Extreme Networks BlackDiamond 6800 MPLS, Инструкция по установке и эксплуатации, Страница: 64 / 176

4-2 MPLS Module Installation and User Guide
Configuring the Label Distribution Protocol
Hello messages must continue to be received periodically for the hello-adjacency to be
maintained. The hold time that specifies the duration for which a hello message remains
valid defaults to 15 seconds in the basic discovery mechanism and can be negotiated by
the peer LSRs as part of the HELLO exchange. During the HELLO exchange, each LSR
proposes a value and the lower of the two is used as the hold time.
Targeted LDP Sessions between nondirectly connected LSRs are supported using an
extended discovery mechanism. In this case, targeted hello messages are periodically
sent to a specific IP address. The default HELLO time for targeted LDP sessions is 45
seconds.
After the hello-adjacency is formed, the active LSR initiates establishment of a TCP
connection to the peer LSR. At this point, an LDP session is initiated over the TCP
connection. The LDP session consists of an exchange of LDP messages that are used to
setup, maintain, and release the session.
Advertising Labels
You can control whether labels are advertised for:
• Direct routes
• RIP routes exported by OSPF
• Static routes exported by OSPF
To conserve label space, the Implicit NULL Label is advertised for RIP and static routes
exported by OSPF. The Implicit NULL Label is advertised for direct routes when PHP is
enabled.
Propagating Labels
LDP propagates labels for FECs that exactly match a routing table entry, with the
exception of mappings for 32-bit prefixes corresponding to OSPF router IDs (where the
router ID IP addresses are dynamically learned from the advertising router field of
received OSPF router and AS external LSAs).