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.

Other related questions:
Why the S series switches do not generate backup information for static routes or BGP routes after configuring IP FRRs
The iterative route does not apply the IP FRR configuration directly, but inherits next-hop route through the backup outbound interface and the backup next hop generated by IP FRR. Static route or BGP route If it is an iterative route and the next hop route does not generate backup information of IP FRR, static route or BGP route does not generate backup information even if it matches IP FRR policy.

Why the ASBR does not generate equal-cost route
Two areas are configured on two devices separately, for example: SwitchA: area 0: area 1: SwitchB: 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.

Why configure OSPF route tag on S series switches
For S series switches supporting OSPF, the OSPF router tag is applied to VPNs and prevents loops of Type 5 LSAs in CE dual-homing networking. If the tag of a received Type 5 LSA is the same as the router tag of OSPF on a PE, the PE neglects this LSA when calculating routes. When a CE is connected to two PEs, PE1 generates a Type 5 LSA based on the imported BGP route and sends the LSA to the CE, and the CE forwards the LSA to PE2. The OSPF route takes precedence over the BGP route, so PE2 replaces the BGP route with the OSPF route. As a result, a loop occurs. If the route tag is configured on a PE, when the PE receives an LSA with the same route tag as its own route tag, it neglects this LSA, avoiding loops. By default, the route tag is calculated using the BGP AS number. If BGP is not configured, the route tag is 0. In OSPF public network instances, router tags cannot be used to prevent loops but can be used as a filtering condition in a policy. When setting the router tag of ASE-LSAs and NSSA-LSAs, not the following: The route-tag command is used in the OSPF multi-instance scenario. The default tag command is used in the OSPF public network instance scenario. The import-route tag command can be used in either of the preceding scenarios.

Why does not an S series switch functioning as an ABR summarize addresses while advertising the routes
Perform the following operations on the S series switch functioning as the ABR to solve the problem. Run the display current configuration command to check whether all network segments in the area are consecutive. If not, divide the network segments into several groups of consecutive network segments. Run the abr-summary command on the ABR to summarize each group of consecutive network segments into one network segment. Check the filter export command in the area view and ensure that the LSA for ABR route summarization is not filtered out.

Why are routes in the OSPF routing table on an S series switch in the inactive state
A policy that filters all OSPF routes may be configured in an OSPF process. As a result, all routes in the OSPF routing table are in the inactive state.

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