I have some confuse about when to use this command "ebgp-multihop" , Is it use when two different brand router.
(e.g. Cisco Router connect to Juniper Router via non directly connection). Then cisco router
need to issue "neighbor x.x.x. ebgp-multihp" to form the bgp neighbor
No, ebgp multihop is used when having eBGP peering with non direct connected eBGP peer ip address (whatever the vendor is), by default eBGP messages are send with a TTL of 1, and accordingly if the eBGP neighbor ip is not directly connected the eBGP session won't get up.
Commonly used when peering using loopbacks, since it is not directly connected we must use ?ebgp-multihop 2? command to set TTL to 2 for the peer connection to be formed (by routing the packets to the loopback, instead of dropping it if the TTL was sent with the default value of 1 ? can?t be routed).
HTH, please rate if it does help,