3Com MSR 50 Series, Configuration Manual

Page: 1324 / 2443

1324 C
HAPTER
76: MPLS B
ASICS
C
ONFIGURATION
After establishing sessions between them, LSRs send Hello messages and
Keepalive messages to maintain those sessions.
LSP establishment and maintenance
Establishing an LSP is to bind FECs with labels and notify adjacent LSRs of the
bindings. This is implemented by LDP. The following takes DoD mode as an
example to illustrate the primary steps:
1
When the network topology changes and an LER finds in its routing table a new
destination address that does not belong to any existing FEC, the LER creates a
new FEC for the destination address and determine the route for the FEC to use.
Then, the LER creates a label request message that contains the FEC requiring a
label and sends the message to its downstream LSR.
2
Upon receiving the label request message, the downstream LSR records this
request message, finds in its routing table the next hop for the FEC, and sends the
label request message to its own downstream LSR.
3
When the label request message reaches the destination node or the egress of the
MPLS network, if the node has any spare label, it validates the label request
message and assigns a label to the FEC. Then, the node creates a label mapping
message containing the assigned label and sends the message to its upstream LSR.
4
Upon receiving the label mapping message, an LSR checks the status of the
corresponding label request message that is locally maintained. If it has
information about the request message, the LSR assigns a label to the FEC, and
adds an entry in its LFIB for the binding, and sends the label mapping message on
to its upstream LSR.
5
When the ingress LER receives the label mapping message, it also adds an entry in
its LFIB. Up to this point, the LSP is established, and packets of the FEC can be
label switched along the LSP.
Session termination
LDP checks Hello messages to determine adjacency and checks Keepalive
messages to determine the integrity of sessions.
LDP uses different timers for adjacency and session maintenance:
■
Hello timer: LDP peers periodically send Hello messages to indicate that they
intend to keep the Hello adjacency. If the timer expires but an LSR still does not
receive any new Hello message from its peer, it removes the Hello adjacency.
■
Keepalive timer: LDP peers keep LDP sessions by periodically sending Keepalive
message over LDP session connections. If the timer expires but an LSR still does
not receive any new Keepalive message, it closes the connection and
terminates the LDP session.
LDP Loop Detection LSPs established in MPLS may be looping. The LDP loop detection mechanism can
detect looping LSPs and prevent LDP messages from looping forever.
The LDP loop detection mechanism must be configured on all LSR for it to work.
However, for an LDP session to be established, LDP loop detection configuration
on LDP peers may be different.
LDP loop detection can be in either of the following two modes: