「BGP」- Update Packet

  CREATED BY JENKINSBOT

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2）同时 Update 还可以携带多条不可达路由，用于告知对方撤销路由，这些保存在 Withdrawn Routes 字段中；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Withdrawn Routes Length（Unfeasible routes length）：标明 Withdrawn Routes 部分的长度，以 Byte 为单位。其值为零时，表示没有撤销的路由；

Withdrawn routes：不可达路由的列表；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：可达路由的前缀和前缀长度二元组；

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    Withdrawn Routes Length: 0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    Withdrawn Routes Length: 9
    Withdrawn Routes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            Withdrawn route prefix length: 24
            Withdrawn prefix: 10.1.12.0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            Withdrawn route prefix length: 32
            Withdrawn prefix: 100.100.100.100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

MP-BGP VPNv4

MP_REACH_NLRI and EXTENDED_COMMUNITIES

Frame 273: 289 bytes on wire (2312 bits), 289 bytes captured (2312 bits) on interface -, id 0
Ethernet II, Src: HuaweiTe_f0:4f:dc (00:e0:fc:f0:4f:dc), Dst: HuaweiTe_ed:04:ec (00:e0:fc:ed:04:ec)
MultiProtocol Label Switching Header, Label: 1026, Exp: 6, S: 1, TTL: 255
Internet Protocol Version 4, Src: 10.1.1.1, Dst: 10.4.4.4
Transmission Control Protocol, Src Port: 179, Dst Port: 50565, Seq: 263, Ack: 96, Len: 231
Border Gateway Protocol - UPDATE Message
Border Gateway Protocol - UPDATE Message
    Marker: ffffffffffffffffffffffffffffffff
    Length: 115
    Type: UPDATE Message (2)
    Withdrawn Routes Length: 0
    Total Path Attribute Length: 92
    Path attributes
        Path Attribute - ORIGIN: INCOMPLETE
        Path Attribute - AS_PATH: empty
        Path Attribute - MULTI_EXIT_DISC: 3
        Path Attribute - LOCAL_PREF: 100
        Path Attribute - EXTENDED_COMMUNITIES
            Flags: 0xc0, Optional, Transitive, Complete
                1... .... = Optional: Set
                .1.. .... = Transitive: Set
                ..0. .... = Partial: Not set
                ...0 .... = Extended-Length: Not set
                .... 0000 = Unused: 0x0
            Type Code: EXTENDED_COMMUNITIES (16)
            Length: 32
            Carried extended communities: (4 communities)
                Route Target: 100:100 [Transitive 2-Octet AS-Specific]
                    Type: Transitive 2-Octet AS-Specific (0x00)
                        0... .... = IANA Authority: Allocated on Standard Action, Early Allocation or Experimental Basis
                        .0.. .... = Transitive across AS: Transitive
                    Subtype (AS2): Route Target (0x02)
                    2-Octet AS: 100
                    4-Octet AN: 100
                OSPF Domain Identifier: 0:0 [Transitive 2-Octet AS-Specific]
                    Type: Transitive 2-Octet AS-Specific (0x00)
                        0... .... = IANA Authority: Allocated on Standard Action, Early Allocation or Experimental Basis
                        .0.. .... = Transitive across AS: Transitive
                    Subtype (AS2): OSPF Domain Identifier (0x05)
                    2-Octet AS: 0
                    4-Octet AN: 0
                OSPF Route Type: Area: 0.0.0.1, Type: Router [Transitive Opaque]
                    Type: Transitive Opaque (0x03)
                        0... .... = IANA Authority: Allocated on Standard Action, Early Allocation or Experimental Basis
                        .0.. .... = Transitive across AS: Transitive
                    Subtype (Opaque): OSPF Route Type (0x06)
                    Area ID: 0.0.0.1
                    Route type: Router (1)
                    Options: 0x00 (Metric: Type-1)
                OSPF Router ID: 10.0.61.1:0 [Transitive IPv4-Address-Specific]
                    Type: Transitive IPv4-Address-Specific (0x01)
                        0... .... = IANA Authority: Allocated on Standard Action, Early Allocation or Experimental Basis
                        .0.. .... = Transitive across AS: Transitive
                    Subtype (IPv4): OSPF Router ID (0x07)
                    Router ID: 10.0.61.1
        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: 32
            Address family identifier (AFI): IPv4 (1)
            Subsequent address family identifier (SAFI): Labeled VPN Unicast (128)
            Next hop:  RD=0:0 IPv4=10.1.1.1
                Route Distinguisher: 0:0
                IPv4 Address: 10.1.1.1
            Number of Subnetwork points of attachment (SNPA): 0
            Network Layer Reachability Information (NLRI)
                BGP Prefix
                    Prefix Length: 112
                    Label Stack: 1032 (bottom)
                    Route Distinguisher: 100:1
                    MP Reach NLRI IPv4 prefix: 192.168.1.0

MP_UNREACH_NLRI

Frame 80: 134 bytes on wire (1072 bits), 134 bytes captured (1072 bits) on interface -, id 0
Ethernet II, Src: HuaweiTe_f0:4f:dc (00:e0:fc:f0:4f:dc), Dst: HuaweiTe_ed:04:ec (00:e0:fc:ed:04:ec)
MultiProtocol Label Switching Header, Label: 1026, Exp: 6, S: 1, TTL: 255
Internet Protocol Version 4, Src: 10.1.1.1, Dst: 10.4.4.4
Transmission Control Protocol, Src Port: 179, Dst Port: 50565, Seq: 20, Ack: 20, Len: 76
Border Gateway Protocol - UPDATE Message
    Marker: ffffffffffffffffffffffffffffffff
    Length: 76
    Type: UPDATE Message (2)
    Withdrawn Routes Length: 0
    Total Path Attribute Length: 53
    Path attributes
        Path Attribute - MP_UNREACH_NLRI
            Flags: 0x90, Optional, Extended-Length, Non-transitive, Complete
            Type Code: MP_UNREACH_NLRI (15)
            Length: 49
            Address family identifier (AFI): IPv4 (1)
            Subsequent address family identifier (SAFI): Labeled VPN Unicast (128)
            Withdrawn Routes
                BGP Prefix
                BGP Prefix
                BGP Prefix
                    Prefix Length: 112
                    Label Stack: 0 (bottom)
                    Route Distinguisher: 100:1
                    MP Unreach NLRI IPv4 prefix: 10.0.61.0

参考文献

BGP Update Message