It is possible for MPLS-TP paths to use both numbered IP numbered interfaces that use ARP/static ARP, or IP unnumbered interfaces. MPLS-TP requires no changes to these interfaces. It is also possible to use a new type of interface that does not require any IP addressing or next-hop resolution.
RFC 7213 provides guidelines for the usage of various Layer 2 next-hop resolution mechanisms with MPLS-TP. If protocols such as ARP are supported, then they should be used. However, in the case where no dynamic next hop resolution protocol is used, it should be possible to configure a unicast, multicast or broadcast next-hop MAC address. The rationale is to minimize the amount of configuration required for upstream nodes when downstream interfaces are changes. A default multicast MAC address for use by MPLS-TP point-to-point LSPs has been assigned by IANA (Value: 01-00-5e-90-00-00). This value is configurable on the router to support interoperability with third-party implementations that do not default to this value, and this no default value is implemented on the router.
To support these requirements, a new interface type, known as an unnumbered MPLS-TP interface is introduced. This is an unnumbered interface that allows a broadcast or multicast destination MAC address to be configured. An unnumbered MPLS-TP interface is configured using the unnumbered-mpls-tp keyword, as follows:
config
router
interface <if-name> [unnumbered-mpls-tp]
port <port-id>[:encap-val]
mac <local-mac-address>
static-arp <remote-mac-addr>
//ieee-address needs to support mcast and bcast
exit
The remote-mac-address may be any unicast, broadcast of multicast address. However, a broadcast or multicast remote-mac-address is only allowed in the static-arp command on Ethernet unnumbered interfaces when the unnumbered-mpls-tp keyword has been configured. This also allows the interface to accept packets on a broadcast or any multicast MAC address. If a packet is received with a unicast destination MAC address, then it is checked against the configured <local-mac-address> for the interface, and dropped if it does not match. When an interface is of type unnumbered-mpls-tp, only MPLS-TP LSPs are allowed on that interface; other protocols are blocked from using the interface.
An unnumbered MPLS-TP interface is assumed to be point-to-point, and therefore users must ensure that the associated link is not broadcast or multicast in nature if a multicast or broadcast remote MAC address is configured.
The following is a summary of the constraints of an unnumbered MPLS-TP interface:
It is unnumbered and may borrow/use the system interface address
It prevents explicit configuration of a borrowed address
It prevents IP address configuration
It prevents all protocols except mpls
It prevents deletion if an MPLS-TP LSP is bound to the Interface
MPLS-TP is only supported over Ethernet ports. The system blocks the association of an MPLS-TP LSP to an interface whose port is non-Ethernet.
If required, the IF_Num is configured under a MEP context under the MPLS interface. The mpls-tp-mep context is created under the interface as shown below. The if-num parameter, when concatenated with the Node ID, forms the IF_ID (as per RFC 6370), which is the identifier of this MEP. It is possible to configure this context whether the interface is IP numbered, IP unnumbered, or MPLS-TP unnumbered:
config
router
mpls
interface <ip-int-name>
mpls-tp-mep
[no] ais-enable
[no] if-num <if-num>
[no] if-num-validation [enable | disable]
...
exit
The if-num-validation command is used to enable or disable validation of the if-num in LSP Trace packet against the locally configured if-num for the interface over which the LSP Trace packet was received at the egress LER. This is because some implementations do not perform interface validation for unnumbered MPLS-TP interfaces and instead set the if-num in the DSMAP TLV to 0. The default is enabled.
AIS insertion is configured using the ais-enable command under the mpls-tp-mep context on an MPLS interface.