「BGP」- Open

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

Filed under: K4NZDROID - @ 5:17 PM