BGP Peering

Question 1

Does BGP discover neighbors?

Answer 1

no, peers are configured manually

Question 2

Do BGP peers need to be on the same subnet?

Answer 2

since “neighbors” do not need to be on the same subnet, they are referred to as peers

Question 3

When the BGP peering is established, does BGP automatically exchange routes?

Answer 3

no, unlike OSPF or EIGRP, BGP does not automatically exchange routes, when the peering relationship is established

Question 4

What are the different BGP Peering States (6)

Answer 4

1. Idle
2. Connect
3. Active
4. Open-Sent
5. OpenConfirm
6. Established

Question 5

Describe the BGP Idle state

Answer 5

• initial state of BGP where a BGP router tries to initiate a TCP connection with a peer
• if both peers are stuck in the idle state, it might mean that the peers do not have IP connectivity

Question 6

Describe the BGP Connect state

Answer 6

• BGP waits for the TCP connection with the peer to be completed
• if the TCP handshake is completed, then the state will transition from Connect to Open-Sent
• if the communication is stuck in this stage, it is likely that security policy is blocking the connection

Question 7

Describe the BGP Active state

Answer 7

• active router (higher IP) starts a new TCP connection with its peer
• the passive router (lower IP) listens for the new connection
• this state, like the previous states is linked to the TCP-connection
• if the peers are in any of the two states (connect or active) , the problem is with the TCP and the issue should be troubleshooted as any other normal application running on port 179

Question 8

Describe the BGP Open-Sent state

Answer 8

• once TCP session is established, BGP moves to the Open-Sent state
• both routers perform sanity checks
  
  • if the checks don’t pass, the routers are moved back to the idle state
• in this state, the BGP_OPEN message would be sent to the peer
• once Open-Sent is sent and an answer from peer is received, the hold-down timer and the keepalive timers will also be negotiated
• this stage and the next that is the open-confirm are very transient stages (seen only when BGP is misconfigured)

Question 9

Describe the BGP Open-Confirm state

Answer 9

• entered after both routers exchange Open messages and agree on the parameters
• routers verify that the received parameters are acceptable, and they prepare to exchange BGP updates

Question 10

Once the BGP Connect is complete, wich router manages the connection?

Answer 10

the one with the higher IP address