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 L2VPNs and mVPNs. RSVP support is also not present as the underlying routing stack (FRR) does not implement it. Currently VyOS implements LDP as described in RFC 5036; other LDP standard are the following ones: RFC 6720, RFC 6667, RFC 5919, RFC 5561, RFC 7552, RFC 4447. Because MPLS is already available (FRR also supports RFC 3031).
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.
Another thing to keep in mind with LDP is that much like BGP, it is a protocol that runs on top of TCP. It however does not have an ability to do something like a refresh capability like BGPs route refresh capability. Therefore one might have to reset the neighbor for a capability change or a configuration change to work.
Use this command to enable MPLS processing on the interface you define.
Use this command to enable LDP on the interface you define.
Use this command to configure the IP address used as the LDP router-id of the local device.
Use this command to set the IPv4 or IPv6 transport-address used by LDP.
Use this command to configure authentication for LDP peers. Set the IP address of the LDP peer and a password that should be shared in order to become neighbors.
Use this command to configure a specific session hold time for LDP peers. Set the IP address of the LDP peer and a session hold time that should be configured for it. You may have to reset the neighbor for this to work.
Use this command to enable, disable, or specify hop count for TTL security for LDP peers. By default the value is set to 255 (or max TTL).
Use these commands if you would like to set the discovery hello and hold time parameters.
Use this command if you would like to set the TCP session hold time intervals.
Use these commands to control the importing of forwarding equivalence classes (FECs) for LDP from neighbors. This would be useful for example on only accepting the labeled routes that are needed and not ones that are not needed, such as accepting loopback interfaces and rejecting all others.
Use these commands to control the exporting of forwarding equivalence classes (FECs) for LDP to neighbors. This would be useful for example on only announcing the labeled routes that are needed and not ones that are not needed, such as announcing loopback interfaces and no others.
Use this command if you would like for the router to advertise FECs with a label of 0 for explicit null operations.
Use this command if you would like to control the local FEC allocations for LDP. A good example would be for your local router to not allocate a label for everything. Just a label for what it’s useful. A good example would be just a loopback label.
Use this command to use a Cisco non-compliant format to send and interpret the Dual-Stack capability TLV for IPv6 LDP communications. This is related to RFC 7552.
Use this command to use ordered label distribution control mode. FRR by default uses independent label distribution control mode for label distribution. This is related to RFC 5036.
Use this command to prefer IPv4 for TCP peer transport connection for LDP when both an IPv4 and IPv6 LDP address are configured on the same interface.
Use this command to enable targeted LDP sessions to the local router. The router will then respond to any sessions that are trying to connect to it that are not a link local type of TCP connection.
Use this command to enable the local router to try and connect with a targeted LDP session to another router.
Use these commands if you would like to set the discovery hello and hold time parameters for the targeted LDP neighbors.
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 interface 'eth1' <--- Enable MPLS for an interface connecting to network 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 LDP for an interface connecting to network set protocols mpls ldp interface 'lo' <--- Enable 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: