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Vito_Corleone
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OSPF Quiz

Sun Oct 23, 2011 3:39 pm

Image

Assume interfaces have correct bandwidth statements and no cost commands have been added. R1 and R2 are redistributing the 192.168.1.0/24 prefix as E2 with a cost of 100.

Which path does R4 take to the 192.168.1.0/24 network? Does it load balance? Explain.

Chris, Jason, James and Darren, please PM the answer. I don't want you guys spoiling it.:P
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Re: OSPF Quiz

Sun Oct 23, 2011 4:10 pm

Huh. My initial answer to this was wrong. Glad I labbed it before I answered. And thanks... My vRack session is borked right now so this gave me something to do while support tries to fix it.

chrismarget
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Re: OSPF Quiz

Sun Oct 23, 2011 4:38 pm

Extra Credit!

Make R4 an ABR:

R1-R4 link: area0
R4-R3 link: area1
R3-R2 link: area1

Then:
router ospf <process-id>
no compatible rfc1583

What happens now, and why?

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Vito_Corleone
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Re: OSPF Quiz

Sun Oct 23, 2011 4:41 pm

slow your roll

:squint:
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mellowd
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Re: OSPF Quiz

Sun Oct 23, 2011 4:45 pm

chrismarget wrote:Extra Credit!

Make R4 an ABR:

R1-R4 link: area0
R4-R3 link: area1
R3-R2 link: area1

Then:
router ospf <process-id>
no compatible rfc1583

What happens now, and why?


Without looking at the actual rfcs, would R4 still not prefer the Area 0 link as that is the backbone area? I think this has something to do with routing loops from getting the same external LSA from 2 sources?

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Vito_Corleone
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Re: OSPF Quiz

Sun Oct 23, 2011 4:46 pm

Keep answering people, but here's another question. What happens if we change it to:

Image

Everything is the same except R2 is now redistributing as E1. Which path and why?
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ZeroZeroFourteen
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Re: OSPF Quiz

Sun Oct 23, 2011 4:51 pm

You are right. Editing for others.

-Vito

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Re: OSPF Quiz

Sun Oct 23, 2011 4:58 pm

In the initial question, I believe R4 would chose R1 as the path, and won't load balance. Although with E2, the metric doesn't increment, the "forward metric" of R1 would be closer, and thus R4 would choose it over R2.

2nd question, still no load balancing as the metric of E1 increments.

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Re: OSPF Quiz

Sun Oct 23, 2011 5:17 pm

chrismarget wrote:Extra Credit!

Make R4 an ABR:

R1-R4 link: area0
R4-R3 link: area1
R3-R2 link: area1

Then:
router ospf <process-id>
no compatible rfc1583

What happens now, and why?

Damn you for making me read an RFC...

RFC 2328 page 173/174 wrote: (c) If the new AS external path is still indistinguishable
from the current paths in the N's routing table entry,
and RFC1583Compatibility is set to "disabled", select
the preferred paths based on the intra-AS paths to the
ASBR/forwarding addresses, as specified in Section
16.4.1.

(d) If the new AS external path is still indistinguishable
from the current paths in the N's routing table entry,
select the preferred path based on a least cost
comparison. Type 1 external paths are compared by
looking at the sum of the distance to the forwarding
address and the advertised type 1 metric (X+Y). Type 2
external paths advertising equal type 2 metrics are
compared by looking at the distance to the forwarding
addresses.


Vito_Corleone wrote:Everything is the same except R2 is now redistributing as E1. Which path and why?

That one's easy :P

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Re: OSPF Quiz

Sun Oct 23, 2011 5:23 pm

Infinite wrote:
chrismarget wrote:Extra Credit!

Make R4 an ABR:

R1-R4 link: area0
R4-R3 link: area1
R3-R2 link: area1

Then:
router ospf <process-id>
no compatible rfc1583

What happens now, and why?

Damn you for making me read an RFC...

I believe this is the applicable paragraph:

RFC 1583 wrote: (3) The inter-area routes are calculated, through examination of
summary link advertisements. If the router is attached to
multiple areas (i.e., it is an area border router), only
backbone summary link advertisements are examined.
Well, the command is: no compatible rfc1583...
:-)
The relevant details are in RFC 2178 section 16.4.1

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Re: OSPF Quiz

Sun Oct 23, 2011 5:28 pm

Yeh... Saw I made an error there. I changed my post.

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Re: OSPF Quiz

Sun Oct 23, 2011 6:42 pm

For question 1: Goes through R1. Metric it gets is the same from both R1 and R2, but the internal path in the area is shorter through R1.

For question 2: I believe it will go through R3-R2 as an E1 route would always be preferred over an E2, even if the E1 has a higher metric. Not 100% sure on that one ;D

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OSPF Quiz

Mon Oct 24, 2011 12:30 am

Vito_Corleone wrote:Image

Assume interfaces have correct bandwidth statements and no cost commands have been added. R1 and R2 are redistributing the 192.168.1.0/24 prefix as E2 with a cost of 100.

Which path does R4 take to the 192.168.1.0/24 network? Does it load balance? Explain.

Chris, Jason, James and Darren, please PM the answer. I don't want you guys spoiling it.:P


Since the type 2 routes won't increment the metric then R4 would see both paths as equal cost and load balance across them.

That's my shot at this.

Dave


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sgtcasey
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OSPF Quiz

Mon Oct 24, 2011 12:37 am

Vito_Corleone wrote:Keep answering people, but here's another question. What happens if we change it to:

Image

Everything is the same except R2 is now redistributing as E1. Which path and why?


The path from R4 will be through R3 and R2 since the type 1 route is preferred over the type 2 route no matter the cost.

Dave


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Re: OSPF Quiz

Mon Oct 24, 2011 12:58 am

a.)path R4->>R1 is preferred. Even though the metrics tie, R4 chooses the best route based on the lowest cost to the ASBR(s) that advertises the metric
b.)path R4->>R3->>R2 E1 is preferred over E2 route regardless of the metric
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Re: OSPF Quiz

Mon Oct 24, 2011 7:08 am

moods wrote:a.)path R4->>R1 is preferred. Even though the metrics tie, R4 chooses the best route based on the lowest cost to the ASBR(s) that advertises the metric
b.)path R4->>R3->>R2 E1 is preferred over E2 route regardless of the metric



This.

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Re: OSPF Quiz

Mon Oct 24, 2011 8:41 am

I think it should load balance
by deafault OSPF doesn't increment the cost of a imported path (metric type 2 I think) if you want it to increment you have to set it to type 1 (could have those backwards though)
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Re: OSPF Quiz

Mon Oct 24, 2011 8:46 am

dlots wrote:I think it should load balance
by deafault OSPF doesn't increment the cost of a imported path (metric type 2 I think) if you want it to increment you have to set it to type 1 (could have those backwards though)


It won't increment, but R4 still makes a decision. What is that decision?

chrismarget
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Re: OSPF Quiz

Mon Oct 24, 2011 9:03 am

I think it should load balance
If R4 load balances, won't R3 also load balance? They're in the same situation: Each has two hops to an ABR going one way, and one hop to the ABR going the other way.

If they're both load balancing, that means that each router could be using the other for transit to 192.168.1.0/24.

That sounds like one looong page of traceroute output for this four router network! :-)

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Re: OSPF Quiz

Mon Oct 24, 2011 10:59 am

Ah, nothing like rooting through an RFC on a Monday morning to overturn a misconception.

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