Hcip - MPLS Technology

MPLS --- Multi protocol label switching technology

Packet exchange --- Data facilitation packet , after , It is continuously transmitted in each network node , Finally, we reach our goal .

The problem of inefficient packet switching and forwarding ：

      1、 In the whole process of packet exchange , You need to query the routing table first and then check ARP Cache two tables to complete the forwarding process , Leading to low efficiency ;

       2、 Matching rules 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 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 , There is no need to view the three-tier content . Because the label itself Short and fixed length , therefore , Its Forwarding efficiency will be higher than 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 only improves the efficiency in the forwarding process , On the whole, the improvement of intersecting packet exchange is not obvious .

Changes in packet exchange

       1、 Process exchange --- process switching --- The earliest packet switching --- Every packet needs to check the router 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 in the cache table . after , Other data in the data stream will be directly compared with the cached five tuples when it comes to the device , identical , Then it will be forwarded directly according to the forwarding method instead of the routing process .

      3、 Cisco's fast switching technology --- Cisco EXpress Forwading --- 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 surface in , and CEF It supports hardware forwarding , therefore , It can greatly improve the forwarding efficiency of routing .

        although CEF technology It's Cisco's proprietary technology , However, major manufacturers have also designed their own express switching technology based on Cisco . 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 in common development with packet exchange ,MPLS It is the product of the joint development of packet exchange and label exchange , Mainly because in some fields , Label switching Can do better ：

          1、 For resolution BGP The routing black hole of

          2、MPLS VPN

          3、MPLS TE --- Flow engineering --- Simply understand that traffic engineering is to 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 him MPLS Domain . We run these MPLS Your device is called LSR --- Label switching router .

        Based on the premise of label exchange, there must be labels first , 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 a 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 The device needs to know other things besides assigning labels by itself 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

Two levels of packet switching and label switching

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 equipment is based on 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 Assigned tags , after , Record generation LIB surface , Then combine FIB Generate LFIB surface . --- This process can be done statically , It can also be done based on dynamic protocols --- LDP agreement --- Label distribution agreement

        Data level --- The device is based on LBIB, Forward according to the tag .

Three LSR

Inbound LSR （ingress）--- MPLS The boundary of the domain , When a packet without a tag comes in LSR On , He needs to press a label into the packet ;

Central station LSR（transit） --- Complete the label replacement action LSR equipment ;

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

LSP --- Label switching path

        The whole process , The flow path of data layer traffic is R1-R2-R3-R4, This path is called LSP --- Label switching path . need Pay attention to is ,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 .

MPLS Tag message ：

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 applied to 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 --- commonly 000, It can be understood as priority , If the priority is high, it can be forwarded first .

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 a 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 three layers TTL Copy values into labels , stay MPLS There is no forwarding in the domain ,TTL reduce 1, after , At exit LSR Pop up the label , And then put TTL The value is copied into the three-level header .

Build static LSP：

One 、 Ensure routing conditions --- Static only requires the border device to have round-trip routing information

Two 、 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 .

       

       2. Activate MPLS

           1、 Global activation MPLS

         

           2、 Interface activation MPLS --- All need to participate MPLS All data receiving and sending interfaces need to be activated

        

     3. Build static LSP  

            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：

in - label --- The inbound label must be the same as the outbound label configured in the previous hop

Configure outbound LSR：

LDP --- Label distribution agreement --- Mainly used in MPLS The control level of

       MPLS Control level The main work to be done is to assign 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

        Not all devices assign labels to all routing entries in their local routing table , Instead, you decide that you are the outbound station of a certain network segment LSR when , Will actively assign labels .

        Of course , In the form of reaching the target network segment LSP In the process , All the middle LSR Need to give the corresponding FEC Assign tags . Then other devices only receive the mapping relationship between the label and the target network segment , Will passively allocate labels . Before assigning labels, you need to complete the inspection , Mainly check whether the local routing table of the target network segment can reach , And whether the next hop is the same as the announcer . Labels can be assigned if the inspection is passed , If you fail, you cannot assign labels .

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

        Departure LSR After assigning labels , It needs to be passed on to others LSR equipment . The prerequisite for transmission is that they must first LDP Peer relationship ,LDP The prerequisite for establishing peer relationship is to establish LDP conversation .

LDP Session classification :

        Local LDP conversation --- A direct connection is established between two devices LDP conversation

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

       LDP agreement Building local LDP conversation when , There is no need to manually specify the neighbor IP Address , It can automatically discover neighbor relationships . Mainly because LDP First, through multicast （224.0.0.2） Send periodically in the form of hello Bag to find neighbors .hello Package usage UDP 646 Port to complete .

Local LDP The process of establishing a session ：

1、 stay LSR Activate on LDP after , First, send periodically hello Package to complete peer discovery .

stay hello Some parameters need to be carried in the package , It includes Transmission address and LDP ID.

Transmission address ： This address is mainly established later TCP The address needed for the conversation , By default , We will use high address devices LSR ID As this address . So ask for ,LSR ID Must be available and accessible IP Address .

LDP ID： All active LDP Agreed LSR The equipment must have a LDP ID, This ID By 48 Bit binary structure , The expression is 32 position ：16 position , front 32 Bit usage LSR ID, after 16 Bit use 0 Express . This is related to label space , All device based tag spaces , It's all worth 0.

hello The default sending cycle of packets is 5s, The survival time is hello Time , Default 15 second .

2、 Both sides are interacting hello After the package , Then go to the other party's transmission address , Based on this exclusive address TCP conversation .

        In establishment TCP Between sessions ,LDP There are also check actions , You need to confirm the receipt of the forward address , Is it reachable in the local routing table , Reachable can be established normally TCP conversation .

        If both sides establish TCP conversation , Two two-way channels will be created , Waste of resources . At the receiving end hello After the transmission address in the packet , Compare with the local transmission address , The party with the largest transmission address , We call it the active party , The active party can take the initiative to establish TCP conversation , The second passive party does not establish .

3、 complete TCP After the session is established , The party with a large delivery address will give priority to the initialization message , It contains some negotiated parameters and sends them to the passive party , If the other party confirms the parameters , Will reply keep live 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 keep live message . Received by both parties keep live message , It means that the parameter negotiation is successful ,LDP Session successfully established .

        after , It also needs to be sent periodically keeplive Keep the message alive , The default sending cycle is 15s, The survival time is 3 Times the cycle sending time , The default is 45 second .

Upstream downstream relationship

        It is determined by the flow direction of data layer traffic , The data sender is upstream , The receiver is downstream . From the perspective of the control layer , Then the direction of label transmission should be from downstream to upstream . after Upstream equipment will collect downstream labels .

Label transfer method

        DU Pattern --- Downstream autonomous mode --- Huawei equipment adopts this mode by default , all LSR After the assignment number label , You can actively transfer labels to others LSR. --- If you choose downstream autonomous mode , Then the device passes the label , Unable to judge the upstream and downstream relationship , The label will be sent to all peer relationships . When the device receives the label , Then the upstream and downstream relationship can be judged based on the route . Huawei equipment , Only the upstream equipment will record the labels sent by the downstream equipment , Labels sent by upstream devices will not be saved .（ Another way to save labels is to save both upstream and downstream labels .）

       DOD Pattern --- Downstream on demand mode --- Only the downstream equipment receives the notification from the upstream equipment , Will send labels to upstream devices .

Data level flow process

        In the process of data level traffic passing , Inbound LSR Equipment and outbound 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 LSR --- Because the inbound packet does not contain labels , therefore , Must view FIB surface . You can directly add the battle tag to FIB In the table , In this way, you will no longer need to view LFPB surface .

        Departure LSR --- PHP --- Second end bounce mechanism --- Departure LSR In giving FEC Assign special labels directly when assigning labels （ 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 is found that the outbound tag is 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 .

Huawei devices enable the next end bounce mechanism by default .

LSB 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

      Global activation

      Interface activation

see LDP Peer relationship ：

see LDP conversation ：

see LFIB surface ：

Be careful ： Huawei equipment only gives /32 Host bit routing assignment label , because MPLS It is not mainly used in data forwarding , If there are too many routing entries in the routing table , Will result in too many LSP. Waste of resources .

MPLS Solve the routing black hole problem ：