LDP related knowledge

LDP
Label distribution agreement , Mainly used in MPLS The control level of
MPLS The main work to be completed at the control level is to allocate labels and transfer labels . The premise of assigning labels is that labels must first exist in the local routing table , The premise of transferring labels is also to have a routing foundation . therefore ,LDP Want to work properly , You need to IGP As the foundation .

1、 Assign tags
LDP How to assign tags to ：
1、 In an orderly way
By default, Huawei devices choose to allocate labels in an orderly manner
2、 Independent way
There is no need to confirm that you are leaving LSR, You can also actively assign labels to the routing entries in the routing table

2、 Transfer label
Local DP conversation
A direct connection is established between two devices LDP conversation .
Local DP The process of establishing a session ：
1、 stay LSR Activate on LDP after , First, send periodically hello Package to complete peer discovery .
2、 Both sides are interacting hello After the package , Will get the other party's transmission address , Will be established according to this transmission address TCP conversation .
3、 complete TCP After the session is established , The party with a large delivery address will give priority to the initialization message , It contains — Send some negotiated parameters to the passive party . If the other party confirms the parameters , Will reply keeplive Message to confirm , At the same time, send your own local initialization message , Carry your own parameters . The active party will also confirm the parameters after receiving , If there is no problem, it will also be sent keeplive message . Received by both parties keeplive message , It means that the parameter negotiation is successful ,LDP Session successfully established .

long-range LDP conversation
Between non directly connected devices , You can create remote LDP conversation .

Data level flow process
In the process of data level traffic passing , Inbound LSR Equipment and battle LSR Both devices need to query two tables , a sheet FIB surface , a sheet LFIB Packets can complete the forwarding process , Resulting in reduced efficiency , Need to optimize .
Optimization idea ：
Inbound SR
Because the inbound packet does not contain labels , therefore , Must view FlIB surface . You can directly add the outbound label to FIB In the table , In this way, you will no longer need to view LFPB surface .
Departure SR ---- PHP ---- Last hop pop-up mechanism
Departure LSR The equipment is giving FEC Assign special labels directly when assigning labels 3( Implicit empty tag ), after , Pass this tag to your upstream device , The upstream will record this tag as an outbound tag in LFIB In the table . When data comes to the device , matching LFIB It was found that the battle tag was 3, Then he will pop up this label directly . Then exit like this LSR The device will receive an unlabeled packet , Then he can directly check FIB Table to realize forwarding .

LDP Configuration of
1、 First of all LSR Start the IGP agreement , Ensure the routing foundation
2、 To configure MPLS Of LSR ID
3、 Activate MPLS and LDP