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ibgp-->ebgp

Experts need your assistance

R12----iBGP--->R5---eBGP----->R4-----eBGP------->R2

i have one router advertised in iBGP R12, that route is injecting into ebgp router R2, please help me to understand why this route injecting into R2. details are below..!!

--------------

R12

router ospf 1

log-adjacency-changes

network 12.12.12.12 0.0.0.0 area 0

router bgp 65000

no synchronization

bgp log-neighbor-changes

bgp suppress-inactive

network 12.12.12.12 mask 255.255.255.255

neighbor 5.5.5.5 remote-as 65000

neighbor 5.5.5.5 update-source Loopback1

neighbor 5.5.5.5 next-hop-self

no auto-summary

--------------------------

R5#

router bgp 65000

no synchronization

bgp log-neighbor-changes

neighbor 12.12.12.12 remote-as 65000

neighbor 12.12.12.12 update-source Loopback1

neighbor 12.12.12.12 next-hop-self

neighbor 192.168.1.4 remote-as 65100

no auto-summary

------------------------------

R4

router bgp 65100

no synchronization

bgp log-neighbor-changes

neighbor 192.168.1.5 remote-as 65000

neighbor 200.200.1.2 remote-as 65200

no auto-summary

-----------------------------------

R2#

BGP table version is 6, local router ID is 200.200.1.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 12.12.12.12/32   200.200.1.4                            0 65100 65000 i

*> 160.160.1.0/24   200.200.1.4              0             0 65100 i

*> 160.160.2.0/24   200.200.1.4              0             0 65100 i

R2#

thanks in advance..!!

6 REPLIES

Re: ibgp-->ebgp

Hi,

eBGP connections don't use the same rules as ibgp. When using ibgp, the ibgp neighbor cannot advertise an ibgp learned route to another ibgp neighbor. This is why it requires a full mesh or route reflectors in order to workaround those rules. Your diagram, however, is an ibgp neighbor advertising to another ibgp neighbor, which then CAN advertise to an ebgp neighbor. You can avoid this if you want by setting the community local-as or no-export, or configuring an outbound route-map on R5 that matches the route and denies it from being advertised to your ebgp peer.

HTH,
John

*** Please rate all useful posts ***

HTH, John *** Please rate all useful posts ***
New Member

Re: ibgp-->ebgp

Hello,

Can u plz share configuration of R2....

Thanks

New Member

ibgp-->ebgp

Appreciate for your response, here is the R2 config. thanks

R2#sh run | se router

router bgp 65200

no synchronization

bgp log-neighbor-changes

neighbor 200.200.1.1 remote-as 65000

neighbor 200.200.1.1 next-hop-self

neighbor 200.200.1.4 remote-as 65100

no auto-summary

R2#sh ip rou 12.12.12.12

Routing entry for 12.12.12.12/32

  Known via "bgp 65200", distance 20, metric 0

  Tag 65100, type external

  Last update from 200.200.1.4 00:00:21 ago

  Routing Descriptor Blocks:

  * 200.200.1.4, from 200.200.1.4, 00:00:21 ago

      Route metric is 0, traffic share count is 1

      AS Hops 2

      Route tag 65100

R2#

Purple

ibgp-->ebgp

Hi,

As John already explained this is normal behaviour and you'll have to filter this route on R5 if you don't want it to go to R2( using a route-map or a prefix-list or distribute-list outbound) or you can use community(setting it to local AS or no-export) when advertsing it to R5 with the network command using a route-map and sending the community to R5.

Regards

Alain

Don't forget to rate helpful posts.

Don't forget to rate helpful posts.
New Member

ibgp-->ebgp

Thanks john for clearifiying me,

it means eBGP doesn't care, it'll take any route from anyone unless we specifically configured to ignore routes.. Thats why i am seeing routes in R2 which is advertised in R12 iBGP. please let me know if my understanding is correct.

Thanks in advance.

Purple

ibgp-->ebgp

Hi,

BGP will  advertise best routes and eBGP peer will accept it as long as it doesn't see its own AS in the AS_PATH.

Regards

Alain

Don't forget to rate helpful posts.

Don't forget to rate helpful posts.
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