BSCI OSPF true or false

6stitches

Source BSCI self-study guide p. 308.

"True or False: OSPF design requires that all areas be directly connect to the backbone."

I answered false, because you can use virtual link, book says it's true. what do you guys think, this is a badly formulated question?

cisco_trooper

I say true. The virtual link is used to connect the far end area to area 0. I do see your point however, because it obviously is not physically connected.....

networker050184

True.

A virtual link still links the area to the backbone like cisco_trooper said.

Mrock4

Interesting FYI that you don't need to necessarily have a physical link in area 0 for the routes to be shared between say, area 1 and area 2. As long as you have at least a loopback being advertised as area 0, you can interconnect areas without a "real" area 0..if that makes sense. I don't know if it makes me a geek or what, but I think that's pretty cool. I know this isn't really completely relevant, I just thought it's something interesting a lot of people don't know.

networker050184

Mrock4 wrote:

Interesting FYI that you don't need to necessarily have a physical link in area 0 for the routes to be shared between say, area 1 and area 2. As long as you have at least a loopback being advertised as area 0, you can interconnect areas without a "real" area 0..if that makes sense. I don't know if it makes me a geek or what, but I think that's pretty cool. I know this isn't really completely relevant, I just thought it's something interesting a lot of people don't know.

What do you mean by not having a "real" area zero? Do you mean not having a router that is in area zero only?

cisco_trooper

Lab time....

GT-Rob

If you want to get picky, you can run OSPF without area 0 at all!

But if you answer false, you are looking too far into the question.

mikej412

Not only is it True, it is ABSOLUTELY TRUE!

If you can't accept that as a basic postulate of OSPF Network Design, then you may as well run IS-IS.

Of course, in actual network life your mileage may vary.

Mrock4

networker050184 wrote:

What do you mean by not having a "real" area zero? Do you mean not having a router that is in area zero only?

I should have been more clear. What I meant by a "real" area zero, was actually having two routers sharing a physical link placed in area zero. To clarify..


RouterA

---

Area-1

---

RouterB

---

Area-2

---

RouterC


This will not work if all you do is place each physical link in their respective areas (Area 1, and Area 2), but if you create a loopback on RouterB, and place it in Area 0..it will work.

Just stirring things up

networker050184

I think people get confused with areas (not you Mrock4 just people I have spoken with in general). They do not seem to grasp the fact that an area is logical. They want to identify area 0 as if it is a router or set of routers, but like in Mrock4's example its purely logical in software. Router B does have an interface in area 0 even if it is a loopback.

I've never ran OSPF without area 0 so I may have to look into that GT-Rob.

Mrock4

networker050184 wrote:

Router B does have an interface in area 0 even if it is a loopback.

I've never ran OSPF without area 0 so I may have to look into that GT-Rob.

It definitely does..but I just brought it up because most people do not think of that scenario. I didn't either..I saw it on a blog a while ago, and thought it may come in use someday- even if it's only in the lab.

I might have to play with some scenarios when I get home in the morning, all this talk of OSPF is raising questions in my head now.

6stitches

I guess when i see the word "directly connect" I wouldn't consider a virtual link a direct connect, so I answer false.

but then one could argue that a virtual link is a direct connect, but it gets a little hazy for me here.

I think that's my problem, like GT-rob said, I read to much into the question.

gorebrush

Also, don't forget that in an ideal world - we shouldn't even be using "Virtual Links", they are only designed to be used in cases where a company has been taken over or integrated, and you have not had the time to properly integrate their OSPF infrastructure (Or any other interior routed structued) in with your own...

I think when you consider this question, you have to discount Virtual Links and think about the basic design principles of OSPF which states, categorically, that all areas must be connected to the backbone.

dtlokee

A virtual link is an extension of area 0, so when you use a virtual link the area is now in fact "directly connected" to area 0. Think of it as a tunnel between the routers that is in area 0, therefore extending area 0 across another area.

ITdude

mikej412 wrote:

If you can't accept that as a basic postulate of OSPF Network Design, then you may as well run IS-IS.

Let's hear it for CLNS!

GT-Rob

networker050184 wrote:

I've never ran OSPF without area 0 so I may have to look into that GT-Rob.

wanR1(config)#router ospf 1
wanR1(config-router)#network 192.168.0.1 0.0.0.0 area 1 
wanR1(config-router)#exit
wanR1(config)#exit
wanR1#telnet R2
R2#conf t
R2(config)#router ospf 1
R2(config-router)#network 192.168.0.2 0.0.0.0 area 1
R2(config-router)#exit
R2(config)#do sh ip ospf neigh

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.0.1       1   FULL/DR         00:00:39    192.168.0.1     Ethernet0/0

R2#exit

wanR1#sh ip ospf neigh

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.0.2       1   FULL/BDR        00:00:30    192.168.0.2     Ethernet0/1

wanR1#sh ip pro
Routing Protocol is "ospf 1"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Router ID 192.168.0.1
  Number of areas in this router is 1. 1 normal 0 stub 0 nssa
  Maximum path: 4
  Routing for Networks:
    192.168.0.1 0.0.0.0 area 1
  Routing Information Sources:

Area 0 essentially is used so that OSPF knows how to get to other areas. If there only is 1 area though, there is no need.

networker050184

I never even thought to try it. I guess there there is nothing stopping it from functioning within the area.

gorebrush

Hmm, I haven't read the RFC's and the proper design on OSPF, but I wonder...

Can Area 0 be substituted with Area 1, as long as Area 1 is recognised as the backbone?

Or will the OSPF Process on the routers require Area 0 to be a backbone?

Time to lab!

EDIT: - somehow though, I think the above is a load of bobbins :Dw

cisco_trooper

The effects of using an area other than area 0 won't be substantially felt until you start playing with all the stub area types...

dtlokee

No area 0 is the backbone. It happens that IF you only have a single area then you can use whatever area value you like, if you have more than one then you NEED an area 0 for it to function and all other areas must have a connection to area 0 either directly or via a virtual link.

cisco_trooper

cisco_trooper wrote:

The effects of using an area other than area 0 won't be substantially felt until you start playing with all the stub area types...

This is what happens when you try to be smart in the morning....

cisco_trooper

dtlokee wrote:

No area 0 is the backbone. It happens that IF you only have a single area then you can use whatever area value you like, if you have more than one then you NEED an area 0 for it to function and all other areas must have a connection to area 0 either directly or via a virtual link.

What he said....

Paul Boz

that is a tricky question because from CCNA you are taught that virtual links are bogus and that they should never be used. That doesn't mean that they CAN'T be used though, so even though the connection is logical, it is still a logical connection to the backbone.

The fact that all nets have to connect to area 0 is a fundamental principal of OSPF.