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Beginner

ospf path selection!!

Given the below topology, where R4 mutually redistributes between EIGRP and OSPF, which path(s) will R1 choose to reach the network 5.5.5.5/32, and why?

  • What will R2′s path selection to 5.5.5.5/32 be, and why?
  • What will R3′s path selection to 5.5.5.5/32 be, and why?
  • Assume R3′s link to R1 is lost.  Does this affect R1′s path selection to 5.5.5.5/32? If so, how?

Please give answer with explanation in detail.

Thank You.

1 ACCEPTED SOLUTION

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VIP Mentor

to figure this out you need

to figure this out you need to go by route path selection rules that go in the process below and also how ospf choses best path selection

standard rules  

  1. Prefix Length - The longest-matching route is preferred first. Prefix length trumps all other route attributes.
  2. Administrative Distance - In the event there are multiple routes to a destination with the same prefix length, the route learned by the protocol with the lowest administrative distance is preferred.
  3. Metric - In the event there are multiple routes learned by the same protocol with same prefix length, the route with the lowest metric is preferred. (If two or more of these routes have equal metrics, load balancing across them may occur.)

ospf path selection rules

Intra-Area (O)
Inter-Area (O IA)
External Type 1 (E1)
External Type 2 (E2)
NSSA Type 1 (N1)
NSSA Type 2 (N2)

One of the sides Is NSSA which will make it least preferred after the path selection rules , AD is the same , prefix length should be the same  , metric cost looks to be the same , so it comes down to ospf path selection , NSSA is least preferred so I would think R1 would take r3 to r4 as its E2

View solution in original post

7 REPLIES 7
Highlighted
VIP Mentor

to figure this out you need

to figure this out you need to go by route path selection rules that go in the process below and also how ospf choses best path selection

standard rules  

  1. Prefix Length - The longest-matching route is preferred first. Prefix length trumps all other route attributes.
  2. Administrative Distance - In the event there are multiple routes to a destination with the same prefix length, the route learned by the protocol with the lowest administrative distance is preferred.
  3. Metric - In the event there are multiple routes learned by the same protocol with same prefix length, the route with the lowest metric is preferred. (If two or more of these routes have equal metrics, load balancing across them may occur.)

ospf path selection rules

Intra-Area (O)
Inter-Area (O IA)
External Type 1 (E1)
External Type 2 (E2)
NSSA Type 1 (N1)
NSSA Type 2 (N2)

One of the sides Is NSSA which will make it least preferred after the path selection rules , AD is the same , prefix length should be the same  , metric cost looks to be the same , so it comes down to ospf path selection , NSSA is least preferred so I would think R1 would take r3 to r4 as its E2

View solution in original post

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Re: to figure this out you need

Since Cisco IOS release 15.1(2)S, Cisco uses the OSPF path selection order is O > O IA > N1 > E1 > N2 > E2

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Beginner

@Peter Paluch  , @Richard

paluchpeter  , rburts - could you please help me out in this question.

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Engager

R4 originates

R4 originates

  • a Type-7 LSA into (NSS)Area 2 (only) and
  • a Type-5 which is flooded unchanged in Areas 1 and 0.

From RFC 3101 2.4 (Originating Type-7 LSAs):

"When an NSSA border router originates both a Type-5 LSA and a Type-7 LSA for the same network, then the P-bit must be clear in the Type-7 LSA so that it isn't translated into a Type-5 LSA by another NSSA border router."

So the routers don't really have a choice. R2 has only a Type-7 LSA for the external prefix [*] and R1/R3 have only one Type-5 LSA because R2 must not translate the Type-7 into a Type-5 LSA.

Assume R3′s link to R1 is lost.  Does this affect R1′s path selection to 5.5.5.5/32? If so, how?

R1 will lose its only path to the external prefix 5.5.5.5/32.

HTH
Rolf


[*] EDIT:

R2 also has an interface in Area 0, so it receives the Type-5 LSA from R1 as well. But the path to the ASBR through Area 0 would be inter-area, so R2 will prefer the (non-backbone) intra-area path through Area 2.

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Beginner

Re: R4 originates

this is the right answer! five starts

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Engager

Would have been nice to tell

Would have been nice to tell us where this comes from:

http://blog.ine.com/2015/05/10/ospf-path-selection-challenge/

They also provide the solution.

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Beginner

I was not aware about that.

I was not aware about that.

My friend  asked me this question..

Thank you so much for your responses.