My BGP Notes – Part 2

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

My BGP Notes – Part 1

My BGP notes. It’s going to be a series of posts here….My BGP Notes 1,2,3,……

(Part 2 is here My BGP Notes – Part 2)

I have been keeping BGP notes on many scattered places. A lot of times when I want to refresh my BGP knowledge – I could not find the notes I kept earlier easily and end up Googling (yes, BGP references are everywhere but I prefer my own way of taking notes). Hence, I am adding my BGP notes on my blog page.

To me, BGP is not only a routing protocol but rather BGP is a big “network application” I use in designing networks in enterprise connectivity, data centre networking, and ISP connectivities.

This is the “part 1” of my notes; I will start with the fundamentals of BGP.

The Basics

-RFC 1654 defines BGP as an EGP “path-vector” routing protocol.
-BGP is designed for IPv4. (But Multiprotocol BGP – MP-BGP works with IPv6).
-BGP configuration requires Autonomous System Numbers (ASN).
-ASN numbering was originally 16-bits long number (2-bytes); 1-65,535.
-Extended ASN ranges are 32-bits (4-bytes) number up to 4,294,967,294.
-ASN 64,512–65,535 are private within 2-bytes range.
-ASN 4,200,000,000–4,294,967,294 are private within 4-bytes range.
-The BGP version we use is BGP v4.
-BGP loop prevention is based on the path-vector mechanism.
-BGP adds its own AS number (AS_PATH) to the prefixes it announces to peers and discards messages if its own AS number is found in a received message.
-Multiprotocol BGP (MP-BGP) supports a wide range of address families besides IPv4 (l2vpn, l3vpn, evpn, unicast, multicast, flow-spec…).

BGP Sessions

-A BGP session refers to the established adjacency between two BGP speaker routers. BGP sessions are always point-to-point between two BGP speakers.
-A BGP session can be iBGP, when a BGP session is established within the same ASN number; both BGP speakers belong to the same ASN.
-A BGP session can be eBGP, when BGP speakers belong to different ASN numbers.
-iBGP administrative distance is 200 whereas eBGP administrative distance is 20.
-BGP speakers do not use Hello packets to discover neighbors like IGP routing protocols.
-A BGP session can not be discovered automatically like OSPF/EIGRP/RIP.
-BGP uses TCP port 179 to communicate with neighbors.
-A BGP session starts with a TCP 3-way handshake.

BGP Messages

BGP speakers use four (04) messages to communicate between themselves.

(01)OPEN message
(02)KEEPALIVE message
(03)UPDATE message
(04)NOTIFICATION message

Some vendor implementations use a fifth message – this is called “Route Refresh” message; however, this is found in the OPEN message for Cisco routers (part of optional capabilities).

BGP messages are easy to identify in captured packets using Wireshark. Let’s see what we found in different BGP messages.

OPEN Message

After the 3-way TCP handshake, the very first BGP message is called “Open”. Both BGP speakers negotiate session capabilities before a BGP peering is established.

Screenshot of OPEN message following.

Based on the OPEN message captured, we found the following items here –

-BGP version
-ASN number
-Hold Time
-BGP identifier (RID)
-BGP capabilities (Multiprotocol extensions, Route refresh capabilities, Graceful Restart capabilities, support for Extended ASN 4-bytes octet)

Notes on Hold Time: BGP default hold time suggested is 90 seconds (3x of keepalive) and keepalives is 30 seconds. BGP default keepalive and hold time are vendor specific these days.

KEEPALIVE Message

Although BGP uses TCP 3-way handshake, it does not rely on TCP connection states (ack mechanism) to check if the peer is still alive.

BGP KEEPALIVE is a simple message format sent every 1/3 of configured Hold Time interval. BGP configuration with 90 seconds Hold Time will send KEEPALIVE every 30 seconds. If Hold Time is set to zero seconds – then there is no KEEPALIVE!

Screenshot of KEEPALIVE message following.

UPDATE Message

BGP network advertisements are included in UPDATE messages. BGP sends both feasible routes and withdrawn routes (previously advertised). Route prefixes and BGP Path Attributes (PA) are found in BGP NLRI. MP_REACH_NLRI and MP_UNREACH_NLRI along with AFI and SAFI details are found in UPDATE messages.

An UPDATE message can act as KEEPALIVE to reduce noise in BGP communications.

Screenshot of the UPDATE message following.

NOTIFICATION Message

NOTIFICATION message is sent if there is an error found in BGP communication between BGP speakers. Notification codes include “Cease”, “Hard Reset” etc.

Screenshot of NOTIFICATION message following.

BGP Message Header

BGP messages header has the following three items –

(01)Marker; this is filled with all fffffffff……16-octets for all message types.
(02)Length; length can be different for different types of message and also based on what information are in the message. The total length is mentioned on the top of the header; breakdowns are in each path attributes section. Min length size is 19 bytes and the max size is 4096 bytes.
(03)Type; Open/Update/Keepalive/Notificaiton.