OSPF Interview Aid
OSPF Interview Questions
A page to visit to refresh the brain about OSPF stuff before you go for an interview
LSA Types & Description
LSA’s
1. Router LSA Each router genererates this to describe its own interface (inluding address/mask, cost, etc). Flooded within the OSPF Area
2. Network LSA Generated by a DR, and lists all the routers in an OSPF area. Flooded within an area
3. Summary LSA Created by the ABR, summarizes network information from one area and advertise to another. This is what enables Inter-Area routing. This doesn’t do network summarization, which is confusing to say the least
4. ASBR LSA An ASBR send this to tell all the routers about itself, and how to get to it.Kind of selfish, really. This enables routing between Autonomous Systems (i.e Eigrp -> OSPF, or BGP -> OSPF)
5. Autonomous System External LSA An ASBR sends this, and it advertises routes that originate outside of OSPF. This shows up as E1 or E2 routes in a routing table. Flooded to all areas except for stub area, Totally Stub Area, NSSA and Totally NSSA
7. NSSA External LSA used in Not-so-stubby areas An ABSR generates thes, and is used to describe routes redistributed into an NSSA.
9. Link-Scope Opaque Used for OSPF GR (Graceful Restart) and is flooded only on the link that the originating interface lives.
10. Area-Scope Opaque Used for MPLS Traffic Engineering (TE), used to advertise information about interfaces (e.g bandwidth) relevant to TE
11.Autonomous System-Scope Opaque I haven’t come across this guy in the wild, and after scouring the internet it looks like alot of other people haven’t either. Apparently reserved for future use… whatever that means.
Area Types
Backbone Area
Is the “core” area for OSPF, also known as area 0.0.0.0. Every other area connects back to this area, and you must use a virtual link if you don’t have direct connectivity for a remote area. Backbone area distributes routing information between non-backbone areas. In modern networking, lots of people just run 1 big Area 0, as the performance constraints of yester-year routers is no longer a concern.
Stub Areas (1,2,3,d)
The most basic of areas, the stub area doesn’t accept LSA type 5 or 7 and is the natural enemy of route redistribution. Type 3 LSA’s still get to him, so he’s still keen to know about things from other OSPF Areas. Think of him a bit like a Bogan Australian Racist; hates type 5’s and 7’s and refuses to know anything about them, but still keen to know about type 3’s so he can make good routing decisions out of his suburb to go to the pub.
In this area, you’ll still have a O IA and have a default route by default - gotta know how to get out of your suburb!
area x stub
The ABR will still have a full routing table, but the routers underneath it won't
Totally Stub (1,2,d)
Alot like his stub area mate, but even worse. Think of him as the Cronulla Riot Aussie bogan. He is small minded and doesn’t care about anything except his own area (and how to get out of it). Type 3,5 & 7 are blocked, so all that is left is a default route on how to get out to the rest of the world. This guy is good (the Total Stub area, not the Cronulla Riot Bogan) when you wanted to have a very small, clear and concise routing table.
area x stub no-summary
Not to Stubby Area (1,2,3, manually add d, 7)
NSSA’s are a little bit different to a stub area, and a little bit the same. They still block type 5 LSA’s like a stub, but will accept type 3’s However, they have no default route to anything else and you have to manually add that. NSSA’s are used when you want the benefits of a stub area, but also have an ABSR in that area that want routes from. In this instance, the ABSR will generate a type 7 LSA, which the ABR will convert back into a type 5 LSA and flood through the network.
You can think of NSSA’s as Kiwi’s who came to Australia but don’t want to go back home. They’re not interested in learning about the other areas of Australia (won’t accpet type 5’s), but are still interested in getting out of their suburb (type 3). We want them to be able to tell us all about NZ (Route redistribution), so we give them a special visa (type 7) so they can tell us about NZ whilst still not exposing them to the rest of Australia. Unfortunately, they aren’t very good at directions so we have to tell them how to get out of their area (manual default route or “area 1 nssa default-information-originate always”)
area x nssa .
Note that you have to convert them back from a stub area first.
Total Not-so-Stubby Area (1,2,d,7)
Total NSSA Area’s are the lovechild of a Kiwi and the Cronulla Bogan Aussie Racist. In a TNSSA, we take the Total Stub’s dislike of type 3&5, and we add the Kiwi’s ability to tell us about NZ (Type 7) and we roll it into one area. The caveat of this brainchild is that it somehow knows how to get out of it’s own suburb (it has a default route we don’t have to add)
area x nssa no-summary
Recap
Stub :1,2,3,d
Total Stub :1,2,d
Not so stubby : 1,2,3,manuald, 7
Totally not so stubby Area : 1,2,d,7
Total Stub v Stub
Total Stub’s block 3,4,5 and have only a default route, whereas Stub just blocks 4,5
O v E
E’s are the most common, can be thought of as “External”, and are used to identify a route external to OSPF.
O’s are only found in NSSA area.
E1/O1 v E2/O2
1 = the metric reflects the cost of the ** entire path to the destination **
2= the cost to the ** ABSR Advertising that route/network