OSPF area 0 questions

DANMOH009

Hi All,

I got asked these questions a couple of weeks back in an interview, and was wondering if you guys would help me answer them.

Q1. From an ISP perspective why should area 0 never be used on a customer network and why?

Q2. From an ISP perspective when should area 0 be used in a customer network and why?

I didnt get the job, so i probably answered wrong, Cant remember 100% what i answered, but i think i went down the lines of saying something like "all routers must connect to area 0 and this would cause a problem because it could be used to transit traffic between areas?"

As you can tell from my answer i dont know much about this topic, still studying switch...

Any help would be great on these.

Thanks

EdTheLad

Some advise to begin with, if you don't know the answer to a question in an interview, tell them you don't know, don't try and bluff through the question, it's not professional. You could say i don't know but if i was to guess blah blah blah...

The question is vague, do they mean joining a customer network to the ISP network? Generally the customer will peer to the ISP via BGP, whatever the customer is doing with their IGP is up to them. But i'll answer this as per the customer merging their ospf area 0 with the ISPs, even though that's never going to happen.

1)
Everytime the customer makes a change in their network, area 0 will need to perform a spf recalculation, traffic forwarding is affected during convergence and hence this will affect all area 0 traffic and other inter-area traffic as all traffic must transit the backbone.
You would have no way to prevent the customer from flooding the backbone with lsa's, a customer misconfiguration could crash your backbone routers.
Security, the customer will be able to see the full l3 topology of your network.

2) Never

DANMOH009

Thanks Ed,

I think i didn't really understand the question, and this i suppose didn't help. but thanks for the advice

bobfromfpl

sometimes i wish the person doing the interview would have a diagram to explain the questions or provide a visual. a few times i've found that some people enjoy playing stump the chump or in other cases I interpret the question differently then what they are really asking. this is definitely a lesson for me if I'm ever interviewing a candidate in the future.

Dieg0M

EdTheLad wrote: »

1)
Everytime the customer makes a change in their network, area 0 will need to perform a spf recalculation, traffic forwarding is affected during convergence and hence this will affect all area 0 traffic and other inter-area traffic as all traffic must transit the backbone.
You would have no way to prevent the customer from flooding the backbone with lsa's, a customer misconfiguration could crash your backbone routers.


2) Never

1) I don't think this question is why the customer should not share the same area as the ISP but why the customer should not be in area 0.
2) How would you interconnect customers if they are not joined atleast on one link to area 0 ?

EdTheLad

If you wanted to interconnect customers via OSPF, you would assign each to a non-zero area, the ABR they are connected to has an interface in area zero.Best practice would be to set each of there area's to stub so that they couldn't flood type 5 lsa's, config some inter-area filtering to limit there type 3 lsa's and possibly do some summarization.At no point are you going to connect any customer to your backbone.

Dieg0M

Ok, you are thinking OSPF from an enterprise perspective. I don't think an ISP would ever peer that way with a customer. I think the question was in a L3VPN situation, if the CE-PE was OSPF.

EdTheLad

If it was L3VPN, you could extend area 0 across the vpn, but you wouldn't join two independent customers on either side of the vpn to area 0 in any case. The question as it stands doesn't make sense.