News
发布日期：2022-04-28 浏览次数：2891 来源：崔志鹏
MPLS LDP会话的建立过程- HCIE Datacom技术文档
WOLF-LAB沃尔夫网络实验室
HCIE培训针对0基础入门学员开设，
LDP协议规定了标签分发过程中的各种消息以及相关的处理过程。通过LDP，LSR可以把网络层的路由信息映射到数据链路层的交换路径上，进而建立起LSP。
LDP会话的建立过程
两台LSR之间交换Hello消息触发LDP会话的建立。
LDP会话的建立过程如图1所示：
两个LSR之间互相发送Hello消息。
Hello消息中携带传输地址（即设备的IP地址），双方使用传输地址建立LDP会话。
传输地址较大的一方作为主动方，发起建立TCP连接。
如图1所示，LSR_1作为主动方发起建立TCP连接，LSR_2作为被动方等待对方发起连接。
TCP连接建立成功后，由主动方LSR_1发送初始化消息，协商建立LDP会话的相关参数。
LDP会话的相关参数包括LDP协议版本、标签分发方式、Keepalive保持定时器的值、最大PDU长度和标签空间等。
被动方LSR_2收到初始化消息后，LSR_2接受相关参数，则发送初始化消息，同时发送Keepalive消息给主动方LSR_1。
如果被动方LSR_2不能接受相关参数，则发送Notification消息终止LDP会话的建立。
初始化消息中包括LDP协议版本、标签分发方式、Keepalive保持定时器的值、最大PDU长度和标签空间等。
主动方LSR_1收到初始化消息后，接受相关参数，则发送Keepalive消息给被动方LSR_2。
如果主动方LSR_1不能接受相关参数，则发送Notification消息给被动方LSR_2终止LDP会话的建立。
当双方都收到对端的Keepalive消息后，LDP会话建立成功。
LDP邻居的发现：
@两台路由器会先进行邻居的发现，当启用了LDP协议，两台路由器会周期性发送hello报文，进行邻居的发现，在hello报文中携带一个传输地址，默认情况下用的是lsr-id；
@接着由传输地址大的一方，以自己传输地址作为源地址，DIP地址是传输地址小的发起TCP的连接完成三次握手，用于建立LDP的会话；
@hello用于LDP的发现，TCP连接用于建立LDP的会话，端口号是TCP的646。hello报文5s发一次，DIP：224.0.0.2，端口号UDP的646；
LDP邻居的建立：
dis：通过hello包进行对等体的发现；
tcp：建立LDP的会话；
session：发起方发送init报文，携带协商参数，对端收到后也会发送init报文，要是协商参数没问题就互相发送keeplive报文作为确认；
advertisement：
@两台路由器会发送address，就是把启用了mpls和mpls ldp的接口地址+自己的LSR-ID携带出去；
@lable mapping标签映射消息，路由器为自己路由表中的路由分配的标签，彼此进行标签信息的交换；
@notification：如果在整个过程中出现问题就发送nof报文
注意：
@传输地址必须可达；
@ldp中因为默认使用LSR-ID充当传输地址，LSR-ID必须可达；
@但是可以修改传输地址，此时只要保证修改后的传输地址可达就行，这时候LSR-ID就跟router-id一样，无需设备上必须有一个接口地址属于LSR-ID；
@假设将下图的AR1的LSR-ID改成3.3.3.3，传输地址改成g0/0/0接口地址，LDP的邻居关系和会话都是可以正常建立的；
LDP配置：
R1:
mpls lsr-id 1.1.1.1
mpls
mpls ldp
interface GigabitEthernet0/0/0
mpls
mpls ldp
R1/R2互相发送的hello报文：
Frame 10: 76 bytes on wire (608 bits), 76 bytes captured (608 bits) on interface -, id 0
Ethernet II, Src: HuaweiTe_ba:13:f8 (00:e0:fc:ba:13:f8), Dst: IPv4mcast_02 (01:00:5e:00:00:02)
Internet Protocol Version 4, Src: 1.1.12.1, Dst: 224.0.0.2
User Datagram Protocol, Src Port: 646, Dst Port: 646
Label Distribution Protocol
Version: 1
PDU Length: 30
LSR ID: 1.1.1.1 //自己的LSR-ID
Label Space ID: 0 //标签空间
Hello Message
0... .... = U bit: Unknown bit not set
Message Type: Hello Message (0x100)
Message Length: 20
Message ID: 0x00000008
Common Hello Parameters
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: Common Hello Parameters (0x400)
TLV Length: 4
Hold Time: 15 //LDP邻居失效时间是15s
0... .... .... .... = Targeted Hello: Link Hello
.0.. .... .... .... = Hello Requested: Source does not request periodic hellos
..0. .... .... .... = GTSM Flag: Not set
[Expert Info (Chat/Protocol): GTSM is not supported by the source]
[GTSM is not supported by the source]
[Severity level: Chat]
[Group: Protocol]
...0 0000 0000 0000 = Reserved: 0x0000
IPv4 Transport Address
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: IPv4 Transport Address (0x401)
TLV Length: 4
IPv4 Transport Address: 1.1.1.1 //传输地址
注意：
@hello包是基于UDP 646的，每隔5s发送一次，LDP邻居的失效时间是15s；
LDP邻居的失效时间是15s，224.0.0.2来维护的；
LDP会话失效的时间是45s，keepalive每隔15s发送一次；
LDP邻居的失效时间和LDP会话的失效时间是两回事
@携带了传输地址：默认LSR-ID
int g0/0/0
mpls ldp transport-address interface //把这个接口的地址作为传输地址，此时在hello包中就看不到传输地址的字段了，看不到就是SIP地址和传输地址一样；
@lable space id：标签空间id，等于0就是基于设备的。
1、基于接口的标签空间：一般用于信源模式下，ATM很少使用了已经。就是不同的FEC可以分配相同的标签，转发报文的时候不能单纯的根据标签，而是根据接口+标签。接口上分配的标签必须唯一，在同一个接口上给两个不同的FEC分配的标签不能相同，但是在不同的接口上可以为不同的FEC分配相同的标签。
2、基于设备的标签空间：现在用的都是基于帧模式的MPLS，长度不固定。就是LSRA来说给不同的邻居为FEC分配的是同一个标签，LSRA给自身设备上1000条路由分配的标签都是唯一的。
@两台路由器彼此发送hello包，就能彼此知道传输地址了；
@完成三次握手，传输地址大的一方发起；
R2首先发送init，因为R2的传输地址大：
Frame 16: 90 bytes on wire (720 bits), 90 bytes captured (720 bits) on interface -, id 0
Ethernet II, Src: HuaweiTe_a8:76:56 (00:e0:fc:a8:76:56), Dst: HuaweiTe_ba:13:f8 (00:e0:fc:ba:13:f8)
Internet Protocol Version 4, Src: 2.2.2.2, Dst: 1.1.1.1
Transmission Control Protocol, Src Port: 49178, Dst Port: 646, Seq: 1, Ack: 1, Len: 36
Label Distribution Protocol
Version: 1
PDU Length: 32
LSR ID: 2.2.2.2
Label Space ID: 0
Initialization Message
0... .... = U bit: Unknown bit not set
Message Type: Initialization Message (0x200)
Message Length: 22
Message ID: 0x00000006
Common Session Parameters
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: Common Session Parameters (0x500)
TLV Length: 14
Parameters
Session Protocol Version: 1
Session KeepAlive Time: 45 //keepalive每隔15s发送一次，会话失效时间为45s。
0... .... = Session Label Advertisement Discipline: Downstream Unsolicited proposed //标签通告的方式DU
.0.. .... = Session Loop Detection: Loop Detection Disabled //环路检测的方式：没有disable
Session Path Vector Limit: 0
Session Max PDU Length: 4096
Session Receiver LSR Identifier: 1.1.1.1
Session Receiver Label Space Identifier: 0
R1回复的init+keepalive：
Frame 18: 98 bytes on wire (784 bits), 98 bytes captured (784 bits) on interface -, id 0
Ethernet II, Src: HuaweiTe_ba:13:f8 (00:e0:fc:ba:13:f8), Dst: HuaweiTe_a8:76:56 (00:e0:fc:a8:76:56)
Internet Protocol Version 4, Src: 1.1.1.1, Dst: 2.2.2.2
Transmission Control Protocol, Src Port: 646, Dst Port: 49178, Seq: 1, Ack: 37, Len: 44
Label Distribution Protocol
Version: 1
PDU Length: 40
LSR ID: 1.1.1.1
Label Space ID: 0
Initialization Message
0... .... = U bit: Unknown bit not set
Message Type: Initialization Message (0x200)
Message Length: 22
Message ID: 0x0000000a
Common Session Parameters
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: Common Session Parameters (0x500)
TLV Length: 14
Parameters
Session Protocol Version: 1
Session KeepAlive Time: 45
0... .... = Session Label Advertisement Discipline: Downstream Unsolicited proposed
.0.. .... = Session Loop Detection: Loop Detection Disabled
Session Path Vector Limit: 0
Session Max PDU Length: 4096
Session Receiver LSR Identifier: 2.2.2.2
Session Receiver Label Space Identifier: 0
Keep Alive Message
0... .... = U bit: Unknown bit not set
Message Type: Keep Alive Message (0x201)
Message Length: 4
Message ID: 0x0000000b
R2回复keepalive+address：
Frame 19: 94 bytes on wire (752 bits), 94 bytes captured (752 bits) on interface -, id 0
Ethernet II, Src: HuaweiTe_a8:76:56 (00:e0:fc:a8:76:56), Dst: HuaweiTe_ba:13:f8 (00:e0:fc:ba:13:f8)
Internet Protocol Version 4, Src: 2.2.2.2, Dst: 1.1.1.1
Transmission Control Protocol, Src Port: 49178, Dst Port: 646, Seq: 37, Ack: 45, Len: 40
Label Distribution Protocol
Version: 1
PDU Length: 36
LSR ID: 2.2.2.2
Label Space ID: 0
Keep Alive Message
0... .... = U bit: Unknown bit not set
Message Type: Keep Alive Message (0x201)
Message Length: 4
Message ID: 0x00000007
Address Message
0... .... = U bit: Unknown bit not set
Message Type: Address Message (0x300)
Message Length: 18
Message ID: 0x00000008
Address List
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: Address List (0x101)
TLV Length: 10
Address Family: IPv4 (1)
Addresses
Address 1: 1.1.12.2 //携带了自己的LSR-ID和所有启用了ldp的接口地址
Address 2: 2.2.2.2
假设R2右边连接着R3，R2的g0/0/1接口也启用了MPLS MPLS LDP，如果将此接口上敲undo mpls ldp，此时R2会发送地址撤销报文：
Frame 1866: 82 bytes on wire (656 bits), 82 bytes captured (656 bits) on interface -, id 0
Ethernet II, Src: HuaweiTe_a8:76:56 (00:e0:fc:a8:76:56), Dst: HuaweiTe_ba:13:f8 (00:e0:fc:ba:13:f8)
Internet Protocol Version 4, Src: 2.2.2.2, Dst: 1.1.1.1
Transmission Control Protocol, Src Port: 49178, Dst Port: 646, Seq: 2977, Ack: 2939, Len: 28
Label Distribution Protocol
Version: 1
PDU Length: 24
LSR ID: 2.2.2.2
Label Space ID: 0
Address Withdrawal Message
0... .... = U bit: Unknown bit not set
Message Type: Address Withdrawal Message (0x301)
Message Length: 14
Message ID: 0x00000263
Address List
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: Address List (0x101)
TLV Length: 6
Address Family: IPv4 (1)
Addresses
Address 1: 1.1.23.2
R1也会发送address消息：
Frame 20: 86 bytes on wire (688 bits), 86 bytes captured (688 bits) on interface -, id 0
Ethernet II, Src: HuaweiTe_ba:13:f8 (00:e0:fc:ba:13:f8), Dst: HuaweiTe_a8:76:56 (00:e0:fc:a8:76:56)
Internet Protocol Version 4, Src: 1.1.1.1, Dst: 2.2.2.2
Transmission Control Protocol, Src Port: 646, Dst Port: 49178, Seq: 45, Ack: 77, Len: 32
Label Distribution Protocol
Version: 1
PDU Length: 28
LSR ID: 1.1.1.1
Label Space ID: 0
Address Message
0... .... = U bit: Unknown bit not set
Message Type: Address Message (0x300)
Message Length: 18
Message ID: 0x0000000c
Address List
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: Address List (0x101)
TLV Length: 10
Address Family: IPv4 (1)
Addresses
Address 1: 1.1.12.1
Address 2: 1.1.1.1
R1和R2彼此交换标签映射消息：
注意：R1和R2都会发送两个标签映射消息
R1发送的两个：
Frame 22: 98 bytes on wire (784 bits), 98 bytes captured (784 bits) on interface -, id 0
Ethernet II, Src: HuaweiTe_ba:13:f8 (00:e0:fc:ba:13:f8), Dst: HuaweiTe_a8:76:56 (00:e0:fc:a8:76:56)
Internet Protocol Version 4, Src: 1.1.1.1, Dst: 2.2.2.2
Transmission Control Protocol, Src Port: 646, Dst Port: 49178, Seq: 77, Ack: 77, Len: 44
Label Distribution Protocol
Version: 1
PDU Length: 40
LSR ID: 1.1.1.1
Label Space ID: 0
Label Mapping Message
0... .... = U bit: Unknown bit not set
Message Type: Label Mapping Message (0x400)
Message Length: 30
Message ID: 0x0000000d
FEC
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: FEC (0x100)
TLV Length: 8
FEC Elements
FEC Element 1
FEC Element Type: Prefix FEC (2) //根据什么分类的，根据路由的前缀
FEC Element Address Type: IPv4 (1) //根据IPV4路由的前缀，进行路由分类的
FEC Element Length: 32 //路由表有一条1.1.1.1/32的路由
Prefix: 1.1.1.1
Generic Label
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: Generic Label (0x200)
TLV Length: 4
.... .... .... 0000 0000 0000 0000 0011 = Generic Label: 3 (0x00003) //分配的标签是3，告诉别人我是倒数第一跳
Unknown TLV type (0x0900)
11.. .... = TLV Unknown bits: Unknown TLV, do Forward (0x3)
TLV Type: Unknown TLV type (0x900)
TLV Length: 2
TLV Value: ffff
-------------------------------------------------------------------------------------------------------------------------------------
Frame 26: 98 bytes on wire (784 bits), 98 bytes captured (784 bits) on interface -, id 0
Ethernet II, Src: HuaweiTe_ba:13:f8 (00:e0:fc:ba:13:f8), Dst: HuaweiTe_a8:76:56 (00:e0:fc:a8:76:56)
Internet Protocol Version 4, Src: 1.1.1.1, Dst: 2.2.2.2
Transmission Control Protocol, Src Port: 646, Dst Port: 49178, Seq: 121, Ack: 165, Len: 44
Label Distribution Protocol
Version: 1
PDU Length: 40
LSR ID: 1.1.1.1
Label Space ID: 0
Label Mapping Message
0... .... = U bit: Unknown bit not set
Message Type: Label Mapping Message (0x400)
Message Length: 30
Message ID: 0x0000000e
FEC
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: FEC (0x100)
TLV Length: 8
FEC Elements
FEC Element 1
FEC Element Type: Prefix FEC (2)
FEC Element Address Type: IPv4 (1)
FEC Element Length: 32
Prefix: 2.2.2.2
Generic Label
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: Generic Label (0x200)
TLV Length: 4
.... .... .... 0000 0000 0100 0000 0000 = Generic Label: 1024 (0x00400) //R1也会给2.2.2.2/32的路由分配标签1024
Unknown TLV type (0x0900)
11.. .... = TLV Unknown bits: Unknown TLV, do Forward (0x3)
TLV Type: Unknown TLV type (0x900)
TLV Length: 2
TLV Value: 05dc
R2发送的两个：
Frame 23: 98 bytes on wire (784 bits), 98 bytes captured (784 bits) on interface -, id 0
Ethernet II, Src: HuaweiTe_a8:76:56 (00:e0:fc:a8:76:56), Dst: HuaweiTe_ba:13:f8 (00:e0:fc:ba:13:f8)
Internet Protocol Version 4, Src: 2.2.2.2, Dst: 1.1.1.1
Transmission Control Protocol, Src Port: 49178, Dst Port: 646, Seq: 77, Ack: 121, Len: 44
Label Distribution Protocol
Version: 1
PDU Length: 40
LSR ID: 2.2.2.2
Label Space ID: 0
Label Mapping Message
0... .... = U bit: Unknown bit not set
Message Type: Label Mapping Message (0x400)
Message Length: 30
Message ID: 0x00000009
FEC
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: FEC (0x100)
TLV Length: 8
FEC Elements
FEC Element 1
FEC Element Type: Prefix FEC (2)
FEC Element Address Type: IPv4 (1)
FEC Element Length: 32
Prefix: 1.1.1.1
Generic Label
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: Generic Label (0x200)
TLV Length: 4
.... .... .... 0000 0000 0100 0000 0000 = Generic Label: 1024 (0x00400)
Unknown TLV type (0x0900)
11.. .... = TLV Unknown bits: Unknown TLV, do Forward (0x3)
TLV Type: Unknown TLV type (0x900)
TLV Length: 2
TLV Value: 05dc
-----------------------------------------------------------------------------------------------------------------------------------
Frame 25: 98 bytes on wire (784 bits), 98 bytes captured (784 bits) on interface -, id 0
Ethernet II, Src: HuaweiTe_a8:76:56 (00:e0:fc:a8:76:56), Dst: HuaweiTe_ba:13:f8 (00:e0:fc:ba:13:f8)
Internet Protocol Version 4, Src: 2.2.2.2, Dst: 1.1.1.1
Transmission Control Protocol, Src Port: 49178, Dst Port: 646, Seq: 121, Ack: 121, Len: 44
Label Distribution Protocol
Version: 1
PDU Length: 40
LSR ID: 2.2.2.2
Label Space ID: 0
Label Mapping Message
0... .... = U bit: Unknown bit not set
Message Type: Label Mapping Message (0x400)
Message Length: 30
Message ID: 0x0000000a
FEC
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: FEC (0x100)
TLV Length: 8
FEC Elements
FEC Element 1
FEC Element Type: Prefix FEC (2)
FEC Element Address Type: IPv4 (1)
FEC Element Length: 32
Prefix: 2.2.2.2
Generic Label
00.. .... = TLV Unknown bits: Known TLV, do not Forward (0x0)
TLV Type: Generic Label (0x200)
TLV Length: 4
.... .... .... 0000 0000 0000 0000 0011 = Generic Label: 3 (0x00003)
Unknown TLV type (0x0900)
11.. .... = TLV Unknown bits: Unknown TLV, do Forward (0x3)
TLV Type: Unknown TLV type (0x900)
TLV Length: 2
TLV Value: ffff
查看R1和R2之间LDP的邻居关系：
@第一个2.2.2.2表示对等体的LSR-ID；
@PEER ID后面的0就是标签空间的意思；
@第二个2.2.2.2表示邻居的传输地址；
查看R1和R2之间LDP的会话：
@status：operational就是正常的；
@ssnrole：passive被动方，R2是active主动方，地址大的就是主动方。 //注意这个passive指的R1是passive（自己在会话中的角色），不是邻居是passive；
@KASENT/REC：keeplive发送和接收的计数器，用来维持TCP会话；
查看R1和R2之间LDP会话的详细信息：里面携带address
查看R1和R2的标签转发表象：
欢迎关注WOLF-LAB网络实验室
HCIE Datacom可联系WOLF-LAB网络实验室
电话：173-1636-2402
邮箱：wolflab@wolf-lab.com
地址：上海市徐汇区漕宝路82号E座1902室WOLFLAB实验室
提供： CCNA/HCIA培训 | CCNP/HCIP培训 | HCIE/CCIE培训 | WOLF | 华为认证咨询 | 思科认证咨询 |
官方微信
关注微信公众号
