Unnumbered interfaces are point-to-point interfaces that are not explicitly configured with a dedicated IP address and subnet; instead, they borrow (or link to) an IP address from another interface on the system (the system IP address, another loopback interface, or any other numbered interface) and use it as the source IP address for packets originating from the interface. For more information about support for unnumbered interfaces, see the 7705 SAR Router Configuration Guide, ‟Unnumbered Interfaces”.
This feature allows LDP to establish a Hello adjacency and to resolve unicast FECs over unnumbered LDP interfaces.
For example, LSR A and LSR B are the two endpoints of an unnumbered link. These interfaces are identified on each system with their unique link local identifier. The combination router ID and link local identifier uniquely identifies the interface in IS-IS throughout the network.
A borrowed IP address is also assigned to the interface to be used as the source address of IP packets that must originate from the interface. The borrowed IP address defaults to the system interface address, A and B in this example. The borrowed IP interface can be configured to any IP interface by using the following CLI command: config> router>interface>unnumbered {ip-int-name | ip-address}.
The fec-originate command, which defines how to originate a FEC for egress and non-egress LSRs, includes a parameter to specify the name of the interface that the label for the originated FEC is swapped to. For an unnumbered interface, this parameter is mandatory because an unnumbered interface does not have its own IP address.
When the unnumbered interface is added into LDP, the follow behavior occurs.
For link LDP (L-LDP) sessions:
The Hello adjacency is brought up using a link Hello packet with the source IP address set to the interface borrowed IP address and a destination IP address set to 224.0.0.2.
Hello packets with the same source IP address should be accepted when received over parallel unnumbered interfaces from the same peer LSR ID. The corresponding Hello adjacencies are associated with a single LDP session.
The transport address for the TCP connection, which is encoded in the Hello packet, is always set to the LSR ID of the node whether the interface option was enabled using the config>router>ldp>interface-parameters> interface>transport-address command.
The local-lsr-id option can be configured on the interface and the value of the LSR ID can be changed to either the local interface or to some other interface name. If the local interface is selected or the provided interface name corresponds to an unnumbered IP interface, the unnumbered interface borrowed IP address is used as the LSR ID. In all cases, the transport address for the LDP session is updated to the new LSR ID value, but the link Hello packets continue to use the interface borrowed IP address as the source IP address.
The LSR with the highest transport address, the LSR ID in this case, bootstraps the TCP connection and LDP session.
The source and destination IP addresses of LDP packets are the transport addresses, that is, the LDP LSR IDs of the LSRs at the endpoints of the link (A and B in the example).
For targeted LDP (T-LDP) sessions:
The source and destination addresses of the targeted Hello packet are the LDP LSR IDs of systems A and B.
The local-lsr-id option can be configured on the interface for the targeted session and the value of the LSR ID can be changed to either the local interface or to some other interface name. If the local interface is selected or the provided interface name corresponds to an unnumbered IP interface, the unnumbered interface borrowed IP address is used as the LSR ID. In all cases, the transport address for the LDP session and the source IP address of the targeted Hello message are updated to the new LSR ID value.
The LSR with the highest transport address, the LSR ID in this case, bootstraps the TCP connection and LDP session.
The source and destination IP addresses of LDP packets are the transport addresses, that is, the LDP LSR IDs of the LSRs at the endpoints of the link (A and B in the example).
FEC resolution:
LDP advertises/withdraws unnumbered interfaces using the Address/Address-Withdraw message. The borrowed IP address of the interface is used.
A FEC can be resolved to an unnumbered interface in the same way as it is resolved to a numbered interface. The outgoing interface and next hop are looked up in the RTM cache. The next hop is the router ID and link identifier of the interface at the peer LSR.
LDP FEC ECMP next hops over a mix of unnumbered and numbered interfaces are supported.
All LDP FEC types are supported.
The fec-originate command is supported when the next hop is over an unnumbered interface.
All LDP features supported for numbered IP interfaces are supported for unnumbered interfaces, with the following exceptions:
BFD is not supported on unnumbered IP interfaces
LDP FRR is not triggered by a BFD session timeout, only by a physical failure or the local interface going down
unnumbered IP interfaces cannot be added into LDP global and peer prefix policies
The unnumbered interface feature also extends the support of LSP ping and LSP traceroute to test an LDP unicast FEC that is resolved over an unnumbered LDP interface.