Hi
eBGP multihop is used when you are building an eBGP peering with other IP address different than the IP addresses connected directly.
It is useful when you are creating a BGP peering using loopbacks known via a NLRI (through a interior gateway protocol like RIP, EIGRP, IS-IS, OSPF).
So the hop value represents the hops to reach that IP, imagine the attached scenario:
CONFIGURATION:
ROUTER 1
router ospf 1
network 10.12.0.1 0.0.0.0 area 0
network 1.1.1.1 0.0.0.0 area 0
router bgp 100
no sync
no auto-summary
neighbor 1.1.1.1 remote 200
neighbor 1.1.1.1 ebgp-multihop 2
neighbor 1.1.1.1 update-source loobpack0
ROUTER 2
router ospf 1
network 10.12.0.2 0.0.0.0 area 0
network 2.2.2.2 0.0.0.0 area 0
router bgp 200
no sync
no auto-summary
neighbor 2.2.2.2 remote 100
neighbor 2.2.2.2 ebgp-multihop 2
neighbor 2.2.2.2 update-source loobpack0
The IP address used to build the eBGP peering is known via OSPF ( NLRI ) so you need to specify the hops to reach the destination.
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:-)
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