MPLS (Multiprotocol Label Switching)¶
MPLS is a packet forwarding paradigm which differs from regular IP forwarding. Instead of IP addresses being used to make the decision on finding the exit interface, a router will instead use an exact match on a 32 bit/4 byte header called the MPLS label. This label is inserted between the ethernet (layer 2) header and the IP (layer 3) header. One can statically or dynamically assign label allocations, but we will focus on dynamic allocation of labels using some sort of label distribution protocol (such as the aptly named Label Distribution Protocol / LDP, Resource Reservation Protocol / RSVP, or Segment Routing through OSPF/ISIS). These protocols allow for the creation of a unidirectional/unicast path called a labeled switched path (initialized as LSP) throughout the network that operates very much like a tunnel through the network. An easy way of thinking about how an MPLS LSP actually forwards traffic throughout a network is to think of a GRE tunnel. They are not the same in how they operate, but they are the same in how they handle the tunneled packet. It would be good to think of MPLS as a tunneling technology that can be used to transport many different types of packets, to aid in traffic engineering by allowing one to specify paths throughout the network (using RSVP or SR), and to generally allow for easier intra/inter network transport of data packets.
For more information on how MPLS label switching works, please go visit Wikipedia (MPLS).
MPLS support in VyOS is not finished yet, and therefore its functionality is limited. Currently there is no support for MPLS enabled VPN services such as L3VPNs, L2VPNs, and mVPNs. RSVP support is also not present as the underlying routing stack (FRR) does not implement it. Currently VyOS can be configured as a label switched router (MPLS P router), in both penultimate and ultimate hop popping operations.
Label Distribution Protocol¶
The MPLS architecture does not assume a single protocol to create MPLS paths. VyOS supports the Label Distribution Protocol (LDP) as implemented by FRR, based on RFC 5036.
LDP is a TCP based MPLS signaling protocol that distributes labels creating MPLS label switched paths in a dynamic manner. LDP is not a routing protocol, as it relies on other routing protocols for forwarding decisions. LDP cannot bootstrap itself, and therefore relies on said routing protocols for communication with other routers that use LDP.
In order to allow for LDP on the local router to exchange label advertisements with other routers, a TCP session will be established between automatically discovered and statically assigned routers. LDP will try to establish a TCP session to the transport address of other routers. Therefore for LDP to function properly please make sure the transport address is shown in the routing table and reachable to traffic at all times.
It is highly recommended to use the same address for both the LDP router-id and the discovery transport address, but for VyOS MPLS LDP to work both parameters must be explicitly set in the configuration.
Sample configuration to setup LDP on VyOS¶
set protocols ospf area 0 network '192.168.255.252/32' <--- Routing for loopback set protocols ospf area 0 network '192.168.0.5/32' <--- Routing for an interface connecting to the network set protocols ospf parameters router-id '192.168.255.252' <--- Router ID setting for OSPF set protocols mpls ldp discovery transport-ipv4-address '192.168.255.252' <--- Transport address for LDP for TCP sessions to connect to set protocols mpls ldp interface 'eth1' <--- Enable MPLS and LDP for an interface connecting to network set protocols mpls ldp interface 'lo' <--- Enable MPLS and LDP on loopback for future services connectivity set protocols mpls ldp router-id '192.168.255.252' <--- Router ID setting for LDP set interfaces ethernet eth1 address '192.168.0.5/31' <--- Interface IP for connecting to network set interfaces loopback lo address '192.168.255.252/32' <--- Interface loopback IP for router ID and other uses
Operational Mode Commands¶
When LDP is working, you will be able to see label information in the outcome
show ip route. Besides that information, there are also specific show
commands for LDP: