BGP - [Part 6] - NEXT HOP SELF

• Before BGP Next Hop Self, we should know the "what is the next hop exactly ?". Next hop is IP to reach the next Autonomous System. For Example check the below topology, if Router R4 will advertise any route to R3, then 192.167.24.1 will be next hop for Router R4.
• IBGP has an issue. When an update send within the same AS, then next hop will be same, sometimes it can cause reach-ability issue, we can change this behavior by using "Next-Hop-Self" command.
• Note that when an update send between different Autonomous System then next hop will be change.

Please check the below example, I will show you both situation.

1. When an update send within the same AS - Next hop will be same (We can change this behavior by using  "Next-Hop-Self" command).
2. When an update send between different AS - Next hop will be change.

So you can see the above topology, there are four routers

• R1 and R2 in AS100
• R3 in AS200 
• R4 in AS300
• We will advertise the network for point to point reach-ability of devices
• We will use "Next Hop Self" command to change next hop in IBGP

Without Next Hop Self

Now below are the configuration  - 

R1 Configuration -
interface FastEthernet0/0
 ip address 192.167.22.1 255.255.255.0
 duplex auto
 speed auto
!
router bgp 100
 no synchronization
 bgp log-neighbor-changes
 neighbor 192.167.22.2 remote-as 100

 no auto-summary

R2 Configuration -
interface FastEthernet0/0
 ip address 192.167.22.2 255.255.255.0
 duplex auto
 speed auto
!
interface FastEthernet1/0
 ip address 192.167.23.1 255.255.255.0
 duplex auto
 speed auto
!
router bgp 100
 no synchronization
 bgp log-neighbor-changes
 neighbor 192.167.22.1 remote-as 100
 neighbor 192.167.23.2 remote-as 200

 no auto-summary

R3 Configuration -
interface FastEthernet1/0
 ip address 192.167.23.2 255.255.255.0
 duplex auto
 speed auto
!
interface FastEthernet2/0
 ip address 192.167.24.1 255.255.255.0
 duplex auto
 speed auto
!
router bgp 200
 no synchronization
 bgp log-neighbor-changes
 neighbor 192.167.23.1 remote-as 100
 neighbor 192.167.24.2 remote-as 300

 no auto-summary

R4 Configuration -

interface Loopback1
 ip address 1.1.1.1 255.255.255.0
!
interface FastEthernet2/0
 ip address 192.167.24.2 255.255.255.0
 duplex auto
 speed auto
!
router bgp 300
 no synchronization
 bgp log-neighbor-changes
 network 1.1.1.0 mask 255.255.255.0
 neighbor 192.167.24.1 remote-as 200
 no auto-summary

Verification - 

R2 has installed 1.1.1.0 /24 in its BGP table and it is a valid route, the next hop is 192.167.23.2 .

Also R3 has installed 1.1.1.0 /24 in its BGP table and it is a valid route, the next hop is 192.167.24.2 .

R1 learns the prefix but it’s unable to install it in the routing table:

R1 does not know about 3.3.3.0/24, so it is impossible to install 3.3.3.0 /24 in the routing table.

So as per above output , we found below things -

1. There is no point to point reach-ability, we cannot ping 1.1.1.1 from R1 and R2 Router.
2. As per IBGP issue, in R1 and R2 same next hop is showing for network 1.1.1.0/24. R1 did not change the next hop.
3. But you can notice there is no any issue with EBGP next hop, R2 has next hop 192.167.23.2 for 1.1.1.0/24 and R3 has next hop 192.167.24.2 for 1.1.1.0/24. So issue only with IBGP .

So now we will advertise the network and will use "next-hop-self" command to improve above things in next part.

Click here for next part

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Anubhav Upadhyay

Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.

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