Open 报文是 TCP 连接建立之后发送的第一个报文，用于建立 BGP 对等体之间的连接关系；
报文格式
报文格式如图所示，主要字段解释如下：
Version
BGP 的版本号。对于 BGP 4 来说，其值为 4；
My AS（AS Number）
本地 AS 号。通过比较两端的 AS 号可以判断对端是否和本端处于相同 AS；
Hold Time
保持时间。在建立对等体关系时两端要协商 Hold Time，并保持一致。如果在 Hold Time 内未收到对端发来的 Keepalive 报文或 Update 报文，则认为 BGP 连接中断；
BGP Identifier
BGP 标识符，以 IP 地址的形式表示，用来识别 BGP 路由器（Route ID）；
Opt Parm Len
Optional parameters 的长度；
Optional parameters
宣告自身对于一些可选功能的支持，比如 认证、多协议支持 等等；
除了 IPv4 单播路由信息，BGP4+还支持多种网络层协议（如 IPv6、组播），在协商时 BGP Peer 之间会通过 Optional parameters 字段协商对网络层协议的支持能力；
报文示例
Open in BGP EVPN with VXLAN
Frame 160: 105 bytes on wire (840 bits), 105 bytes captured (840 bits) on interface -, id 0
Ethernet II, Src: 38:3b:75:01:01:00 (38:3b:75:01:01:00), Dst: HuaweiTechno_a3:61:d7 (00:e0:fc:a3:61:d7)
Internet Protocol Version 4, Src: 1.1.1.1, Dst: 3.3.3.3
Transmission Control Protocol, Src Port: 58166, Dst Port: 179, Seq: 1, Ack: 1, Len: 51
Border Gateway Protocol - OPEN Message
Marker: ffffffffffffffffffffffffffffffff
Length: 51
Type: OPEN Message (1)
Version: 4
My AS: 200
Hold Time: 180
BGP Identifier: 10.0.19.1
Optional Parameters Length: 22
Optional Parameters
Optional Parameter: Capability
Parameter Type: Capability (2)
Parameter Length: 20
Capability: Multiprotocol extensions capability
Type: Multiprotocol extensions capability (1)
Length: 4
AFI: IPv4 (1)
Reserved: 00
SAFI: Unicast (1)
Capability: Multiprotocol extensions capability
Type: Multiprotocol extensions capability (1)
Length: 4
AFI: Layer-2 VPN (25)
Reserved: 00
SAFI: EVPN (70)
Capability: Route refresh capability
Type: Route refresh capability (2)
Length: 0
Capability: Support for 4-octet AS number capability
Type: Support for 4-octet AS number capability (65)
Length: 4
AS Number: 200
IPv4 Labeled Unicast and IPv4 Unicast
Frame 85: 105 bytes on wire (840 bits), 105 bytes captured (840 bits) on interface -, id 0
Ethernet II, Src: 38:39:cc:05:01:00 (38:39:cc:05:01:00), Dst: 38:39:cc:06:01:01 (38:39:cc:06:01:01)
Internet Protocol Version 4, Src: 6.6.6.6, Dst: 4.4.4.4
Transmission Control Protocol, Src Port: 63159, Dst Port: 179, Seq: 1, Ack: 1, Len: 51
Border Gateway Protocol - OPEN Message
Marker: ffffffffffffffffffffffffffffffff
Length: 51
Type: OPEN Message (1)
Version: 4
My AS: 200
Hold Time: 180
BGP Identifier: 192.168.20.254
Optional Parameters Length: 22
Optional Parameters
Optional Parameter: Capability
Parameter Type: Capability (2)
Parameter Length: 20
Capability: Multiprotocol extensions capability
Type: Multiprotocol extensions capability (1)
Length: 4
AFI: IPv4 (1)
Reserved: 00
SAFI: Unicast (1)
Capability: Multiprotocol extensions capability
Type: Multiprotocol extensions capability (1)
Length: 4
AFI: IPv4 (1)
Reserved: 00
SAFI: Labeled Unicast (4)
Capability: Route refresh capability
Type: Route refresh capability (2)
Length: 0
Capability: Support for 4-octet AS number capability
Type: Support for 4-octet AS number capability (65)
Length: 4
AS Number: 200