OSPF Simulation – AS Border Router (Interactive, Visual)

Q1. What is ASBR?

Answer. It stands for AS Border Router. It sits on the boundary of an OSPF AS and a non-OSPF AS. The main job of AS Birder Router is to import external addresses from non-OSPF AS into OSPF network.

Q2. Why ASBR?

Answer: In a typical OSPF network, majority of network addresses in routing tables are not learned from other OSPF routers. They are imported from outside of OSPF AS. We need a router that runs both OSPF and non-OSPG routing protocols in order to import external addresses into OSPF AS. This is the job of AS Border Router.

Q3. What is ASBR Summary LSA?

Answer: This LSA describes the ASBR that imports external addresses into OSPF AS. This SLA’s link-state ID is the router ID of the ASBR. ABR creates an ASBR Summary LSA when it receives AS Border Router’s Router LSA. Then ABR floods ASBR Summary LSA into its areas.

Q4. Why do we need ASBR Summary LSA?

Answer: For a router in an OSPF AS to forward packets to a destination outside its AS, it needs the following:

1) Destination subnet IP/mask in the alien AS.

2) The gateway router (ASBR) which connects to the alien AS.

3) How to reach AS Border Router.

External LSAs created by ASBR describes 1) and 2). ASBR LSA describes 3).

Q5. How does an ABR know which router is an ASBR?

Answer: An ABR receives Router LSAs from other routers in its area. If a Router LSA’s E bit is 1, then that router is an ASBR. ABR creates an ASBR Summary LSA to specify the ASBR and floods it into its areas.

Q6. What is the flooding scope of ASBR Summary LSA?

Answer: Area only.

Q7: How does an internal router forward packets to external destinations?

Answer: It takes both ASBR and ABR to provide routing paths to reach external addresses.

1) AS Border Router creates an External LSA for each external destination it has learned from an alien AS.

2) ABR flood External LSAs in OSPF AS to let all routers learn external destinations.

3) When ABR receives the Router LSA from ASBR, it creates ASBR Summary LSA and floods it to other areas.

4) Now an internal router can create routing paths to reach external addresses as follows:

Router > ABR > ASBR > external address.

Where Router > ABR: All routers in an area have the ABR’s Router LSA and know how to reach to ABR.

ABR > ASBR: They are in the same area. ABR has ASBR’s Router LSA and knows how to reach to ASBR.

ASBR > external address: ASBR has learned the external address from the alien AS. It knows how to reach the external address.

This article is the FAQ part of an OSPF simulation: ASBR. To see how an ASBR announce external addresses across OSPF AS, and ABR floods ASBR Summary LSA to inform router the ASBR, go to the external link listed below.

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