain the assignment of a particular packet a particular Forward Equivalence Class is done just once, as the packet enters the network. The FEC to which the packet is assigned is encoded as a short fixed length value known as a “label”. When a packet is forwarded to the next hop, the label is sent along with it; that is, the packets are “labeled” before they are forwarded.

A router capable of receiving and forwarding MPLS frames is called “Label Switch Router” or LSR. Label scope is generally router-wide meaning that a certain label has a specific meaning only for a certain LSR.

Currently, NetBSD supports MPLS over Ethernet interfaces and GRE tunnels. For these kind of interfaces, a label is contained by a fixed sized “shim” that precedes any network layer headers, just after data link layer headers.

MPLS shim header structure

In network bit order:

|               |        |       |        | 
| Label         | TC     | BoS   | TTL    | 
| 20 bits       | 3 bits | 1 bit | 8 bits | 
|               |        |       |        | 
20 bits representing FEC, consequently the only information used to forward the frame to next-hop
Traffic Class Field
3 bits that are used for specifying a traffic class, usually used for defining a type of service. This field was named the "Experimental Field" in most early (pre-RFC 5462) documents.
Bottom of Stack
One bit that is set for the last entry in the shim stack and 0 for all others. An MPLS frame may contain more than one shim, the last one before the network headers being marked by setting the BoS bit.
8 bits, representing Time to Live, decremented at every LSR.


The MPLS behavior is controlled by the net.mpls sysctl(8) tree:
If zero, MPLS frames are dropped on sight on ingress interfaces.
If zero, MPLS frames are not forwarded to next-hop.
The default ttl for self generated MPLS frames.
If set, TTL field from IP header will be mapped into the MPLS shim on encapsulation, and the TTL field from MPLS shim will be copied into IP header on decapsulation.
The IPv6 version of the above.
If set, precedence field from IP header will be mapped into MPLS shim in TC field on encapsulation, and the MPLS TC field will be copied into IP Precedence field on decapsulation.
The IPv6 version of the above.
Returns ICMP TTL exceeded in transit when an MPLS frame is dropped because of TTL = 0 on egress interface.
Pop the Explicit Null labels as specified by RFC 4182
In order to encapsulate and decapsulate to and from MPLS, an mpls pseudo-interface must be created and packets that should be encapsulated must be routed to that interface.

MPLS routes may be created using AF_MPLS sa_family sockaddrs for destination and tag fields. Other protocols can be encapsulated using routes pointing to mpls pseudo-interfaces, and AF_MPLS sockaddrs for tags. Decapsulation can be made using values of reserved labels set in the tag field (see below). For more information about doing this using userland utilities see the EXAMPLES section of this manual page.

The netstat(1) and route(8) utilities should be used to manage routes from userland.

The NetBSD implementation stores route tagging information into a sockaddr_mpls structure that is referenced by the rt_tag field of rtentry struct. For storing multiple labels associated with the next-hop, the current implementation abuses the sockaddr_mpls structure, extending it in order to fit a stack of labels.

ldpd(8) should be used in order to automatically import, manage and distribute labels among LSRs in the same MPLS domain.


MPLS labels 0 through 15 are reserved. Out of those, only four are currently defined:
IPv4 Explicit NULL label. This label value is only legal at the bottom of the label stack. It indicates that the label stack must be popped, and the forwarding of the packet must then be based on the IPv4 header.
Router Alert Label. Currently not implemented in NetBSD.
IPv6 Explicit NULL label. It indicates that the label stack must be popped, and the forwarding of the packet must then be based on the IPv6 header.
Implicit NULL label. This is a label that an LSR may assign and distribute, but which never actually appears in the encapsulation. When an LSR would otherwise replace the label at the top of the stack with a new label, but the new label is “Implicit NULL”, the LSR will pop the stack instead of doing the replacement. In this case, the LSR will have to deduce by itself what is the original address family of the encapsulated network packet. Currently, NetBSD implementation is assuming that the latter address family is equal to the next-hop address family specified in the Implicit Null Label MPLS route.


  1. Create an MPLS interface and set an IP address:

    # ifconfig mpls0 create up 
    # ifconfig mpls0 inet
  2. Route IP packets into MPLS domain with a specific tag

    # route add -ifp mpls0 -tag 25 -inet
  3. Create a static MPLS forwarding rule - swap the incoming label 50 to 33 and forward the frame to and verify the route

    # route add -mpls 50 -tag 33 -inet 
    add host 50: gateway 
    # route -n get -mpls 50 
       route to: 50 
    destination: 50 
            Tag: 33 
     local addr: 
      interface: sk0 
          flags: <UP,GATEWAY,HOST,DONE,STATIC> 
    recvpipe  sendpipe  ssthresh  rtt,msec    rttvar  hopcount      mtu     expire 
          0         0         0         0         0         0         0         0 
    sockaddrs: <DST,GATEWAY,IFP,IFA,TAG>
  4. Route IP packets into MPLS domain but use a different source address for local generated packets.

    # route add -ifa -ifp mpls0 -tag 25 -inet
    For the latter example, setting an IP address for the mpls0 interface is not necessary.
  5. Route MPLS packets encapsulated with label 60 to and POP label

    # route add -mpls 60 -tag 3 -inet
  6. Route IP packets into MPLS domain and prepend more tags

    # route add 10/8 -ifa -ifp mpls0 -tag 20,30,40 -inet
    For the above example, tag 20 will be inserted at Bottom of Stack, while tag 40 will be set into the outermost shim.
  7. Replace label 60 with label 30, prepend two more labels: 40 and 41 (in this order) and forward the result to

    # route add -mpls 60 -tag 30,40,41 -inet


netstat(1), route(4), ldpd(8), route(8), sysctl(8)

Multiprotocol Label Switching Architecture, RFC 3031.

MPLS Label Stack Encoding, RFC 3032.

Removing a Restriction on the use of MPLS Explicit NULL, RFC 4182.

MPLS Label Stack Entry: EXP Field Renamed to Traffic Class Field, RFC 5462.


The mpls support appeared in NetBSD 6.0.


User must be aware that encapsulating IP packets in MPLS implies a major security effect when using firewalls. Currently neither ipf(4) nor pf(4) implement the heuristics in order to look inside an MPLS frame. Moreover, it's technically impossible in most cases for an LSR to know information related to encapsulated packet. Therefore, MPLS Domains should be strictly controlled and, in most cases, limited to trusted connections inside the same Autonomous System.

Users must be aware that the MPLS forwarding domain is entirely separated from the inner (IP, IPv6 etc.) forwarding domain and once a packet is encapsulated in MPLS, the former forwarding is used. This could result in a different path for MPLS encapsulated packets than the original non-MPLS one.

IP or IPv6 forwarding is not necessary for MPLS forwarding. Your system may still forward IP or IPv6 packets encapsulated into MPLS if net.mpls.forwarding is set.

July 24, 2013 NetBSD 7.0