FA of the type-7 LSA

Hello,

As shown in the following figure, the router ID of R3 is greater than that of R1. When R5 imports a route, Type 7 LSAs are translated to Type 5 LSAs on R3. On R3, set the FA of the Type 5 LSAs translated from Type 7 LSAs to 0.0.0.0 using the NSSA suppress-forwarding-address command. R3 forwards the type 5 LSA to R4 and R4 forwards the LSA to R1. 

Therefore, R1 has the Type 7 LSA received from R5 and the Type 5 LSA received from R4. If the cost of the Type 7 LSA is greater than that of the Type 5 LSA, R1 learns the external route from Type 5 LSAs. That is, the next hop of the route from R1 to the external route is R4.

So when R4 accesses the external route, a loop occurs(R4->R3->R1->R4).

<R4>tracert 5.5.5.55

 traceroute to  5.5.5.55(5.5.5.55), max hops: 30 ,packet length: 40,press CTRL_C

 to break 

 1 10.0.34.3 30 ms  10 ms  10 ms 

 2 10.0.13.1 30 ms  20 ms  20 ms 

 3 10.0.14.4 20 ms  20 ms  20 ms 

 4 10.0.34.3 30 ms  30 ms  40 ms 

 5 10.0.13.1 30 ms  40 ms  40 ms 

 6 10.0.14.4 40 ms  20 ms  50 ms