Your company has more offices and each of them has a separate internet connection. The default route for each office points towards the ISP. Also, within each office you run iBGP using private AS numbers.
In the below diagram, you have Office-1 running AS 65100 and Office-2 running AS 65200.

You have been asked to configure an eBGP peering between Office-1 and Office-2. You did that by establishing a BGP session, using each border routers' loopbacks (as shown below).

quiz-4

Soon, you notice that the new eBGP peering does not come up. You perform the following troubleshooting: 

Office-1#sh ip bgp summ
BGP router identifier 1.1.1.1, local AS number 65100
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 65200       0       0        0    0    0 never    Active
Office-1#
Office-1#
Office-1#ping 2.2.2.2 source lo0

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
Packet sent with a source address of 1.1.1.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/16/56 ms
Office-1#
Office-1#
Office-1#
Office-1#traceroute 2.2.2.2 source lo0

Type escape sequence to abort.
Tracing the route to 2.2.2.2

  1 155.1.1.2 4 msec 4 msec 16 msec
  2 155.2.2.1 32 msec *  52 msec
Office-1#
Office-1#
Office-1#
Office-1#sh ip cef 2.2.2.2
0.0.0.0/0, version 9, epoch 0, cached adjacency to Serial0/0
0 packets, 0 bytes
  via 155.1.1.2, 0 dependencies, recursive
    next hop 155.1.1.2, Serial0/0 via 155.1.1.0/30
    valid cached adjacency

As you can see, you can ping each others' loopback, but the BGP does not come up.
Note that Office-2 Border Router displays similar output

What is the problem ?

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