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OSPF route vs eBGP route

lvvp
Beginner
Beginner
 
 

1.png

Refer to the exhibit. An engineer is bringing up a new circuit to the MPLS provider on the Gi0/1 interface of Router 1. The new circuit uses eBGP and learns the route to VLAN25 from the BGP path.
What is the expected behavior for the traffic flow for route 10.10.13.0/25?

  • C. Route 10.10.13.0/25 is updated in the routing table as being learned from interface Gi0/1.
  • D. Route 10.10.13.0/25 learned via the Gi0/0 interface remains in the routing table.


I'm stuck in this question. The answer on this question is D which means the OSPF route remains. However, in theory and in packet tracer, the route should be updated to BGP since it has a lower administrative distance. Did I understand the question correctly?

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1 Accepted Solution

Accepted Solutions

Giuseppe Larosa
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

Hello @lvvp ,

the correct answer is

C: Route 10.10.13.0/25 is updated in the routing table as being learned from interface Gi0/1.

As eBGP route is preferred over OSPF route. ( AD 20 < AD 110)

Only way to avoid this is to use a  command under

router bgp  <yourASN>

network 10.10.13.0 255.255.255.128 backdoor

 

on the router that receives the BGP prefix.

when this is used the AD is increased to 200 for the received eBGP prefix like it was an iBGP session.

But this is above CCNA level

 

be prepared to the fact that some questions are unclear and the "correct" answers  in exam test may be wrong as you have also tested in lab with packet tracer.

 

Hope to help

Giuseppe

 

 

 

View solution in original post

8 Replies 8

Georg Pauwen
VIP Master VIP Master
VIP Master

Hello,

 

in theory, the eBGP route would take precedence (20 vs 110 admin distance). However, it looks like R1 and R2 are directly connected via OSPF ?

The question is actually not clear about that. I've assumed that Site A and Site B are directly connected via OSPF and both of them are also directly connected via eBGP.

paul driver
VIP Expert VIP Expert
VIP Expert

Hello
i am assuming your are redistributing ospf into bgp at both rtrs? 

if so then by default the same prefix that is a redistributed igp route and an a ebgp route received on the same rtr The rtr will choose the redistributed igp route due to the fact in it bgp rib table that redistributed route has a default weight of 32768 and a ebgp route from the mpls would have a weight of 0 

Bgp path selection weight takes precedence.


To rectify this add a default higher weight than 32768 on the peering facing the mpls for any received routes.

example:
neighbour <mpls> weight 40000


Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

The question was not stating that clearly, however, redistributing the OSPF route into BGP makes a lot of sense. I've also tried it in Packet Tracer and no OSPF route remained. All of the OSPF routes became BGP routes.

 

4.png

So to confirm, the answer should be C, right?

  • C. Route 10.10.13.0/25 is updated in the routing table as being learned from interface Gi0/1.

P.S. This question is for CCNA and I don't see BGP in the current blueprint of CCNA 200-301.

Giuseppe Larosa
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

Hello @lvvp ,

the correct answer is

C: Route 10.10.13.0/25 is updated in the routing table as being learned from interface Gi0/1.

As eBGP route is preferred over OSPF route. ( AD 20 < AD 110)

Only way to avoid this is to use a  command under

router bgp  <yourASN>

network 10.10.13.0 255.255.255.128 backdoor

 

on the router that receives the BGP prefix.

when this is used the AD is increased to 200 for the received eBGP prefix like it was an iBGP session.

But this is above CCNA level

 

be prepared to the fact that some questions are unclear and the "correct" answers  in exam test may be wrong as you have also tested in lab with packet tracer.

 

Hope to help

Giuseppe

 

 

 

Thank you @Giuseppe Larosa for confirming my doubts.

Hello,

 

as I understood your original post, the answer was D and you were looking for an explanation why ? 

Yes Georg. It was unclear to me why the route (OSPF) remains unchanged. Looks like the correct answer actually is C.

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