Control Transport Address Used for Targeted-LDP Session
To establish a TCP session between two devices, each device must learn the other device’s transport address. The transport address is an IP address used to identify the TCP session over which the LDP session operates. Earlier, this transport address could only be the router-ID or an interface address. With the LDP transport-address feature, you can explicitly configure any IP address as the transport address for targeted LDP neighbors for Layer 2 circuit, MPLS, and VPLS adjacencies. This enables you to control the targeted-LDP sessions using transport-address configuration.
Benefits of Controlling Transport Address Used for Targeted-LDP Session
Configuring transport address for establishing targeted-LDP sessions has the following benefits:
Flexible interface configurations—Provides the flexibility of configuring multiple IP addresses for one loopback interface without interrupting the creation of LDP session between the targeted-LDP neighbors.
Ease of operation—Transport address configured at the interface-level, allows you to use more than one protocol in the IGP backbone for LDP. This enables smooth and easy operations.
Targeted-LDP Transport Address Overview
Prior to Junos OS Release 19.1R1, LDP provided support only for router-ID or the interface address as the transport address on any LDP interface. The adjacencies formed on that interface used one of the IP addresses assigned to the interface or the router-ID. In case of targeted adjacency, the interface is the loopback interface. When multiple loopback addresses were configured on the device, the transport address could not be derived for the interface, and as a result, the LDP session could not be established.
Starting in Junos OS Release 19.1R1, in addition to the default
IP addresses used for transport address of targeted-LDP sessions,
you can configure any other IP address as the transport address under
the session, session-group, and interface configuration statements. The transport address configuration is
applicable for configured neighbors only including Layer 2 circuits,
MPLS, and VPLS adjacencies. This configuration does not apply to discovered
adjacencies (targeted or not).
Transport Address Preference
You can configure transport address for targeted-LDP sessions at the session, session-group, and interface level.
After the transport address is configured, the targeted-LDP session is established based on the transport address preference of LDP.
The order of preference of transport address for targeted neighbor (configured through Layer 2 circuit, MPLS, VPLS, and LDP configuration) is as follows:
Under
[edit protocols ldp session]hierarchy.Under
[edit protocols ldp session-group]hierarchy.Under
[edit protocols ldp interfcae lo0]hierarchy.Under
[edit protocols ldp]hierarchy.Default address.
The order of preference of transport address for the discovered neighbors is as follows:
Under
[edit protocols ldp interfcae]hierarchy.Under
[edit protocols ldp]hierarchy.Default address.
The order of preference of transport address for auto-targeted neighbors where LDP is configured to accept hello packets is as follows:
Under
[edit protocols ldp interfcae lo0]hierarchy.Under
[edit protocols ldp]hierarchy.Default address.
Troubleshooting Transport Address Configuration
You can use the following show command outputs to troubleshoot targeted-LDP sessions:
show ldp sessionshow ldp neighborThe
detaillevel of output of theshow ldp neighborcommand displays the transport address sent in the hello messages to the targeted neighbor. If this address is not reachable from the neighbor, the LDP session does not come up.show configuration protocols ldp
You can also enable LDP traceoptions for further troubleshooting.
If the configuration is changed from using a transport address that is invalid (non reachable) to transport address that is valid, the following traces can be observed:
May 29 10:47:11.569722 Incoming connect from 10.55.1.4 May 29 10:47:11.570064 Connection 10.55.1.4 state Closed -> Open May 29 10:47:11.570727 Session 10.55.1.4 state Nonexistent -> Initialized May 29 10:47:11.570768 Session 10.55.1.4 state Initialized -> OpenRec May 29 10:47:11.570799 LDP: Session param Max PDU length 4096 from 10.55.1.4, negotiated 4096 May 29 10:47:11.570823 Session 10.55.1.4 GR state Nonexistent -> Operational May 29 10:47:11.669295 Session 10.55.1.4 state OpenRec -> Operational May 29 10:47:11.669387 RPD_LDP_SESSIONUP: LDP session 10.55.1.4 is up
If the configuration is changed from using a transport address that is valid to transport address that is invalid (non reachable),the following traces can be observed:
May 29 10:42:36.317942 Session 10.55.1.4 GR state Operational -> Nonexistent May 29 10:42:36.318171 Session 10.55.1.4 state Operational -> Closing May 29 10:42:36.318208 LDP session 10.55.1.4 is down, reason: received notification from peer May 29 10:42:36.318236 RPD_LDP_SESSIONDOWN: LDP session 10.55.1.4 is down, reason: received notification from peer May 29 10:42:36.320081 Connection 10.55.1.4 state Open -> Closed May 29 10:42:36.322411 Session 10.55.1.4 state Closing -> Nonexistent
In case of faulty configuration, perform the following troubleshooting tasks:
Check the
address family. The transport address that is configured under thesessionstatement must belong to the same address family as the neighbor or session.The address that is configured as the transport address under a
neighbororsessionstatement must be local to the router for the targeted hello messages to start. You can check if the address is configured. If the address is not configured under any interface, the configuration is rejected.