This is part 2 of “My BGP Notes” series; the part 1 link is here My BGP Notes – Part 1

Part 2 is also my notes on BGP fundamentals; this covers “BGP Neighbor States” and “BGP AFI, SAFI”.

Part 2.1 – BGP Neighbor States

BGP uses the Finite State Machine (FSM) to maintain a table of all BGP peers and their operational status; the FSM model defines – “what actions” should be taken by the BGP engine and “when” in the simplest manner.
 
BGP sessions are peer-to-peer sessions between neighbors; BGP neighbor states are the followings:
 
i. Idle
ii. Connect
iii. Active
iv. OpenSent
v. OpenConfirm
vi. Established

“Idle” State
 
->This is the first stage of the BGP FSM; the Idle state occurs when someone configures a new BGP neighbor or resets an “Established” peer session.
->In this state, BGP detects a start event, listens for a new connection (TCP/179) from a peer, and initiates a TCP connection to remote peer.
->When successful, BGP moves onto the next state “Connect” state.
->If an error causes BGP to go back to the “Idle” state for a second time, the “ConnectRetryTimer” is set to 60 seconds and must decrement to zero before the connection is initiated again. Further failures leave the “Idle state” result in the “ConnectRetryTimer” doubling in length from the previous time.

“Connect” State
 
->In this state, BGP waits for the 3-way TCP handshake to complete successfully.
->Upon a successful TCP connection, BGP sends an “OPEN” message to peer and moves onto next “OpenSent” state.
->If the above TCP connection fails, then BGP goes next to “Active” state and resets the “ConnectRetryTimer” timer.
->If any other input is received, BGP goes back to “Idle” state.
 
During this stage, the neighbor with the “higher IP address” manages the connection. The router initiating the request uses a dynamic source port, but the destination port is always TCP/179.

“Active” State
 
->In this state, BGP speaker tries to connect to peer by initiating “another” new TCP 3-way handshake.
->If a TCP connection is established, an “OPEN” message is sent, the Hold Timer is set to 4 minutes (on Cisco), and the state moves to next “OpenSent” state.
->If this attempt for TCP connection fails, the state moves back to the “Connect” state and resets the “ConnectRetryTimer”.
->If any other input is received, BGP goes back to “Idle” state.

“OpenSent” State
 
->In this state, an “OPEN” message has been sent from the originating router and is awaiting an “OPEN” message from the peer. After the originating router receives the “OPEN” message from the peer, both “OPEN” messages are checked and compared for errors.
->The items are being compared “what is configured” on the peers are: BGP Versions/RID/ASN/Security Params/TTL/SourceIP/and similar params.
->Upon successful “OPEN” messages exchange, BGP sets the Hold Time (using the lower value) and a “KEEPALIVE” message is sent; BGP then goes onto next “OpenConfirm” state.
->If an error is found in the “OPEN” message, BGP sends a “NOTIFICATION” message and the state is moved back to “Idle” state.
->If TCP receives a disconnect message, BGP closes the connection, resets the “ConnectRetryTimer”, and sets the state back to “Active”.
->If any other input is received, BGP goes back to “Idle” state.

“OpenConfirm” State
 
->In this state, BGP waits for a “KEEPALIVE” message or “NOTIFICATION” message from peer.  
->Upon successful receipt of a peer’s “KEEPALIVE” message, the state moves next to “Established” state.
->If the hold timer expires, a stop event occurs, or a “NOTIFICATION” message is received, then the state is moved back to “Idle” state.

“Established” State

->In this state, the BGP session is established.
->BGP neighbors exchange routes via “UPDATE” messages.
->As “UPDATE” and “KEEPALIVE” messages are received, the Hold Timer is reset.
->If the Hold Timer expires or an error is detected (a “NOTIFICATION” message), BGP moves the neighbor state back to the “Idle” state.

In summary; if no error, then BGP neighbor state progressions are the followings:

Idle -> Connect -> OpenSent -> OpenConfirm -> Established.
 
If there any error occurs, then BGP neighbor state progressions “could be” the followings:

Idle -> Connect (back to “Idle”) -> Active (back to “Connect” or “Idle”) -> OpenSent (back to “Active” or “Idle”) -> OpenConfirm (back to “Idle”) -> Established (back to “Idle”).

The following screenshot is taken on a Cisco CSRv router – BGP “Idle” state:

The following screenshot is taken on a Cisco CSRv router – BGP “Established” state:

Part 2.2 – BGP AFI and SAFI

AFI means “Address Family Indicator”
SAFI means “Subsequent Address Family Indicator”.

They are used in the “Multiprotocol Extensions” to BGP (MP-BGP) and are exchanged during neighbor capability exchange (in the BGP “OPEN” message) during the process of establishing the peers. They basically tell the remote peer what address families (IPv4, IPv6, VPNv4, VPNv6…) and what specific sub-address family (multicast, unicast, vrf, evpn, vpls, flow-spec…) the local BGP router will transport the routes for.

AFI is 16-bit.
SAFI is 8-bit.

A few well-known AFI-SAFI are the followings:

1-1 is for IPv4 (AFI:1) unicast forwarding (SAFI:1)
1-2 is for IPv4 (AFI:1) multicast forwarding (SAFI:2)
1-128 is for IPv4 (AFI:1) VPNv4 (MPLS-labeled VPN address SAFI:128)
1-132 is for IPv4 (AFI:1) VRF – Route Target constrains (SAFI:132)
1-133 is for IPv4 (AFI:1) Flow-spec (SAFI:133)

2-1 is for IPv6 (AFI:2) unicast forwarding (SAFI:1)
2-2 is for IPv6 (AFI:2) multicast forwarding (SAFI:1)
2-128 is for IPv6 (AFI:2) VPNv6 (MPLS-labeled VPN address SAFI:128)

25-70 is for L2VPN (AFI:25) EVPN (SAFI:70)

AFI: 0 is reserved
AFI: 32-16383 are unassigned
AFI: 16400-65534 are unassigned

In the future, there will be many new AFI and SAFI adopted in MP-BGP as new capabilities!

The following screenshot is taken on a Cisco CSRv router – showing available AFIs:

The following screenshot is taken on a Cisco CSRv router – showing available SAFIs within IPv4:

The following screenshot is taken on a Cisco CSRv router – showing available SAFIs within L2VPN:

MP-BGP AFI and SAFI References:
https://www.iana.org/assignments/address-family-numbers/address-family-numbers.xhtml
https://www.iana.org/assignments/safi-namespace/safi-namespace.xhtml