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New Member

BGP issue

Hi expert,

R27,R28,R5,R6 form ibgp together

R27,R28,R5,R6,Sw27,Sw28,Sw5,Sw6 form ospf together

R27, R5, R6 can learn 1.1.1.1 via ebgp

If I config BGP redistribute ospf in R5,R6, then routing table of R27 to

ip address 1.1.1.1 will be learned from ospf R6. Why ospf (AD110) will win ebgp (AD20) in R27 ?

Thanks your help!

3 REPLIES
Hall of Fame Super Silver

Re: BGP issue

Hello Jack,

>> ip address 1.1.1.1 will be learned from ospf R6. Why ospf (AD110) will win ebgp (AD20) in R27 ?

because R27 is learning the BGP advertisement over the iBGP session with RTE6-1 and RTE5_1.

As a result of this:

in sh ip bgp you should see on R27 something like:

> i 1.1.1.1 RTE6_1

*i 1.1.1.1 RTE5_1

the i at the beginning of the line means learned via an iBGP session.

When BGP proposes its prefix it is classified as iBGP with AD 200 on R27.

the end result is that OSPF external route is installed.

only sessions directly configured on R27 are considered true eBGP sessions in your case the one with AS 789.

Hope to help

Giuseppe

New Member

Re: BGP issue

Hi Gouseppe,

Thanks for your explanation. But I still not understand. May I ask more question?

Why R27 choose the best path from R6 about route 1.1.1.1 In R6 router there is no local preference set on the link (connect to AS123) only weight is set. Therefore, I expect R27 will not affect by other ibgp router 's attribute.

As a result R27 still to go 1.1.1.1 via AS789

Hall of Fame Super Silver

Re: BGP issue

Hello Jack,

you should post

sh ip bgp 1.1.1.1

to see all possible paths on R27 and why that path is chosen as best path.

Also you can use the BGP hierarchy to help understand results

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

Hope to help

Giuseppe

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