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Community Member

A router should not see this default route concept ?

I have got Router3 connected between two different domains OSPF and EIGRB (Router3 is ASBR), if the requirement like this:

Configure your network so that Router3 advertises a default route to the rest of your OSPF enabled devices. However in order to help prevent traffic black holes, it has been suggested that Router3 should not see this default route. Instead Router3 must have a longer match for all destination.

What did he meant by : “Router3 should not see this default route” and “Instead Router3 must have a longer match for all destination.” ?

9 REPLIES

Re: A router should not see this default route concept ?

HI

if u want the rest of your ospf network to see a default route out configure all the routers in a ospf stub area which prevents the flow of LSA 3 and 4 in those areas.and you can implement route summarization on your ASBR to go out for other destinations.Router3 is going to advertise the default route in your ospf domain.

Hope this helps

Mahmood

Community Member

Re: A router should not see this default route concept ?

((if u want the rest of your ospf network to see a default route out configure all the routers in a ospf stub area which prevents the flow of LSA 3 and 4 in those areas))

1- You meant that I have to configure the OSPF area as stub area to prevent type 3 and type 4 ? did not you ?,,,,

1- Why do I need to do that ?

2- There is another problem raised, stub area or totally stub area can not be connected to ASBR,,,Am I right,,,,because ASBR means External route Type 5,,,and both areas do not accept type 5 LSAs ?

I will respond to the rest of thread when I get your reply for the two point above.

Thanks

Community Member

Re: A router should not see this default route concept ?

Hi,

Another solution is to issue the default information-originate always command on the router under the ospf process. This will force the router to inject a default into the ospf domain but itself will not have a dafault route. In this scenario you do not need to redistribute eigrp into ospf, instead ospf gets a default. As router3 has routes to both ospf and eigrp networks it can use the longest match prefix to the destination.

HTH

E.

Community Member

Re: A router should not see this default route concept ?

((This will force the router to inject a default into the ospf domain but itself will not have a dafault route))

1- Will that value for default be 0.0.0.0/0 ?

2- What do you mean by itself will not have a default route ?

(( it can use the longest match prefix to the destination.))

I did not get what you meant here ?could you please explain more

Community Member

Re: A router should not see this default route concept ?

yes the default is 0.0.0.0/0

The router will issue a default into the ospf domain but when you check r3's routing table there will not be a default route...This instructs all other routers that have no specific route to a destination network, to send their traffic to r3.

When a router makes a routing decision it looks for the routing entry that best matches the destiantion. For example if it needs to route a packet to 10.1.2.0 and has the following entries in the routing table:

1. 10.0.0.0/16 next-hop A

2. 10.1.0.0/16 next-hop B

3. 0.0.0.0 next-hop C

It will chose B, because it is a longer match to the destination.

for a destination net 192.168.0.0 it would use the default

Community Member

Re: A router should not see this default route concept ?

((This instructs all other routers that have no specific route to a destination network, to send their traffic to r3)).

Now if there no default route in the routing table of R3,,,,Can all other routers that have no specific route to a destination network, to send their traffic to R3 ?

Community Member

Re: A router should not see this default route concept ?

yes, because the other routers in the ospf domain all have a default route pointing to r3

Community Member

Re: A router should not see this default route concept ?

That is fine , now can R3 reach the unknow destination if the default route has not been installed in his routing table ?

Community Member

Re: A router should not see this default route concept ?

The answer is in your original question...-)

If taking a Cisco test, always read the question very carefully. The wording is very important.

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