BGP biplane architecture

requirement ：PC1,PC3,PC5 Route for the telecommunication plane ,PC2,PC4,PC6 Route for the connection surface . Under normal circumstances , The telecommunication route goes AS1, The Unicom route goes AS2, When R9 And R1 In case of line failure between , The route of telecommunication plane is changed R4 Then back AS1

Route analysis

• Normal condition
  【1】PC1,PC2 visit PC5 Route through R9 Came to AS1 Of R1 On , As a result of the full interconnection to build neighborhoods ,R1 Will receive two PC5 The routing , One is R5（IBGP） Given , The other is R2（EBGP） Given . stay BGP Under the default configuration of , because R2 to R1 Routing of AS-PATH The attribute is long , therefore R1 Will be preferred R5 The routing . therefore PC1,PC2 Of the traffic will pass through R1 Forwarding to R5,R5 In comparison R12 and R6 To arrive PC5 After routing, the priority is R12, Final PC1,PC2 The route of R9–R1–R5–R12 arrive PC5
  【2】PC3,PC4 visit PC6 The flow routing principle is the same as above , The final course R10–R4–R8–R12 arrive PC6
  【3】 Under default configuration ,R12 From R5 and R8 Received from PC1,2,3,4 The routing , according to BGP According to the route selection rules of R12 It is better to choose from R5 What we learned from routing . therefore PC5,6 Of traffic will only pass through AS1 arrive PC1,2,3,4

• R9 And R1 line fault
  【1】 After line failure PC1,2,3,4 Of traffic can only pass through AS2 Came to PC5,6. according to AS2 The right way , The flow path is (R9)–R10–R4–R8–R12
  【2】 Link failure ,AS1 adopt AS2 Learning to PC1,PC3 After routing, it will be transmitted to R12, because AS-PATH The attribute is long ,R12 Will be preferred R8 The routing , Lead to PC5,6 The traffic will only be R8 transit ,PC1,2,3,4 The traffic will only be R4 transit

Custom strategy

• Normal condition
  After analysis, we get , Under normal circumstances PC2,PC3 To the flow and PC6 The return flow is not as expected , Route selection intervention is required . To prevent mutual interference between attributes , It is recommended to use PreVal Attribute intervention routing .

 Modify the telecommunication interface (PC1,3,5) The routing community attribute is 100：*
 Connecting surface (PC2,4,6) The routing community attribute is 200：*
 The purpose of the community attribute is to match multiple routes at one time 

R9 Grasp the route

route-polocy Modify the community attribute

In the neighborhood R10 The outgoing call of route-policy

R10,R12 Empathy
After the R10 Hold on to 100.* The routing , And in the neighborhood R9 Call the policy up



R10 Empathy
R12 You need to put R8 The given community attribute is 200 The routing of PreVal enlarge

test ：
PC1,PC3 The flow to meet the expectations

PC5 The return flow is as expected

PC2,PC4 The flow to meet the expectations

PC6 The return flow is as expected

• R9 And R1 Link failure
  After analysis, we get , When the link fails ,R12 Preferably from R8 Learned routes , Must let 100 The route continues R5 Will be in R5 And R12 Change the direction of entry PreVal
  
  
  
  stay R5 And R12 After the policy is called in the direction of , It does R5 Yes 100 The right way , But because of R5 Will receive IBGP Neighbors and EBGP Neighbors' about 100 The routing . When the line is OK ,EBGP The routing of neighbors AS-PATH Long , therefore R5 Will choose IBGP The routing of neighbors , At this time, the line fails ,AS1 Only from AS2 Received information about 100 The routing , Route selection rules EBGP>IBGP, therefore R5 It is better to choose from EBGP Routes learned from neighbors , The problem is that the route will go through R5 arrive R6, Not in line with expectations .
  To solve this problem, we need to AS1 Increase on 100 route PreVal Let the route go back to R3 On
  AS1 All modified PreVal, The last configured router will really become the router through which the traffic passes ,R3 Finally, configure it
  R1, The same goes for other routers
  
  PC5 After the route intervention of the return route is successful, it still needs to intervene PC1,2 The route to . Go through as expected R4 go to R3, So you need to be in R4 Catch up 100 The routing , And in R3 And R4 The input increases upward PreVal, Give Way 100 The route of R4 To AS1
  
  
  test ：
  PC1,PC3 The flow to meet the expectations
  
  PC5 The return flow is as expected
  
  PC2,4,6 Unaffected
  
• R9 And R1 Link returns to normal
  After returning to normal 100 Will be routed by R9,R2,R3 Give them to R1, Because I was just AS1 Modified on 100 Of PreVal, therefore R1 Will choose R3 Route to , Will lead to PC5 The return flow will pass through R3 Round to R4 As a result, the route is not selected
  

Observe BGP You can see the route to let R1 Believe in R9 The routing , Only need R9 And R1 The input is modified upwards PreVal The value of and R3 The consistency of , According to the routing rules EBGP>IBGP,R1 It is better to choose from R9 Received at about 100 The routing


test
PC5 The return flow returns to normal