BGP next hop behavior

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BGP next hop behavior: – BGP is an AS by AS routing protocol.

By default, BGP only change the next hop behavior, when it changes the AS number.

  • When EBGP neighbor send an update to another EBGP neighbor, change the next hop behavior.
  • When IBGP neighbor send an update to another IBGP neighbor the next hop behavior will remain same.

If next hop behavior will remain same, then problem might be reachability issue.

BGP next hop behavior Lab: –

On Router R1,R2, & R3 basic ip addressing, and BGP are configured as per diagram.

R1#

R1#show running-config | section bgp
router bgp 100
no synchronization
bgp log-neighbor-changes
network 12.12.12.0 mask 255.255.255.0
network 192.168.1.0
neighbor 12.12.12.2 remote-as 100
no auto-summary

R2#

R2#show running-config | section bgp
router bgp 100
no synchronization
bgp log-neighbor-changes
network 12.12.12.0 mask 255.255.255.0
network 23.23.23.0 mask 255.255.255.0
network 192.168.2.0
neighbor 12.12.12.1 remote-as 100
neighbor 23.23.23.3 remote-as 200
no auto-summary

R3#

R3#show running-config | section bgp
router bgp 200
no synchronization
bgp log-neighbor-changes
network 23.23.23.0 mask 255.255.255.0
network 192.168.3.0
neighbor 23.23.23.2 remote-as 100
no auto-summary

Lets verify the next hop behavior:-

See, On Router R1, for 192.168.3.0/24 network, next hop ip address is 23.23.23.3

See above topology, Router R3 advertise network 192.168.3.0/24 with next hop address 23.23.23.3 to R2.

Router R2 will receive that network and advertise to the Router R1, without changing the next hop address, the next hop address is 23.23.23.3, for same.

Let me change the next hop behavior on Router R2, Majorly its done on border router, pointing to its internal neighbor.

Now lets verify again, next hop behavior.

See above, for 192.168.3.0/24 network, next hop address is 12.12.12.2

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