MPLS knowledge points

multi-protocol-lable-switching（ Multi protocol label switching technology ）

Packet exchange —— Data consists of packets , after , It is continuously transmitted in each network node , Finally, we reach our goal .

The reason why packet switching forwarding efficiency is not high ：

1. In the whole process of packet exchange , You need to query the routing table before viewing ARP Cache two tables to complete the forwarding process , Leading to low efficiency ;

2. Matching principle of routing table —— The longest matching principle causes the query efficiency of the routing table not to be too high

3. There is a recursive lookup process in the routing table

4.IP The head is a variable length head , Lead us to IP When the head is read, it needs to be processed by software , Unable to find fast based on hardware .

Label switching

Label switching is between two-layer and three-layer packaging , Add a label that has a mapping relationship with the routing entry , Then maintain a table that records the correspondence and forwarding interface , Bring tagged data to the device , Look at the label first , Then forward based on the maintained tag forwarding table , Instead of viewing the three-tier content . Because the label itself is short and fixed length , therefore , Its forwarding efficiency will be higher than that of packet switching .

But label exchange needs to label the packet first , Then you need to pop up the tag before reaching the target , therefore , Label switching improves the efficiency in the forwarding process , On the whole, compared with packet switching, the improvement is not obvious .

Changes in packet exchange

1. Process exchange （process switching)—— The earliest packet switching —— Every packet needs to check the routing table first , I'll see later ARP Cache the table to determine the content of the two-layer package .

2. Fast packet switching （fast switching）—— Stream based packet switching —— One time routing , Exchange many times

When a stream of data comes to the device , We only perform the routing process for the first packet , Route the results and packet characteristics （ Quintuples ） Record to cache . after , When other data in the data stream comes to the device, it will directly compare with the five tuples in the cache , identical , Then it will be forwarded directly according to the forwarding method without the routing process .

3. Cisco's express switching technology ——Cisco Express Forwarding —— abbreviation CEF technology . It is mainly to combine the routing table and ARP Pre read the contents of the cache table （ Is to recursively perform the recursive operation , Should check ARP Check the cache table in advance ）, Then record the results in CEF In the table , and CEF The table supports hardware forwarding , therefore , It can greatly improve the forwarding efficiency of routing .

although CEF Technology is Cisco's proprietary technology , However, major manufacturers have also designed their own express exchange technology based on their ideas . Huawei devices are based on FIB（ Forwarding information table ）, This table supports hardware forwarding .

Although label switching cannot replace packet switching as the mainstream forwarding technology , however , Still exist and develop together with packet switching ,MPLS It is the product of the joint development of packet exchange and label exchange , Mainly because in some fields , Label exchange can be done better ：

1. For resolution BGP The routing black hole of

2.MPLS VPN

3.MPLS TE（ Flow engineering ）—— Control the path of traffic forwarding .

A forwarding network consisting of all routers running label switching , We can see it as a MPLS The Internet , We call it MPLS Domain . We run these MPLS Your device is called LSR（ Label switching router ）

Based on the premise of label exchange ： First of all, there must be a label , The label is for each LSR It is generated based on the routing entries in the local routing table . We will all have the same characteristics （ All packets matching the same routing entry ） The packet is called FEC—— Equivalent forwarding class . The same FEC You need to configure the same label .

Each station LSR Put your label and FEC The mapping information of is recorded in a local table , This table is called LIB（ Label information table ）.

after , Will and FIB Combine tables , Generate LFIB surface （ Label forwarding information table ）

In the whole process ,LSR In addition to assigning labels to devices themselves , There are other things you need to know LSR The device is for the same FEC Assigned tags , therefore , Label switching and packet switching are also divided into two levels —— Control level 、 Data level

Packet exchange

Control level —— The data flow direction of the routing protocol , The purpose is to obtain the routing information of unknown network segments , Generate routing table .

Data level —— The device is based on the improved routing table （FIB） To forward specific data information , Its direction is exactly opposite to the flow of the control layer .

Label switching

Control level —— be based on FEC Assign tags , And get other LSR To the same FEC Assign tags , after , Record generation LIB surface , Then combine FIB Generate LFIB surface .—— This process can be done statically , It can also be done dynamically ——（ repair ）！！！！！！！！！！！！！！！

Data level —— The device is based on LFIB surface , Forward according to the tag .

Inbound LSR——MPLS The boundary of the domain , When a packet without a tag comes in LSR On , They need to push labels into packets .

transit LSR—— Complete the label replacement action LSR equipment .

Departure LSR—— It's also MPLS The boundary of the domain , Complete the pop-up action of the label .

The whole process , The flow path of data layer traffic is R1-R2-R3-R4, This path is called LSP（ Label switching path ）. It should be noted that ,LSP Is one-way , If you want data to go back and forth in two directions , You also need to build a R4-R3-R2-R1 The path of

Label——20 position —— 0-2 Of 20 Power —— We call the value range of tag number as tag space , Each station LSR The label space of the device is independent .

 among ：

0 - 15 —— Special tag value —— When we allocate, we cannot directly allocate , Because every special label has its special meaning

16 - 1023 —— Generally used for static LSP build

1024 - 2 Of 20 Power —— be applied to LDP And other dynamic label exchange protocols

EXP—— Occupy 3 position —— Do strategic use , It's usually 000, It can be understood as priority , High priority allows limited forwarding .

S—— Occupy 1 position —— At the bottom of the stack —— There is more than one label , You can play more than one , We call an ordered set of multiple tags Tag stack .

If there are multiple tags , You need to use this flag bit to identify . If it is the last tag , Then the position 1, If not , Then put 0.

TTL—— Time to live —— And the third floor TTL Have the same value , The main purpose is to prevent data packets from circulating wirelessly in the loop . The data comes to the inbound LSR when , Will put the third floor TTL Copy values into labels , stay MPLS After each forwarding in the domain ,TTL reduce 1, after , At exit LSR Pop up the label , Then put the label TTL The value is copied into the three-tier header .

Build static LSP：
1. Ensure routing conditions —— Static only requires the border device to have round-trip routing information
2. To configure LSR

Inbound LSR
1. to LSR To configure LSR-ID （32 Bit binary structure , This is the IP Address format to write , however , It can only be configured manually ）

[r1]mpls lsr-id 1.1.1.1  #LSR-ID It is recommended to use the loopback interface IP Address 

2. Activate MPLS
1. Global activation MPLS

[r1]mpls

2. Interface activation MPLS （ All need to participate MPLS All data receiving and sending interfaces need to be activated ）

[r1-GigabitEthernet0/0/0]mp

3. Build static LSP

[r1]static-lsp ingress 1to4 destination 4.4.4.0 24 nexthop 12.0.0.2 out-label 1000

1. Next jump —— It must be the same as the next hop in the routing table
2.out-label （ Outbound label ） —— After the next hop device accepts it, it needs to judge which network segment the data reaches according to this tag .

Configure transit LSR：

[r2]static-lsp transit 1to4 incoming-interface GigabitEthernet 0/0/0 in-label 1000 nexthop 23.0.0.2 out-label 1001

1in-label — Inbound tag — The inbound tag must be the same as the battle tag configured in the previous hop
Configuration battle LSR：
[r4]static-lsp egress 1to4 incoming-interface GigabitEthernet 0/0/0 in-label 1002
[r1]display mpls static-lsp — View static LSP

Configure outbound LSR：

[r4]static-lsp egress 1to4 incoming-interface GigabitEthernet 0/0/0 in-label 1002
[r1]display mpls static-lsp # View static LSP