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Equal-cost Routes Cannot Be Created Because the NE80E Fills the FA in a Type-7 LSA

Publication Date:  2012-07-27 Views:  37 Downloads:  0
Issue Description
The NE80E is connected to a downstream S8505 through two links. OSPF is enabled on the device. The NE80E serves as the ABR device in an NSSA. The S8505 belongs to the NSSA. After the nssa default-route-advertise command is configured on the NE80E, no equal-cost routes are created on the S8505.
The corresponding configurations on the NE80E are as follows:
area 0.0.0.10
network 10.0.0.16 0.0.0.3
network 10.0.0.192 0.0.0.3
nssa default-route-advertise
The default route on the S8505 has only one next hop 10.0.0.17. 
Alarm Information
Null
Handling Process
Solution 1: Configure a loopback interface on the NE80E and add it to the NSSA. After that, the NE80E automatically updates the LSA and fills the loopback-interface address as the FA. After that, the equal-cost route can be created on the S8505.
Solution 2: Run the nssa no-summary command on the NE80E. After that, the NE80E distributes Type-3 LSAs by default. Thus, no FA padding is involved. Note that, you need to configure this command on all ABRs in the NSSA, so that equal-cost routes to multiple ABRs are created.  
Root Cause
1. After you use the nssa default-route-advertise command on the NE80E running OSPF, a Type-7 LSA is created. According to the current VRP implementation, the FA of the LSA must be filled.
2. The NE80E preferentially uses the loopback address of the network in the NSSA to fill the FA. If no loopback address is available, the NE80E fills the FA with the lowest IP address of actual interfaces. Because the NSSA contains two network segments only and both are connected to the S8505, 10.0.0.17 is filled in the FA in this example.
3. When calculating the route, the downstream device S8505 selects only 10.0.0.17 as the next hop of the default route, because the FA of the LSA is one of the interconnected addresses. 
Suggestions
On a device based on the VRP, OSPF fills the FA of a Type-7 LSA and the device does not support the variables of a non-Huawei device. You need to pay attention to the impact exerted by the FA on route selection. 

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