OSPF
Topic 8 of 26. 30 questions: 14 pick one, 4 pick all, 4 type it, 3 put in order, 5 interview
What this topic covers
- link state and SPF
- router ID selection
- hello and dead timers
- neighbor states Down to Full
- what must match to form an adjacency
- DR and BDR election and why
- network types
- LSA types 1 to 7
- areas and why area 0
- ABR and ASBR
- stub totally stubby NSSA
- cost and reference bandwidth
- summarization at ABR and ASBR
- virtual links
- authentication
- stuck in EXSTART or 2-WAY
- passive interfaces
- default-information originate
- OSPF vs EIGRP vs IS-IS
Study it in depth on Octet.
- 1OSPF is a link state protocol. Explain what that means and how a router gets from hearing its neighbors to a full routing table.
- 2Put the ways Cisco IOS chooses an OSPF router ID in order of precedence, first choice first.
- 3OSPF process 1 starts on R1 for the first time and there is no router-id command. Which router ID does R1 choose?
- 4R1 runs OSPF with router ID 10.255.0.1 and has full adjacencies. On Cisco IOS you enter router-id 1.1.1.1 under router ospf 1. What happens?
- 5On Cisco IOS, what is the default OSPF dead interval on an Ethernet interface with the broadcast network type?
- 6On Cisco IOS, what are the default OSPF hello and dead intervals on an interface with the point-to-multipoint network type?
- 7Put the OSPF neighbor states in the order two routers on a point-to-point link pass through, first to last.
- 8Two OSPF routers share an Ethernet segment and both keep the default interface priority. Which one becomes the DR?
- 9R1 shows this neighbor for several minutes and it never reaches FULL. What is the most likely cause?
- 10Which of these must match on both ends of a link for two OSPF routers to become neighbors? Choose all that apply.
- 11R1 shows this neighbor entry and it never moves on. R2 shows no OSPF neighbors at all on the link. What is the most likely cause?
- 12R3 shares an Ethernet segment with four other OSPF routers. Is anything wrong in this output?
- 13Why does OSPF elect a DR and a BDR on an Ethernet segment, and what do they actually do?
- 14These four routers start OSPF on the same Ethernet segment at the same moment. Which router becomes the BDR? Give its name.
- 15A segment already has an elected DR and BDR, both with priority 1. You connect a new router with interface priority 255. What role does it take?
- 16On Cisco IOS, which OSPF network types elect a DR and BDR? Choose all that apply.
- 17Why would you configure ip ospf network point-to-point on an Ethernet link between two routers, and what do you have to watch for?
- 18R1's Loopback0 is configured as 10.1.1.1/24 and runs OSPF in area 0 with default settings. R2 learns it as shown. Why is it a /32?
- 19Go through the OSPF LSA types 1 to 7. Who originates each one and how far does it travel?
- 20On a broadcast segment, which LSA type does the DR originate to describe the segment and the routers attached to it? Give the type number.
- 21Why does OSPF need Type 4 LSAs at all?
- 22Area 10 is totally stubby. Which LSA types can appear in the database of a router inside area 10? Choose all that apply.
- 23Why does OSPF use areas, and why must every area connect to area 0?
- 24R4 has interfaces in area 0 and area 20, and it also redistributes static routes into OSPF. What is R4?
- 25Which statements about a plain OSPF stub area, not totally stubby, are true? Choose all that apply.
- 26Area 20 must not receive external routes from the rest of the OSPF domain, though inter-area routes are fine. A router inside area 20 must redistribute static routes toward a partner network. Which area type fits?
- 27Area 30 is a plain NSSA, with area 30 nssa on every router in it, on Cisco IOS. Internal routers have inter-area routes but no default route. Why?
- 28R1, the ABR for area 10, has area 10 stub no-summary. A new router R5 in area 10 forms no adjacency with R1. Given this debug on R5, what fixes it?
- 29OSPF learns the same prefix in four different ways. Put the route types in order of preference, most preferred first, whatever their metrics.
- 30ASBRs 5.5.5.5 and 6.6.6.6 both redistribute 172.16.0.0/16 into OSPF with default settings, so both routes are E2 with metric 20. How does R1 route to 172.16.0.0/16?