I Decided to post a Cheat Sheet that summarizes the Label Distribution Modes for those who suffer; like myself, to remember :D
So what is
a Label Distribution Protocol? When Multi-Protocol Label Switching (MPLS) was
created, it needed some sort of protocol to be able to populate Labels and bind
them to the prefix in the local routing table which is also called Local Label
Binding, and at the same time exchange those labels with the surrounding
neighbors. The received Labels are Called Remote Bindings
The use of
those exchanged labels that are handled by LDP is called a Labeled Switched
Paths (LSPs) which is used by MPLS to forward packets based on their labels
instead of the prefix itself.
A Reminder
for some acronyms
LSR: Label
Switch Router
FEC:
Forward Equivalency Class
MPLS Labels
are handled either during the Binding, Exchange, Local Storage or Forwarding based
on the following criteria:
MPLS
Label Space
|
|
Per
Interface Label Space
|
Per-Platform
Label Space
|
MPLS
Labels are assigned per Interface per FEC. This result in a forwarding behaviour
based on the incoming interfaces AND the incoming label
|
MPLS
labels are assigned per Platform per FEC. This results in a forwarding behaviour
based on the incoming label regardless of the incoming interface
|
MPLS
Label Distribution Modes
|
|
Unsolicited
Downstream
|
Downstream
On Demand
|
Each
LSR automatically sends its local labels binding to its neighbours without
them asking for a label. And also accepting incoming labels exchange from
adjacent LSRs
|
Each
LSR requests a label binding for a FEC
from its downstream router only
|
MPLS
Label Retention Modes
|
|
Conservative
Label Retention
|
Liberal
Label Retention
|
Only
Labels that are associated with an adjacent LSR are kept in LIB for a certain
FEC.
|
The
LSR will keep all remote bindings in its Label Information Base. Either those
binding are from direct neighbour or a remote LSR all the way down the LSP.
But here's the catch. The LSR will
only install those Labels which are binded to the next-hop in the LFIB while
all other labels will remain in the LIB
|
LSP
Control Modes
|
|
Independent
LSP Control Mode
|
Ordered
LSP Control Mode
|
A
LSR will Instantaneously create a Label binding for a FEC as soon as it
realizes that there is FEC, which of course is independent from its adjacent
LSRs as the name of the mode implies.
|
A
LSR will only create a local binding to a FEC if it receives a label
binding from downstream LSR or if it's
the Egress LSR for that FEC
|
Hopefully that clears up some of the confusion, I’ll prepare
a more detailed post on those modes and how are they implemented in both Juniper
and Cisco in upcoming posts. Cheers!
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