Quick question on iBGP between internal border router

Answered Question
Mar 5th, 2010
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Hi folks,


Imagine R2 and R3 are my border routers. Both have eBGP sessions with R4 (AS 200, ISP).


I am doing redistribution from BGP into OSPF on R2 and R3.


Detailed diagram is attached.


Question:

What is the benefit or need to have iBGP between R2 and R3 again?

From looking at the BGP design documentation, I believe that is recommended, but if I am doing redistribution BGP into OSPF, isn't that enough to let my internal routers find R3 in case R2 goes down for example?

Correct Answer by Giuseppe Larosa about 7 years 2 months ago

Hello Marlon,

in real world edge routers R2 and R3 will peer with two different ISP routers for redundancy and fault tolerance.


iBGP session is recommended and necessary if the edge routers are simply generating a default route in OSPF telling they are exit points from OSPF domain.


Again in real world this choice of simply injecting a default route usually pays avoiding to inject BGP routes into OSPFdatabase.


OSPF provides useful tools for advertising a default route only under specific conditions by using


router ospf 10

default-information originate route-map check_BGP type 1


where you can check that a default route is sent to you on an eBGP session.


in any case either for the default route or for injecting BGP routes you should use O E1 routes because you have two exit points (ASBR)


going back to your question, if you have the iBGP session you should check BGP next-hop


access-list 11 permit host ebgp-address

route-map BGPtoOSPF permit 10

match ip next-hop 11

! other matches here


this to avoid that after one eBGP session fails the router tries to redistribute iBGP routes into OSPF


note: this is not necessary unless you have enabled redistribute-internal under router bgp process




Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Fri, 03/05/2010 - 11:21
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Hello Marlon,

in real world edge routers R2 and R3 will peer with two different ISP routers for redundancy and fault tolerance.


iBGP session is recommended and necessary if the edge routers are simply generating a default route in OSPF telling they are exit points from OSPF domain.


Again in real world this choice of simply injecting a default route usually pays avoiding to inject BGP routes into OSPFdatabase.


OSPF provides useful tools for advertising a default route only under specific conditions by using


router ospf 10

default-information originate route-map check_BGP type 1


where you can check that a default route is sent to you on an eBGP session.


in any case either for the default route or for injecting BGP routes you should use O E1 routes because you have two exit points (ASBR)


going back to your question, if you have the iBGP session you should check BGP next-hop


access-list 11 permit host ebgp-address

route-map BGPtoOSPF permit 10

match ip next-hop 11

! other matches here


this to avoid that after one eBGP session fails the router tries to redistribute iBGP routes into OSPF


note: this is not necessary unless you have enabled redistribute-internal under router bgp process




Hope to help

Giuseppe

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