BGP Backdoor

In the above network, we are running following protocol between routers

eBGP between RTA and RTC

eBGP between RTB and RTC

IGP protocol between RTA and RTB

What is the distance eBGP updates

20

Less that IGP distances

What are the default IGP distances

RIP  120

IGRP 100

EIGRP 90

OSPF 110

How router RTA receives updates for 160.10.0.0 network

RTA receives update for 160.10.0.0 network via two routing protocol

• eBGP with a distance of 20
• IGP with a distance of more than 20

What are the default BGP distance values

• External distance 20
• Internal distance 200
• Local distance 200

What is the command to change the default distance of BGP

Distance bgp external-distance internal-distance local-distance

Which route RTA will pick

eBGP route via RTC because of shorter distance

How can we allow router RTA to learn route 160.10.0.0 via IGP

There are two options

• Change external eBGP distance or IGP distance
• Use BGP backdoor

What is bgp backdoor

Makes IGP route the preferred route

What would be the configuration on RTA router to use bgp backdoor

RTA#

router eigrp 10

network 150.10.0.0

router bgp 100

neighbor 2.2.2.1 remote-as 300

network 160.10.0.0 backdoor

• RTA learns 160.10.0.0 from RTB via EIGRP with distance 90.
• RTA also learns the address from RTC via eBGP with distance 20.
• eBGP is the preference
• Because of the network backdoor command, EIGRP is the preference

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