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New Member

questions about ospf and nssa

question 1:

Is overflow supported by cisco's ospf implememtation?

question 2:

Please refer to the following topology:

|area 0

__|__10.1.1.1/24

|_RT1_|

|

_________area 0_________

| |

__|__ __|__

|_RT2_| |_RT3_|

| |area 10(nssa)

| __|__

area 20 |_RT4_|

| _____ |

|___|_RT5_|__|area 10(nssa)

RT1 is an ASBR,it is importing some static routes into ospf.The outing interface of these static routes is 10.1.1.1/24,which is in ospf area 0.Area 10 is a nssa,and area 20 is a type-5 capable area.Cost of all links is 1 except RT2's link to area 0,its cost is 10.

RT1 originates type 5 lsas for these static routes with forwarding address set to 10.1.1.1,these type 5 lsas are flooded to RT5.RT5 is calculating AS external routes.

RT5 concludes that packets for these as external destinations should be forwarded to 10.1.1.1,RT5 looks up ospf routing table for 10.1.1.1,it concludes that the best route to 10.1.1.1 is via RT4,with a metric 4.Route via RT2 to 10.1.1.1 has a metric 12.But RT5 notes that these lsas are type 5 and RT4 is in nssa,so RT5 believes that 10.1.1.1 is unreachable via type 5 capable area.For this reason,RT5 WILL NOT find routes to these as external destination,though route via RT2 is available.

If area 10 is a stub area,RT5 will forward these as external packets to RT4,but RT4 has no information about these destinations(except default route),because RT4 is in a stub area.

  • Other Network Infrastructure Subjects
3 REPLIES
Gold

Re: questions about ospf and nssa

First question--what do you mean by overload? Do you mean what happens when the ospf database is "too full," and the router cannot route? OSPF doesn't have the concept of overload, just the concept of max-metric, which means the router can set its metric to max whenever it doesn't want to transit traffic.

This is called a "stub router advertisement:"

http://www.cisco.com/en/US/products/sw/iosswrel/ps5012/products_feature_guide09186a0080134a44.html

For the second question, this is normal ospf behaviour. What you have is an external learned in one area, and you're trying to route to the next hop through the other area. Since there's no type 4 generated into the second area, where the traffic is flowing, there's no way to resolve the border router or the next hop correctly.

Russ.W

New Member

Re: questions about ospf and nssa

Thanks for your reply!Thanks!

About my first question,"overflow" comes from rfc1765,it means "some routers may be unable to keep the entire database due to resource shortages".I can't find overflow command in cisco's CLI,so I ask this question.

The second question,I surely learned external info via one area,and trying to route to the next hop via another area.But there is no reason that both of these areas must be the same.In fact,these destinations are certainly reachable.Can't ospf deal this question correctly?

Gold

Re: questions about ospf and nssa

Oh, you mean setting the metric to max, so the router is no longer a transit. :-)

http://www.cisco.com/en/US/products/sw/iosswrel/ps5012/products_feature_guide09186a0080134a44.html

I think, is what you are after.

On the second question, nope, because you are removing the information the router needs to route to the next hop through the stubby area.

Russ.W

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