Why the ASBR does not generate equal-cost route


Two areas are configured on two devices separately, for example:

area 0:
area 1:
area 0:
area 1:
Adjacencies are formed on the devices separately.

Thus, the next-hop of a route that SwitchB learns from SwitchA should be or However, the route on SwitchB goes through only in area 1 as the gateway and no load sharing can be performed.

This is because when multiple equal-cost routes exist, an ASBR selects the route with the greatest area ID.

Other related questions:
Why cannot interfaces of different types generate equal-cost routes and establish OSPF load balancing
OSPF can automatically calculate the cost of an interface based on the interface bandwidth. The calculation formula is: Interface cost = Bandwidth reference value/Interface bandwidth. The bandwidth reference value is configurable. The default value is 100 Mbit/s. Routes generated by OSPF have the same priority (10 by default). The interface cost, however, depends on the interface type. Interfaces of different types have different costs; therefore, the interfaces cannot generate equal-cost routes, leading to failure in establishing load balancing.

Number of equal-cost routes the firewall supports
The NGFW supports a maximum of eight equal-cost routes.

Modifying the number of equal-cost OSPF routes on the USG2000&USG2000 series
Perform as follows to configure the maximum number of equal-cost OSPF routes on the USG2000 series: 1. In the user view, run the system-view command to enter the system view. 2. Run the ospf [ process-id ] command to access the OSPF process view. 3. Run the maximum load-balancing number command to configure the maximum number of equal-cost routes. A maximum of eight equal-cost OSPF routes are supported.

Why do not S series switches functioning as ASBRs generate equivalent routes
Question: Why do not ASBRs generate equivalent routes? Answer: Two areas are enabled on two S series switches supporting OSPF, for example: Switch A: Area 0: Area 1: Switch B: Area 0: Area 1: Then the switches set up an adjacency relationship. When Switch B imports a route from Switch A, the next-hop address of the route can be or However, only the route through Area 1 with the IP address is displayed on Switch B. This is because when multiple equal-cost routes exist, the route with the largest area ID is preferentially selected.

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