BGP Disable Connected Check and EBGP Multi Hop

Figure 1.1

In this scenario I use OSPF for TCP reachability. We can see the difference 'disable connected' check and 'EBGP multihop'. We can use both for EBGP neighbor connection with loopback address.

Disable connected check
This command used to disable the connection verification process for ebgp.
Use for ebgp neighbor are directly connected and they using the loopback address for peering. It not increment TTL but neighbor must reachable at single hop.

EBGP multihop
This command can use for ebgp neighbor are not directly connected. It increment TTL.

R1
router bgp 100
 neighbor 2.2.2.2 remote-as 200
 neighbor 2.2.2.2 disable-connected-check
 neighbor 2.2.2.2 update-source Loopback0
 neighbor 3.3.3.3 remote-as 300
 neighbor 3.3.3.3 ebgp-multihop 2
 neighbor 3.3.3.3 update-source Loopback0

R2

router bgp 200

 neighbor 1.1.1.1 remote-as 100

 neighbor 1.1.1.1 disable-connected-check

 neighbor 1.1.1.1 update-source Loopback0

R3

router bgp 300

 neighbor 1.1.1.1 remote-as 100

 neighbor 1.1.1.1 ebgp-multihop 2

 neighbor 1.1.1.1 update-source Loopback0

Verify
R1#sh ip bgp nei 2.2.2.2 | s established | TTL
  Connections established 1; dropped 0
Connection is ECN Disabled, Mininum incoming TTL 0, Outgoing TTL 1

R1#sh ip bgp nei 3.3.3.3 | s established | TTL
  Connections established 1; dropped 0
Connection is ECN Disabled, Mininum incoming TTL 0, Outgoing TTL 2

If the R1's f0/0 is down R2 neighbor will down and R3 will survive.

R1#sh ip bgp sum
BGP router identifier 11.11.11.11, local AS number 100
BGP table version is 1, main routing table version 1

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
2.2.2.2         4   200     171     175                  0    0    0      00:00:51    Active
3.3.3.3         4   300     176     174                  1    0    0      00:11:51        0