A Routing Loop Occurs After Routes Are Summarized on an AR2240

A routing loop occurs after an AR2240 summarizes routes. The following figure shows the networking.



In the figure, Router2 connects to the carrier network (Internet) through an EBGP connection. OSPF runs between two AR2240s (Router1 and Router2) and two ME60s. Router2 summarizes routes to the network segment 192.168.128.160/30 using the ip route-static 192.168.128.160 255.255.255.248 Null 0 command and re-advertises the summarized static route into OSPF.

Run the display ip routing-table command to check the IP routing table of Router2.

<Huawei> display ip routing-table
Route Flags: R - relay, D - download to fib                                     
------------------------------------------------------------------------------  
Routing Tables: Public                                                          
         Destinations : 9        Routes : 9                                     
                                                                                
Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface      
                                                                     
192.168.128.160/29   Static  60   0           D   0.0.0.0         NULL0        
192.168.128.160/29   OSPF    10   2           D   192.168.128.158  GigabitEthernet2/0/0      
    ……

The IP routing table shows that the path of traffic from Router2 to 192.168.128.161 is Router2--ME60B--ME60A. In this situation, Router2 can ping 192.168.128.161 successfully.

<Huawei> ping 192.168.128.161
  PING 192.168.128.161: 56  data bytes, press CTRL_C to break
    Reply from 192.168.128.161: bytes=56 Sequence=1 ttl=255 time=4 ms
    Reply from 192.168.128.161: bytes=56 Sequence=2 ttl=255 time=2 ms
    Reply from 192.168.128.161: bytes=56 Sequence=3 ttl=255 time=3 ms
    Reply from 192.168.128.161: bytes=56 Sequence=4 ttl=255 time=2 ms
    Reply from 192.168.128.161: bytes=56 Sequence=5 ttl=255 time=2 ms

  --- 192.168.128.161 ping statistics ---
    5 packet(s) transmitted
    5 packet(s) received
    0.00% packet loss
    round-trip min/avg/max = 2/2/4 ms

Run the tracert command to tracert 192.168.128.161.

<Huawei>tracert 192.168.128.161                                                 
traceroute to  192.168.128.161(192.168.128.161), max hops: 30 ,packet length: 4
0,press CTRL_C to break                                                         
1  192.168.128.158 9 ms  3 ms  4 ms                                                                         
2  192.168.128.161 9 ms  3 ms  4 ms                                 
                 
After the OSPF cost of GE2/0/0 on Router2 is changed to 10, the IP routing table of Router2 is changed:
<Huawei> display ip routing-table  
Route Flags: R - relay, D - download to fib                                     
------------------------------------------------------------------------------  
Routing Tables: Public                                                          
         Destinations : 9        Routes : 9                                     
                                                                                
Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface      
                                                                     
192.168.128.160/29   Static  60   0           D   0.0.0.0         NULL0        
192.168.128.160/29   OSPF    10   3           D   192.168.128.138   GigabitEthernet3/0/0   
  ……

The IP routing table shows that the path of traffic from Router2 to 192.168.128.161 becomes Router2--Router1--ME60A. In this situation, Router2 cannot ping 192.168.128.161.

Run the tracert command to tracert 192.168.128.161.

<Huawei> tracert 192.168.128.161                                                 
traceroute to  192.168.128.161(192.168.128.161), max hops: 30 ,packet length: 4
0,press CTRL_C to break                                                         
1  192.168.128.138 9 ms  3 ms  4 ms                                                                         
2  192.168.128.137 9 ms  3 ms  4 ms   

3  192.168.128.138 9 ms  3 ms  4 ms                                                                         
4  192.168.128.137 9 ms  3 ms  4 ms     

5  192.168.128.138 9 ms  3 ms  4 ms                                                                         
6  192.168.128.137 9 ms  3 ms  4 ms          
                     
The tracert result shows that packets remain transmitted between Router1 and Router2 without being forwarded to the destination. However, Router1 can ping 192.168.128.161 successfully.

Whether packets from Router2 to 192.168.128.161 pass through Router1 or ME60B depends on the 24GE cards connected between Router1 and Router2. 24GE cards are high-end LAN cards, which provide the route forwarding function (enabled by default) but cannot download a complete IP routing table. Router1 learns routes to 192.168.128.160/29 through OSPF. When packets reach Router1, Router1 sends the packets to Router2, causing a loop. To break the loop, disable the route forwarding function of the high-end LAN cards. Subsequently, packets received by the high-end LAN cards are sent to the CPU's sub-cores for forwarding.

To disable the route forwarding function of high-end LAN cards, run the set workmode lan-card l3centralize command in the system view. Router2 then can ping 192.168.128.161 successfully.

24GE cards are high-end LAN cards, which provide the route forwarding function (enabled by default) but cannot download a complete IP routing table. You need to disable the route forwarding function of high-end LAN cards so that packets received by the high-end LAN cards can be sent to the CPU's sub-cores for forwarding.

Run the set workmode lan-card l3centralize command to disable the route forwarding function of high-end LAN cards so that packets received by 24GE cards can be sent to the CPU's sub-cores for forwarding.