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OSPF Path Selection in Juniper

[ Edited ]
‎03-12-2020 01:37 AM

Hi there!

 

I know that OSPF internal route has a preference of 10 and an external route of 150 in Juniper.

However, I am unable to find the document stating the path selection priority between Intra-area route and Inter-area route.

 

Could you please tell me the page with this content?

Or Could you please tell me more about this?

Thank you for reading me.

 

Best Regards,

Seunghyun Lee

3 REPLIES 3
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Solution
Accepted by topic author seunghyun lee
‎03-12-2020 09:46 PM

Re: OSPF Path Selection in Juniper

[ Edited ]
‎03-12-2020 01:50 AM

Hi  seunghyun lee,

 

I hope you are doing great!

 

I believe this DOC will clear all the questions you have, if not please let me know :

 

https://www.juniper.net/documentation/en_US/junos/topics/topic-map/ospf-overview.html

 

A) When there are multiple routes available to the same network with different route types, routers use this order of preference (from highest to lowest): 1. Intra-area routes. 2. Inter-area routes. 3.External Type-1 routes. 4. External Type-2 routes.
B) If there are multiple routes to a network with the same route type, the OSPF metric calculated as cost based on the bandwidth is used for selecting the best route. The route with the lowest value for cost is chosen as the best route.
C) If there are multiple routes to a network with the same route type and cost, it chooses all the routes to be installed in the routing table, and the router does equal cost load balancing across multiple paths.

 

If this solves your problem, please mark this post as "Accepted Solution" Pura vida 😄

Warm regards,

Pablo Restrepo -

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Re: OSPF Path Selection in Juniper

‎03-12-2020 11:04 AM

Hi Seunghyun Lee,

 

This is always a confusing topic at some point when we are learning it. I have this very useful link with some examples and how cost will be taken into account when selecting routes, also it is included the rules for inter-area (different areas) or intra-area (same area) route selection, it basically depends on where the destination is located and the type of LSA for that prefix.

 

> intra area - a route to a subnet or network inside the same area as the router. LSA type 1 and 2 fall in here / router and network LSAs

> inter area - a route to a subnet in an area of which the router is not a part. LSA tpe 3 falls in here / summary LSAs

 

I hope this link helps you understanding more this behavior:

 

OSPF inter-area and intra-area routing rules

https://cciethebeginning.wordpress.com/2015/01/21/ospf-inter-area-and-intra-area-routing-rules/

 

it is not possible to select Inter Area route over Intra Area route or External routes over inter/intra area route for an OSPF process.

 

RFC2328, Section 11 explains the order of preference which is:

 

1) Intra Area

2) Inter Area

3) External routes E1

4) External routes E2

 

https://tools.ietf.org/html/rfc2328

 

Hope it helps, and good luck comprehending the material, feel free to ask more if you need to, I will try to help as needed.

 

Cheers,

 

Benjamin

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Re: OSPF Path Selection in Juniper

‎03-12-2020 09:53 PM

Thank both of you for all your help on this topic.

My curiosity has been solved.

 

I am sorry that I have to choose only one answer.

I would like to thank you again for your great answers.

 

Best Regards,

Seunghyun Lee

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