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OSPF LSA5 /external route manupulation

vinodjad1234
Level 2
Level 2

       Hi Experts ,

I have created one lab to understand LSA 5 & LSA 4 in better manner.....

LSA4 & LSA5.jpg

If you look at my topology , i have configured ospf and eigrp redistribution on R2 and injected LSA 5 into Area 1 .....

Now , if i want to reach R6 routes  from R3 router , it goes via ABR router R1 > R2 > R6 however i want to manupulate route such a way that , To reach R6 from R3 , it should go via R4 > R6

But i am unable to find an option to do this ....

Even I tried to chage the cost value on R3 / Ethernet 0/1 interface to 100 , it goes via R1 only ......

Could you please suggest me on this .... i am working on this lab since long time without getting any soln.

It would be really helpful for me to understand this .......

1 Accepted Solution

Accepted Solutions

Peter Paluch
Cisco Employee
Cisco Employee

Hello,

Recall that in OSPF, inter-area traffic must go through the backbone. The connection between R3 and R4 is, according to OSPF rules, illegal - it connects two non-backbone areas together. As a result, OSPF will not consider this link in the path calculation. So in other words, without modifying your topology or configuring virtual links, it is not possible to accomplish your goal of routing through the R3-R4-R2-R6 path.

A possible solution would be to configure a virtual link between R1 and some other router, say, R4. However, this complicates things and does not really make the goal of understanding LSA-4 and LSA-5 simpler.

Perhaps the easiest modification of your network to see the working of LSA-4 and LSA-5 would be as follows:

Renumber the existing Area 25 to Area 0

Renumber the existing Area 0 to Area 2

Leave the existing Area 1 as is

Best regards,

Peter

View solution in original post

2 Replies 2

Peter Paluch
Cisco Employee
Cisco Employee

Hello,

Recall that in OSPF, inter-area traffic must go through the backbone. The connection between R3 and R4 is, according to OSPF rules, illegal - it connects two non-backbone areas together. As a result, OSPF will not consider this link in the path calculation. So in other words, without modifying your topology or configuring virtual links, it is not possible to accomplish your goal of routing through the R3-R4-R2-R6 path.

A possible solution would be to configure a virtual link between R1 and some other router, say, R4. However, this complicates things and does not really make the goal of understanding LSA-4 and LSA-5 simpler.

Perhaps the easiest modification of your network to see the working of LSA-4 and LSA-5 would be as follows:

Renumber the existing Area 25 to Area 0

Renumber the existing Area 0 to Area 2

Leave the existing Area 1 as is

Best regards,

Peter

Hi Peter ,

Its really very informative and satisfied with this answer ............

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