OSPF Beyond Stump-the-Chump_Interview_Questions - Part 01 -
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The document discusses OSPF (Open Shortest Path First) network configurations, particularly focusing on OSPF neighbor formation and verification commands. It explains the significance of Link State Advertisements (LSAs) and details various types of LSAs, as well as OSPF route selection criteria based on area types. The summary of OSPF LSA types clarifies their roles in routing decisions and highlights how they affect network design and traffic flow.
OSPF Beyond Stump-the-Chump_Interview_Questions - Part 01 -
- 1. OSPF Beyond STUMP The CHUMP Interview Questions Putting it into Your Network – Part -1 - OSPF Neighbor Formation
- 2. OSPF Verification Commands show ip protocols show ip ospf int brief show ip ospf router topology show ip ospf neighbor show ip route ospf show ip ospf events LSA rib sh ip ospf database external “prefix number”
- 3. So What Is Administrative Distance Versus OSPF Metric In A Route Selection Decision? Administrative Distance: Determines what will be installed into the global routing table when 2 different routing protocols have routes to the same destination! OSPF Metric: Determines route that will be chosen when 2 different routers within OSPF are advertising routes to the same destination!
- 4. Link State Advertisements What are Those? Why are they Important? So it is essential understand Link State Advertisements LSA type 1 - Router LSAs are sent from a router to other routers in the same area. It contains information regarding the routers interfaces in the same area, relevant interfaces IPs, its adjacent routers on those interfaces and sub networks LSA type 2 - Network LSAs are generated by the DR on a multi access segment, and provides similar information to an LSA type 1 for the multi access segment and subnet which it belongs LSA type 3 - Network Summary LSAs are generated by ABRs and contain the subnets & costs but omit the topological data from all subnets in one area and sent to another area via the ABR LSA type 4 -ASBR summary LSAs are from ASBRs and are identical in structure to a type 3 LSA and sent when crossing an AS boundary LSA type 5 -Are AS EXTERNAL LSAs which are originated by ASBRs and describe external networks LSA type 6 - Is defined as a Group Membership LSA but not used in Cisco devices LSA type 7 -NSSA External LSAs are generated by the ASBR in an NSSA area LSA type 8 -Is defined as a External Attribute LSA but not used in Cisco devices LSA types 9 to 11 - Defined as Opaque LSAs and are reserved for future expansion
- 5. LSA Types Why Do I Need To Know Them? Complete Understanding of LSA Types Is The Cornerstone To Understanding OSPF Areas and OSPF Area Behaviors Complete Understanding of OSPF Area Behaviors Is The Cornerstone of Good OSPF Network Design and Traffic Flow
- 6. How Does OSPF Select Routes? Let’s review ….. Regardless of a route’s metric or administrative distance, OSPF will choose routes in the following order: 1st) Intra-Area (O) 2nd) Inter-Area (O IA) 3rd) External Type 1 (E1) 4th) External Type 2 (E2) 5th) NSSA Type 1 (N1) 6th) NSSA Type 2 (N2)
- 7. So What Type of LSA Does Each OSPF Area Have? Area Type Router LSA Network LSA Network Summary LSA External LSA NSSA LSA Default Route Backbone 1 2 3 4 5 Stub 1 2 3 0/0 as Type 3 Totally Stub 1 2 0/0 as Type 3 NSSA 1 2 3 7 (Replaces 5) Understand Why? No Automatic 0/0 Totally NSSA 1 2 7 (Replaces 5) Understand Why? If you use "default-information originate" on NSSA command Line, it will come in as an N2 route Separate default-information originate command line brings in the 0/0 as an E2 or N2 route depending on area type.
- 8. Default Route and LSA Type Generated By Each OSPF Special Area: The Stub areas that put in a 0/0 default route Stub Area Automatic 0/0 as a type 3 Totally Stub Area Automatic 0/0 as a type 3 Not So Stubby Area NO automatic 0/0 Total NSSA Automatic 0/0 as a type 3 And LSA Type 3 is What? LSA type 3 - Network Summary LSAs are generated by ABRs and contain the subnets & costs but omit the topological data from all subnets in one area and sent to another area via the ABR
- 9. LSA Type 1 (Router LSA) Generated by all routers in an area to describe their directly attached links (Intra-area routes). These do not leave the area.
- 10. LSA Type 2 (Network LSA) Generated by the DR of a broadcast or Nonbroadcast segment to describe the neighbors connected to the segment. These do not leave the area.
- 11. LSA Type 3 (Summary LSA) Generated by the ABR to describe a route to neighbors outside the area. (Inter-area routes)
- 12. LSA Type 4 (Summary LSA) Generated by the ABR to describe a route to an ASBR to neighbors outside the area.
- 13. LSA Type 5 (External LSA) {Redistribution} Generated by ASBR to describe routes redistributed into the area. These routes appear as E1 or E2 in the routing table. E2 (default) uses a static cost throughout the OSPF domain as it only takes the cost into account that is reported at redistribution. E1 (always preferred) uses a cumulative cost of the cost reported into the OSPF domain at redistribution plus the local cost to the ASBR.
- 14. LSA Type 6 (Multicast LSA) Not supported on Cisco routers NOT Every Router on the Internet was made by Cisco
- 15. LSA Type 7 (NSSA LSA) Generated by an ASBR inside a NSSA to describe routes redistributed into the NSSA. LSA 7 is translated into LSA 5 as it leaves the NSSA. These routes appear as N1 or N2 in the ip routing table inside the NSSA. Much like LSA 5, N2 is a static cost while N1 is a cumulative cost that includes the cost up to the ASBR.
- 16. So What Type of LSA Does Each OSPF Area Have? Area Type Router LSA Network LSA Network Summary LSA External LSA NSSA LSA Default Route Backbone 1 2 3 4 5 Stub 1 2 3 0/0 as Type 3 Totally Stub 1 2 0/0 as Type 3 NSSA 1 2 3 7 (Replaces 5) Understand Why? No Automatic 0/0 Totally NSSA 1 2 7 (Replaces 5) Understand Why? If you use "default-information originate" on NSSA command Line, it will come in as an N2 route Separate default-information originate command line brings in the 0/0 as an E2 or N2 route depending on area type.
- 17. OSPF LSA Types Explained
- 18. OSPF Verification Commands show ip protocols show ip ospf int brief show ip ospf router topology show ip ospf neighbor show ip route ospf show ip ospf events LSA rib sh ip ospf database external “prefix number”