Whether the prefix list contains one or more specific /32 addresses or a range of addresses, an external trigger is required to indicate to LDP to instantiate a targeted Hello adjacency to a node which address matches an entry in the prefix list. The objective of the feature is to provide an automatic creation of a T-LDP session to the same destination as an auto-created RSVP LSP to achieve automatic tunneling of LDP-over-RSVP. The external trigger is when the router with the matching address appears in the Traffic Engineering database. In the latter case, an external module monitoring the TE database for the peer prefixes provides the trigger to LDP. As a result of this, the user must enable the traffic-engineering option in ISIS or OSPF.
Each mapping of a targeted session peer parameter template to a policy prefix which exists in the TE database results in LDP establishing a targeted Hello adjacency to this peer address using the targeted session parameters configured in the template. This Hello adjacency then either gets associated with an LDP session to the peer if one exists or it triggers the establishment of a new targeted LDP session to the peer.
The SR OS supports multiple ways of establishing a targeted Hello adjacency to a peer LSR:
User configuration of the peer with the targeted session parameters inherited from the config>router>ldp>targ-session>ipv4 in the top level context or explicitly configured for this peer in the config>router>ldp>targ-session>peer context and which overrides the top level parameters shared by all targeted peers. Allow us to refer to the top level configuration context as the global context. Some parameters only exist in the global context; their value is always inherited by all targeted peers regardless of which event triggered it.
User configuration of an SDP of any type to a peer with the signaling tldp option enabled (default configuration). In this case the targeted session parameter values are taken from the global context.
User configuration of a (FEC 129) PW template binding in a BGP-VPLS service. In this case the targeted session parameter values are taken from the global context.
User configuration of a (FEC 129 type II) PW template binding in a VLL service (dynamic multi-segment PW). In this case the target session parameter values are taken from the global context
User configuration of a mapping of a targeted session peer parameter template to a prefix policy when the peer address exists in the TE database. In this case, the targeted session parameter values are taken from the template.
Features using an LDP LSP, which itself is tunneled over an RSVP LSP (LDP-over-RSVP), as a shortcut do not trigger automatically the creation of the targeted Hello adjacency and LDP session to the destination of the RSVP LSP. The user must configure manually the peer parameters or configure a mapping of a targeted session peer parameter template to a prefix policy. These features are:
BGP shortcut (next-hop-resolution shortcut-tunnel option in BGP)
IGP shortcut (igp-shortcut option in IGP)
LDP shortcut for IGP routes (ldp-shortcut option in router level)
static route LDP shortcut (ldp option in a static route)
VPRN service (auto-bind-tunnel resolution-filter ldp option)
Because the above triggering events can occur simultaneously or in any arbitrary order, the LDP code implements a priority handling mechanism to decide which event overrides the active targeted session parameters. The overriding trigger becomes the owner of the targeted adjacency to a specific peer and is shown in show router ldp targ-peer.
Table 1 summarizes the triggering events and the associated priority.
| Triggering Event | Automatic Creation of Targeted Hello Adjacency | Active Targeted Adjacency Parameter Override Priority |
|---|---|---|
Manual configuration of peer parameters (creator=manual) |
Yes |
1 |
Mapping of targeted session template to prefix policy (creator=template) |
Yes |
2 |
Manual configuration of SDP with signaling tldp option enabled (creator=service manager) |
Yes |
3 |
PW template binding in BGP-AD VPLS (creator=service manager) |
Yes |
3 |
PW template binding in FEC 129 VLL (creator=service manager) |
Yes |
3 |
LDP-over-RSVP as a BGP/IGP/LDP/Static shortcut |
No |
— |
LDP-over-RSVP in VPRN auto-bind |
No |
— |
LDP-over-RSVP in BGP Label Route resolution |
No |
— |
Any parameter value change to an active targeted Hello adjacency caused by any of the above triggering events is performed by having LDP immediately send a Hello message with the new parameters to the peer without waiting for the next scheduled time for the Hello message. This allows the peer to adjust its local state machine immediately and maintains both the Hello adjacency and the LDP session in UP state. The only exceptions are the following:
The triggering event caused a change to the local-lsr-id parameter value. In this case, the Hello adjacency is brought down which also causes the LDP session to be brought down if this is the last Hello adjacency associated with the session. A new Hello adjacency and LDP session then get established to the peer using the new value of the local LSR ID.
The triggering event caused the targeted peer shutdown option to be enabled. In this case, the Hello adjacency is brought down which also causes the LDP session to be brought down if this is the last Hello adjacency associated with the session.
Finally, the value of any LDP parameter which is specific to the LDP/TCP session to a peer is inherited from the config>router>ldp>session-params>peer context. This includes MD5 authentication, LDP prefix per-peer policies, label distribution mode (DU or DOD), and so on.