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VPN Routes Were Not Normally Learned In Cross-Domain Option B Mode Because the Mask of the Loopback Address of the Intermediate Router Was Incorrect

Publication Date:  2012-07-27 Views:  46 Downloads:  0
Issue Description
1. Networking and version:
R0, R1, R2, R3, and R4 routers are all based on the V500R003 platform. R0 and R1 are installed in AS100, R2 and R3 in AS200, and R4 in AS300. Two loopback interfaces are added to R0, numbered respectively as 1 and 2 to simulate two VPN users. The two users implement the hub-spoke interconnection in cross-domain Option B mode. R0 serves as the spoke-PE and R3 as the hub-PE. 
       AS100             |                 AS200                      |        AS300
                                           |                                     vpn_in  |
         R0------------R1------------R2--------------R3===========R4
vpna|    |vpnb                                                          vpn_out     
         |    |
     L0    L1
2. The address of interface loopback 1 of R0 is 2.2.2.2, which is bound to VPN A. R4 and R4 are interconnected through BGP. The address of interface loopback 0 of R4 is 1.1.1.5 and the interface loopback 0 runs BGP.
3. The 2.2.2.2 route of R0 could be learned in VPN_IN of R4, but R0 could not learn the 1.1.1.5 route of R4.
4. For details about the configuration, see the appendix. 
 
Alarm Information
Null
Handling Process
1. Change the address of interface loopback 0 of R1 into 1.1.1.2 255.255.255.255.
2. Restart the mpls ldp process of R0 by running the reset mpls ldp command.
3. R0 could normally learn the 1.1.1.5 route, which could be viewed in VPN A. The 1.1.1.5 route could be successfully pinged with the ping -vpn vpna -a 2.2.2.2 command.
4. Troubleshoot the fault as follows:
To set up the LSP through LDP, by default, the router allocates the label to only the 32-bit host route. Because the address of interface loopback 0 of R1 was configured as a 30-bit address, which did not meet the condition for allocating LDP labels, the label could not be allocated. Therefore, the LSP necessary for label forwarding could not be set up. Having learned the VPNv4 route, R0 checked whether the LSP forwarding path of the VPNv4 route took effect. If the LSP was not set up (that is, the path label was not completely allocated), the VPNv4 route would not be saved to the VPN routing table. 
 
Root Cause
1. Generally, the unidirectional learning of routes is impossible. For cross-domain Option B networking, because the intermediate the ASBR-PE needs to reserve VPN routes, you should find out where the route is lost by checking the BGP VPNv4 routes on the devices.
2. Running the display bgp vpnv4 all routing-table command successively on R3, R2, R1, and R0 to check the 1.1.1.5 route, the engineer found that all these four routers received the route. However, the 1.1.1.5 route on R0 was not the preferential one. Therefore, this route was not added to the routing table of VPN A. After the display ip rou vpn vpna command was run, the 1.1.1.5 route could not be viewed. The related information is as follows:
<R0>dis ip r vpn vpna                                                           
Routing Tables: vpna                                                            
         Destinations : 1        Routes : 1                
Destination/Mask    Proto  Pre  Cost       NextHop         Interface            2.2.2.2/32  Direct 0    0          127.0.0.1       InLoopBack0 
<R0>disp bgp vpnv4 all rou                               
 BGP Local router ID is 1.1.1.1                                                 
 Status codes: * - valid, > - best, d - damped,                                 
               h - history,  i - internal, s - suppressed, S - Stale            
               Origin : i - IGP, e - EGP, ? - incomplete          
 Total number of routes from all PE: 5                                          
 Route Distinguisher: 400:1                                         
      Network            NextHop        MED        LocPrf    PrefVal Path/Ogn   
 *>i  1.1.1.5/32         1.1.1.2                    100        0      200 300?  
 *>i  2.2.2.2/32         1.1.1.2                    100        0      200 300 20
0 100?                                                                          
 *>i  3.3.3.3/32         1.1.1.2                    100        0      200 300 20
0 100?                                                                          
 *>i  100.0.34.0/30      1.1.1.2                    100        0      200 300?  
 *>i  100.0.34.1/32      1.1.1.2                    100        0      200 300?  
 BGP Local router ID is 1.1.1.1                                                 
 Status codes: * - valid, > - best, d - damped,                                 
               h - history,  i - internal, s - suppressed, S - Stale            
               Origin : i - IGP, e - EGP, ? - incomplete   
 Total routes of vpn-instance vpna: 6                                           
      Network            NextHop        MED        LocPrf    PrefVal Path/Ogn   
 * i  1.1.1.5/32         1.1.1.2                    100        0      200 300?  
 *>   2.2.2.2/32         0.0.0.0         0                     0      ?         
 * i  2.2.2.2/32         1.1.1.2                    100        0      200 300 20
0 100?                                                                          
 * i  3.3.3.3/32         1.1.1.2                    100        0      200 300 20
0 100?                                                                          
 * i  100.0.34.0/30      1.1.1.2                    100        0      200 300?  
 * i  100.0.34.1/32      1.1.1.2                    100        0      200 300?  
 Total routes of vpn-instance vpnb: 6                                           
      Network            NextHop        MED        LocPrf    PrefVal Path/Ogn   
 * i  1.1.1.5/32         1.1.1.2                    100        0      200 300?  
 * i  2.2.2.2/32         1.1.1.2                    100        0      200 300 20
0 100?                                                                          
 *>   3.3.3.3/32         0.0.0.0         0                     0      ?         
 * i  3.3.3.3/32         1.1.1.2                    100        0      200 300 20
0 100?                                                                          
 * i  100.0.34.0/30      1.1.1.2                    100        0      200 300?  
 * i  100.0.34.1/32      1.1.1.2                    100        0      200 300? 
3. Checking the device configuration, the engineer found that the address of interface loopback 0 of R1 was 1.1.1.2 255.255.255.252. 
 
Suggestions
Null

END