Update 报文用于在 BGP Peer 间的传递路由信息，其用于发布路由、撤销路由；
报文格式
单个 Update 报文：
1）可以通告具有相同路径属性的多条路由，这些路由保存在 NLRI（Network Layer Reachable Information，网络层可达信息）中；
2）同时 Update 还可以携带多条不可达路由，用于告知对方撤销路由，这些保存在 Withdrawn Routes 字段中；
报文格式如图所示，主要字段解释如下：
Withdrawn Routes Length（Unfeasible routes length）：标明 Withdrawn Routes 部分的长度，以 Byte 为单位。其值为零时，表示没有撤销的路由；
Withdrawn routes：不可达路由的列表；
Total path attribute length：路径属性字段的长度，以 Byte 为单位。如果为 0，则表示无 Path Attributes 字段；
Path attributes：与 NLRI 相关的所有路径属性列表，每个路径属性由一个 TLV（Type-Length-Value）三元组构成；
NLRI：其为二元组（可达路由前缀、前缀长度）。简而言之，该字段保存的是被通告的网段。
报文示例
更新路由的报文
Frame 28: 109 bytes on wire (872 bits), 109 bytes captured (872 bits) on interface -, id 0
Ethernet II, Src: HuaweiTe_2e:4a:35 (00:e0:fc:2e:4a:35), Dst: HuaweiTe_0b:50:b5 (00:e0:fc:0b:50:b5)
Internet Protocol Version 4, Src: 10.1.12.1, Dst: 10.1.12.2
Transmission Control Protocol, Src Port: 50115, Dst Port: 179, Seq: 65, Ack: 65, Len: 55
Border Gateway Protocol - UPDATE Message
Marker: ffffffffffffffffffffffffffffffff
Length: 55
Type: UPDATE Message (2)
Withdrawn Routes Length: 0
Total Path Attribute Length: 27
Path attributes
Path Attribute - ORIGIN: IGP
Path Attribute - AS_PATH: 200
Path Attribute - NEXT_HOP: 10.1.12.1
Path Attribute - MULTI_EXIT_DISC: 0
Network Layer Reachability Information (NLRI)
10.1.1.1/32
带有撤销路由的报文
Frame 22: 86 bytes on wire (688 bits), 86 bytes captured (688 bits) on interface -, id 0
Ethernet II, Src: HuaweiTe_2e:4a:35 (00:e0:fc:2e:4a:35), Dst: HuaweiTe_0b:50:b5 (00:e0:fc:0b:50:b5)
Internet Protocol Version 4, Src: 10.1.12.1, Dst: 10.1.12.2
Transmission Control Protocol, Src Port: 179, Dst Port: 50969, Seq: 250, Ack: 65, Len: 32
Border Gateway Protocol - UPDATE Message
Marker: ffffffffffffffffffffffffffffffff
Length: 32
Type: UPDATE Message (2)
Withdrawn Routes Length: 9
Withdrawn Routes
10.1.12.0/24
Withdrawn route prefix length: 24
Withdrawn prefix: 10.1.12.0
100.100.100.100/32
Withdrawn route prefix length: 32
Withdrawn prefix: 100.100.100.100
Total Path Attribute Length: 0
BGP-4 Update
BGP-4 中 IPv4 地址族特有的三个信息是 NEXT_HOP 属性、AGGREGATOR、IPv4 NLRI；
R1 向 R2 通告 BGP-4 Update 报文。通告网段为 10.0.0.0/8，下一跳地址为 1.1.1.1；
BGP Update 报文中在 NLRI 中携带 IPv4 网段信息，在 Path attributes 的 NEXT_HOP 中携带下一跳地址；
MP-BGP VPNv4
MP-BGP 为了支持新的地址族并兼容 IPv4，IETF 规定 MP-BGP 增加 MP_REACH_NLRI 和 MP_UNREACH_NLRI 两种新的路径属性，分别用于表示可达的目的信息和不可达的目的信息；
MP-BGP Update 报文结构与 BGP 相比略有区别。如图 R3 向 R2 通告 VPNv4 路由信息如下。其 RD 值和 IPv4 路由信息被存放于新的 NLRI 字段中，RT 值则被存放于路径属性的团体属性中；
参考文献
报文示例
IPv4 Labeled Unicast
Frame 12: 123 bytes on wire (984 bits), 123 bytes captured (984 bits) on interface -, id 0
Ethernet II, Src: 38:39:cc:01:01:01 (38:39:cc:01:01:01), Dst: 38:39:cc:02:01:00 (38:39:cc:02:01:00)
MultiProtocol Label Switching Header, Label: 16003, Exp: 6, S: 1, TTL: 255
Internet Protocol Version 4, Src: 1.1.1.1, Dst: 3.3.3.3
Transmission Control Protocol, Src Port: 179, Dst Port: 64776, Seq: 1, Ack: 20, Len: 65
Border Gateway Protocol - UPDATE Message
Marker: ffffffffffffffffffffffffffffffff
Length: 65
Type: UPDATE Message (2)
Withdrawn Routes Length: 0
Total Path Attribute Length: 42
Path attributes
Path Attribute - ORIGIN: IGP
Flags: 0x40, Transitive, Well-known, Complete
0... .... = Optional: Not set
.1.. .... = Transitive: Set
..0. .... = Partial: Not set
...0 .... = Extended-Length: Not set
.... 0000 = Unused: 0x0
Type Code: ORIGIN (1)
Length: 1
Origin: IGP (0)
Path Attribute - AS_PATH: empty
Flags: 0x40, Transitive, Well-known, Complete
0... .... = Optional: Not set
.1.. .... = Transitive: Set
..0. .... = Partial: Not set
...0 .... = Extended-Length: Not set
.... 0000 = Unused: 0x0
Type Code: AS_PATH (2)
Length: 0
Path Attribute - MULTI_EXIT_DISC: 0
Flags: 0x80, Optional, Non-transitive, Complete
1... .... = Optional: Set
.0.. .... = Transitive: Not set
..0. .... = Partial: Not set
...0 .... = Extended-Length: Not set
.... 0000 = Unused: 0x0
Type Code: MULTI_EXIT_DISC (4)
Length: 4
Multiple exit discriminator: 0
Path Attribute - LOCAL_PREF: 100
Flags: 0x40, Transitive, Well-known, Complete
0... .... = Optional: Not set
.1.. .... = Transitive: Set
..0. .... = Partial: Not set
...0 .... = Extended-Length: Not set
.... 0000 = Unused: 0x0
Type Code: LOCAL_PREF (5)
Length: 4
Local preference: 100
Path Attribute - MP_REACH_NLRI
Flags: 0x90, Optional, Extended-Length, Non-transitive, Complete
1... .... = Optional: Set
.0.. .... = Transitive: Not set
..0. .... = Partial: Not set
...1 .... = Extended-Length: Set
.... 0000 = Unused: 0x0
Type Code: MP_REACH_NLRI (14)
Length: 17
Address family identifier (AFI): IPv4 (1)
Subsequent address family identifier (SAFI): Labeled Unicast (4)
Next hop: 1.1.1.1
IPv4 Address: 1.1.1.1
Number of Subnetwork points of attachment (SNPA): 0
Network Layer Reachability Information (NLRI)
Label Stack=48120 (bottom) IPv4=1.1.1.1/32
MP Reach NLRI Prefix length: 56
MP Reach NLRI Label Stack: 48120 (bottom)
MP Reach NLRI IPv4 prefix: 1.1.1.1