在IOS-XR上的一个BGP邻居上标记和未标记

本文档介绍在Cisco IOS® XR中通过一个BGP会话接收和通告未标记和已标记^的路径的行为。

先决条件

要求

本文档没有任何特定的要求。

使用的组件

本文档特定于Cisco IOS® ^XR，但不限于特定软件版本或硬件。

本文档中的信息都是基于特定实验室环境中的设备编写的。本文档中使用的所有设备最初均采用原始（默认）配置。如果您使用的是真实网络，请确保您已经了解所有命令的潜在影响。

简介

地址系列标识符(AFI)表示BGP路由的类型。例如，IPv4为1,IPv6为2。

后续地址系列标识符(SAFI)是路由类型的进一步指示。示例为1表示未标记的路由，4表示已标记的路由。

IPv4的未标记单播是AFI 1和SAFI 1。

IPv4的单播标记为AFI 1和SAFI 4。

IPv6的未标记单播是AFI 2和SAFI 1。

IPv6的单播标记为AFI 2和SAFI 4。

标记单播(LU)通常称为RFC 3107“在BGP-4中传输标签信息”。

下面，U表示未标记的单播，因此SAFI 1和LU表示已标记的单播，因此SAFI 4。

请注意，Cisco IOS^® XR需要“allocate-label all| route-policy ...”，否则路由不会作为SAFI 4发起或传播到下一个BGP发言者。

Cisco IOS® XR不支持一个BGP会话上的IPv4/v6单播和标记单^播。

在Cisco IOS® ^XR上，在6.2.1中完成了对在一个BGP会话上同时进行未标记和标记的支持。

当不支持在一个会话上同时运行时，会出现问题，因为上次收到的更新/撤消会覆盖前一个会话，即使它们是在另一个SAFI上收到的。

在运行IOS-XR 6.2.1之前的IOS-XR代码的路由器上，在同一BGP会话上配置SAFI 1和SAFI 4时，路由器会发出以下警告：

bgp[1051]: %ROUTING-BGP-4-INCOMPATIBLE_AFI : IPv4 Unicast and IPv4 Labeled-unicast Address families together are not supported under the same neighbor.

IOS-XR 5.3.0和IOS-XR 5.2.2中引入了此警告消息。

功能交换

BGP对等体之间交换的功能必须匹配。否则，BGP会话不会启动。

这是BGP打开消息中为AFI 1/SAFI 1和AFI 1/SAFI 4交换的功能的Wireshark捕获：

图1

以下是仅在与仅配置了U的IOS的会话上配置了LU的IOS-XR的示例。

IOS-XR:

RP/0/0/CPU0:R4#show bgp neighbor 10.100.1.8
 
BGP neighbor is 10.100.1.8
 Remote AS 65003, local AS 65003, internal link
 Remote router ID 0.0.0.0
  BGP state = Idle
…

  Connections established 0; dropped 0
  Local host: 10.100.1.4, Local port: 179, IF Handle: 0x00000000
  Foreign host: 10.100.1.8, Foreign port: 33396
  Last reset 00:00:14, due to BGP Notification sent: unsupported/disjoint capability
  Time since last notification sent to neighbor: 00:00:14
  Error Code: unsupported/disjoint capability
  Notification data sent:
    None

IOS路由器会针对此配置错误打印系统日志消息：

*Aug  8 12:40:44.719: %BGP-3-NOTIFICATION: received from neighbor 10.100.1.4 active 2/7 (unsupported/disjoint capability) 0 by

IOS-XR实施细节

配置U或LU

在BGP neighbor命令下配置“address-family ipv4 unicast”，以为BGP会话启用ipv4单播。

在BGP neighbor命令下配置“address-family ipv6 unicast”，为BGP会话启用ipv6 unicast。

在BGP neighbor命令下配置“address-family ipv4 labeled-unicast”，以启用BGP会话的ipv4 labeled unicast。

在BGP neighbor命令下配置“address-family ipv6 labeled-unicast”，为BGP会话启用ipv6 labeled unicast。

在IOS-XR中，AFI/SAFI组合是按BGP对等体配置的。

以下是同时具有SAFI 1和4的BGP会话的示例：

router bgp 65003
 address-family ipv4 unicast
  redistribute connected
  allocate-label all unlabeled-path
   …
  neighbor 10.100.1.7
   remote-as 65003
   update-source Loopback0
   address-family ipv4 unicast
    route-reflector-client
   !
   address-family ipv4 labeled-unicast
    route-reflector-client

请注意，路由器BGP下仍然只有“地址系列单播”，而不是“地址系列标签单播”。SAFI 1和4路径都存储在此BGP表中。

BGP表

无论IOS-XR是早于还是高于6.2.1，只有一个BGP表用于存储U和LU路由。这一点显而易见，因为您只能在路由器bgp下配置（启用）“地址系列ipv4单播”或“地址系列ipv6单播”。您不能在路由器bgp下配置“address-family ipv4 labeled-unicast”或“address-family ipv6 labeled-unicast”。

U和LU路径可以相同。在IOS-XR 6.2.1之前，再次接收同一路径，但这一次，无论是否带有标签，都将覆盖之前收到的路径。在IOS-XR 6.2.1之后，如果两条相同的路径仅因标签而不同，则它们会被视为不同。路径添加、删除或修改由不同的SAFI执行。

以下是AFI 1/SAFI 4的BGP表中路由的示例。由于为所有前缀启用了标签分配，因此此路径将与本地标签一起存储。由于只有一个BGP表用于存储U和LU路由，因此前缀会同时显示命令“show bgp ipv4 unicast”和“show bgp ipv4 labeled-unicast”！

RP/0/0/CPU0:R4#show bgp ipv4 unicast 10.100.1.1/32
BGP routing table entry for 10.100.1.1/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                  5           5
    Local Label: 24000
Last Modified: Aug  6 15:03:59.574 for 16:06:13
Paths: (1 available, best #1)
  Advertised to update-groups (with more than one peer):
    0.3 0.4
  Advertised to peers (in unique update groups):
    10.1.45.5
  Path #1: Received by speaker 0
  Advertised to update-groups (with more than one peer):
    0.3 0.4
  Advertised to peers (in unique update groups):
    10.1.45.5
  65002 65001
    10.1.24.2 from 10.1.24.2 (10.100.1.2)
      Received Label 24003
      Origin IGP, localpref 100, valid, external, best, group-best, labeled-unicast
      Received Path ID 0, Local Path ID 0, version 5
      Origin-AS validity: not-found

请注意，路径标有“labeled-unicast”。

RP/0/0/CPU0:R4#show bgp ipv4 labeled-unicast 10.100.1.1/32
BGP routing table entry for 10.100.1.1/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                  5           5
    Local Label: 24000
Last Modified: Aug  6 15:03:59.574 for 16:08:41
Paths: (1 available, best #1)
  Advertised to update-groups (with more than one peer):
    0.3 0.4
  Advertised to peers (in unique update groups):
    10.1.45.5
  Path #1: Received by speaker 0
  Advertised to update-groups (with more than one peer):
    0.3 0.4
  Advertised to peers (in unique update groups):
    10.1.45.5
  65002 65001
    10.1.24.2 from 10.1.24.2 (10.100.1.2)
      Received Label 24003
      Origin IGP, localpref 100, valid, external, best, group-best, labeled-unicast
      Received Path ID 0, Local Path ID 0, version 5
      Origin-AS validity: not-found

请注意，路径标有“labeled-unicast”。

如果路径同时作为U和LU存在，则本地路径ID不同。

RP/0/0/CPU0:R4#show bgp ipv4 labeled-unicast 10.100.1.1/32 detail
BGP routing table entry for 10.100.1.1/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                 30          30
    Local Label: 24003 (no rewrite);
    Flags: 0x00003028+0x00010000;
Last Modified: Aug 30 10:45:50.502 for 00:01:59
Paths: (2 available, best #1)
  Advertised IPv4 Unicast paths to peers (in unique update groups):
    10.100.1.8      10.100.1.9
  Advertised IPv4 Labeled-unicast paths to update-groups (with more than one peer):
    0.8
  Path #1: Received by speaker 0
  Flags: 0x4000000009060205, import: 0x20
  Advertised IPv4 Unicast paths to peers (in unique update groups):
    10.100.1.8      10.100.1.9
  Advertised IPv4 Labeled-unicast paths to update-groups (with more than one peer):
    0.8
  65001, (Received from a RR-client)
    10.100.1.9 (metric 2) from 10.100.1.9 (10.100.1.9)
      Origin IGP, metric 0, localpref 100, valid, internal, best, group-best
      Received Path ID 0, Local Path ID 1, version 29
  Path #2: Received by speaker 0
  Flags: 0x4080000008020205, import: 0x20
  Not advertised to any peer
  65001, (Received from a RR-client)
    10.100.1.9 (metric 2) from 10.100.1.9 (10.100.1.9)
      Received Label 24001
      Origin IGP, metric 0, localpref 100, valid, internal, labeled-unicast
      Received Path ID 0, Local Path ID 0, version 0

启用本地标签分配

您必须配置命令“allocate-label”，以便BGP中的接收或源路径具有本地MPLS标签。如果没有此命令，路由将没有本地标签。

RP/0/0/CPU0:R4#conf t
RP/0/0/CPU0:R4(config)#router bgp 65003
RP/0/0/CPU0:R4(config-bgp)# address-family ipv4 unicast
RP/0/0/CPU0:R4(config-bgp-af)#allocate-label ?
  all           Allocate labels for all prefixes
  route-policy  Use a route policy to select prefixes for label allocation

标签分配适用于所有路由或根据已配置的路由策略进行。

IOS-XR上的新实施

在IOS-XR的旧实施中，在同一BGP会话上配置U和LU时会发出警告。警告在IOS-XR版本5.3.0和5.2.2中引入。警告在IOS-XR版本6.2.1中删除，因为同一BGP会话支持标记和未标记。

示例：

RP/0/0/CPU0:ios#conf t
RP/0/0/CPU0:ios(config)#router bgp 65001
RP/0/0/CPU0:ios(config-bgp)#add ipv4 unicast
RP/0/0/CPU0:ios(config-bgp-af)#exit
RP/0/0/CPU0:ios(config-bgp)#neighbor 10.0.0.1
RP/0/0/CPU0:ios(config-bgp-nbr)#remote-as 65001
RP/0/0/CPU0:ios(config-bgp-nbr)#exit
RP/0/0/CPU0:ios(config-bgp)#neighbor 10.0.0.1
RP/0/0/CPU0:ios(config-bgp-nbr)#address-family ipv4 unicast
RP/0/0/CPU0:ios(config-bgp-nbr-af)#exit
RP/0/0/CPU0:ios(config-bgp-nbr)#address-family ipv4 labeled-unicast
RP/0/0/CPU0:ios(config-bgp-nbr-af)#commit

RP/0/0/CPU0:Aug 21 14:14:22.222 : bgp[1052]: %ROUTING-BGP-4-INCOMPATIBLE_AFI : IPv4 Unicast and IPv4 Labeled-unicast Address families together are not supported under the same neighbor. 

此错误消息的说明：

此消息表明用户已在同一邻居下同时配置了IPv4单播和IPv4 Labeled-unicast或IPv6 Unicast和IPv6 Labeled-unicast地址系列。不支持此特定配置。 

建议的操作：配置到路由器的两个邻居会话。在第一个邻居会话下配置单播地址系列，在第二个邻居会话下配置标记单播地址系列。

一对IOS-XR路由器之间两个BGP会话的配置示例。对每个BGP会话使用不同的（环回）地址。

hostname R1

interface Loopback0
 ipv4 address 10.100.1.1 255.255.255.255
!
interface Loopback1
 ipv4 address 10.100.1.101 255.255.255.255
!
router bgp 65001
 address-family ipv4 unicast
 !
 neighbor 10.100.1.2
  remote-as 65001
 update-source Loopback0
  address-family ipv4 unicast
  !
 !
 neighbor 10.100.1.102
  remote-as 65001
  update-source Loopback1
  address-family ipv4 labeled-unicast
  !
 ! 
   
hostname R2

interface Loopback0
 ipv4 address 10.100.1.2 255.255.255.255
!
interface Loopback1
 ipv4 address 10.100.1.102 255.255.255.255
!
router bgp 65001
 address-family ipv4 unicast
 !
neighbor 10.100.1.1
  remote-as 65001
 update-source Loopback0
  address-family ipv4 unicast
  !
 !
 neighbor 10.100.1.101
  remote-as 65001
  update-source Loopback1
  address-family ipv4 labeled-unicast
  !
 !

在IOS-XR 6.2.1中，U和LU在默认VRF上的同一BGP会话上都受支持!

无论BGP会话是内部BGP还是外部BGP都无关紧要。

在任何非默认VRF中，BGP扬声器不支持同一会话上的U和LU。

更新组

在IOS-XR 6.2.1之前，所有U、LU和U + LU BGP扬声器都保留在单独的更新组中。在IOS-XR版本6.2.1之后，情况不再如此。一个更新组中的某些BGP扬声器只能是U或LU，或同时是U和LU。

路由传播

下表显示了不同场景的广告和撤消行为。有16种方案。

除非注释列中另有说明，否则所有内容均适用于IOS-XR版本6.2.1及更高版本。

案例	最佳路径/地址路径类型	存在本地标签？	NHS或NHU	更新组SAFI	广告还是退出？	备注
1	未标记路径，即无接收器标签	Yes	NHS	SAFI-1	默认情况下通告 退回 advertise local-labeled-route(safi-unicast)disable命令	仅在6.5.1之后可能。
2				SAFI-4	通告	仅在6.5.1之后可能。 IPv4/v6 redist路由和6PE:隐式NHS始终
3			NHU	SAFI-1	通告	仅在6.5.1之后可能。
4				SAFI-4	退回	仅在6.5.1之后可能。 IPv4/v6 redist路由和6PE:NHU被忽略；隐式NHS始终
5		无	NHS	SAFI-1	通告
6				SAFI-4	退回
7			NHU	SAFI-1	通告
8				SAFI-4	退回
9	标记路径，即带有rx标签	Yes	NHS	SAFI-1	默认情况下通告 使用advertise local-labeled-route(safi-unicast)disable命令撤消	6.2.1之前：默认行为是通告。
10				SAFI-4	通告
11			NHU	SAFI-1	退回	6.2.1之前： 行为是通告。
12				SAFI-4	通告
13 个		无	NHS	SAFI-1	通告
14				SAFI-4	退回
15			NHU	SAFI-1	退回	6.2.1之前： 行为是通告。
16				SAFI-4	通告

表1 iBGP和eBGP会话的通告行为

NHS =下一跳自己

NHU =下一跳未更改。

如果NHU生效，这意味着未为iBGP会话配置下一跳自己。

请注意，BGP扬声器向eBGP对等体发送消息时，NHS始终是这种情况。

NHS或NHU可能针对iBGP扬声器，具体取决于下一跳自身的配置。对iBGP对等体的默认行为是NHU。

第二列：请注意，仅当最佳路径或标记有add-path的路径之一未标记或标记时，路径才被视为未标记或标记。

对于路由传播，最佳路径的特征至关重要。根据特征（列2到4），它确定路径是通告为U还是LU，还是同时通告为两者。

如果启用了功能Additional Paths(ADD-PATH)，并且路径标有“add-path”，则该路径的特征也将扮演如何通告该路径的角色。

“存在本地标签？:“否”表示：可能收到带有已接收更新的标签，但标签未安装。如果“allocate-label”命令不存在，则不会安装本地标签。

您可以通过详细查看前缀来验证本地标签是否存在。使用“show bgp <prefix> detail”或“show route <prefix> detail”。

在以下示例中，接收前缀时不带标签（通过SAFI 1对等），并且不分配本地标签：

RP/0/0/CPU0:R2#show bgp ipv4 labeled-unicast 10.100.1.5/32 detail
BGP routing table entry for 10.100.1.5/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                  3           3
    Flags: 0x04001001+0x00000000;
Last Modified: Sep  5 03:44:45.647 for 01:01:27
Paths: (1 available, best #1)
  Advertised to update-groups (with more than one peer):
    0.3
  Path #1: Received by speaker 0
  Flags: 0x4000000001040207, import: 0x00
  Advertised to update-groups (with more than one peer):
    0.3
  Local, (Received from a RR-client)
    10.100.1.1 (metric 2) from 10.100.1.1 (10.100.1.1)
      Origin incomplete, metric 0, localpref 100, valid, internal, best, group-best
      Received Path ID 0, Local Path ID 1, version 3

RP/0/0/CPU0:R2#show route 10.100.1.5/32 detail

Routing entry for 10.100.1.5/32
  Known via "bgp 65001", distance 200, metric 0, type internal
  Installed Sep  5 03:44:45.480 for 01:01:37
  Routing Descriptor Blocks
    10.100.1.1, from 10.100.1.1
      Route metric is 0
      Label: None
      Tunnel ID: None
      Extended communities count: 0
      NHID:0x0(Ref:0)
  Route version is 0x23 (35)
  No local label
  IP Precedence: Not Set
  QoS Group ID: Not Set
  Flow-tag: Not Set
  Route Priority: RIB_PRIORITY_RECURSIVE (12) SVD Type RIB_SVD_TYPE_LOCAL
  Download Priority 4, Download Version 52
  No advertising protos.

分配本地标签

为未标记路径分配标签

默认情况下，无标记路径(SAFI 1)从不进行标记，即使配置了“allocate-label”命令也是如此。

自IOS-XR版本6.5.1起，“allocate-label”命令使用关键字“unlabeled-path”，因此也可以为未标记的路径分配标签。

RP/0/0/CPU0:R4#conf t
RP/0/0/CPU0:R4(config)#router bgp 65003
RP/0/0/CPU0:R4(config-bgp)# address-family ipv4 unicast
RP/0/0/CPU0:R4(config-bgp-af)#allocate-label all ?
  unlabeled-path  Allocate label for unlabeled paths too
  <cr>
RP/0/0/CPU0:R4(config-bgp-af)#allocate-label all unlabeled-path ?
  <cr>
RP/0/0/CPU0:R4(config-bgp-af)#allocate-label all unlabeled-path
RP/0/0/CPU0:R4(config-bgp-af)#commit

该路径是SAFI 1路径，因此没有收到标签。

由于使用了“unlabeled-path”命令，现在有一个本地标签。

RP/0/0/CPU0:R4#show bgp ipv4 labeled-unicast 10.100.1.1/32 detail
BGP routing table entry for 10.100.1.1/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                 16          16
    Local Label: 24003 (no rewrite);
    Flags: 0x01303028+0x00000000;
Last Modified: Aug 27 19:08:47.502 for 00:00:59
Paths: (1 available, best #1)
  Advertised IPv4 Unicast paths to update-groups (with more than one peer):
    0.3
  Advertised IPv4 Labeled-unicast paths to update-groups (with more than one peer):
    0.7
  Advertised IPv4 Labeled-unicast paths to peers (in unique update groups):
    10.1.45.5
  Path #1: Received by speaker 0
  Flags: 0x4000000009040207, import: 0x20
  Advertised IPv4 Unicast paths to update-groups (with more than one peer):
    0.3
  Advertised IPv4 Labeled-unicast paths to update-groups (with more than one peer):
    0.7
  Advertised IPv4 Labeled-unicast paths to peers (in unique update groups):
    10.1.45.5
  65001, (Received from a RR-client)
    10.100.1.10 (metric 2) from 10.100.1.10 (10.100.1.10)
      Origin IGP, metric 0, localpref 100, valid, internal, best, group-best
      Received Path ID 0, Local Path ID 1, version 16

RP/0/0/CPU0:R4#show route 10.100.1.1/32 detail

Routing entry for 10.100.1.1/32
  Known via "bgp 65003", distance 200, metric 0
  …
  Route version is 0x4 (4)
  Local Label: 0x5dc3 (24003)
  IP Precedence: Not Set
  …

这将允许表1中的问题1到4。

要了解删除“allocate-label”命令时仍分配本地标签的原因，请运行debug bgp label。

以下是一个示例：

RP/0/0/CPU0:R4#debug bgp label

RP/0/0/CPU0:R4#show debug

####  debug flags set from tty 'con0_0_CPU0'  ####
ip-bgp default label flag is ON with value '##########'

最好为特定前缀或前缀组启用此调试。示例如下：

RP/0/0/CPU0:R4#sh running-config route-policy match-prefix
route-policy match-prefix
  if destination in (10.100.1.1/32) then
    pass
  else
    drop
  endif
end-policy
!

RP/0/0/CPU0:R4#debug bgp label route-policy match-prefix 

RP/0/0/CPU0:R4#show debug

####  debug flags set from tty 'con0_0_CPU0'  ####

ip-bgp default label flag is ON with value '######match-prefix####'

RP/0/0/CPU0:R4#con t
RP/0/0/CPU0:R4(config)#router bgp 65003
RP/0/0/CPU0:R4(config-bgp)# address-family ipv4 unicast
RP/0/0/CPU0:R4(config-bgp-af)#no   allocate-label all
RP/0/0/CPU0:R4(config-bgp-af)#commit

RP/0/0/CPU0:Aug 23 12:43:02.786 : bgp[1048]: [default-lbl] (ip4u): Label computation done: table=TBL:default (1/1), net=10.100.1.1/32: netfl=0x05043001, path=0x1073ed5c(10.1.24.2/32,10.1.24.2,0,0x400000000d060001), pathrcvdlabel=24002: asbr=1, rr=0/1, nhselfcount=1: result="label required"

您可以看到，此路由器收到前缀10.100.1.1/32的标签，是ASBR，不是RR，并且至少有一个BGP会话具有下一跳自己。这会导致此前缀需要本地标签。

本地标签仍保留：

RP/0/0/CPU0:R4#show bgp ipv4 unicast 10.100.1.1/32 detail
BGP routing table entry for 10.100.1.1/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                 13          13
    Local Label: 16002 (no rewrite);
    Flags: 0x05043001+0x00000200;
Last Modified: Aug 23 12:37:11.133 for 00:05:53
Paths: (1 available, best #1)
  Advertised to update-groups (with more than one peer):
    0.6
  Advertised to peers (in unique update groups):
    10.1.46.6       10.100.1.8      10.100.1.7
  Path #1: Received by speaker 0
  Flags: 0x400000000d060001, import: 0x1f
  Advertised to update-groups (with more than one peer):
    0.6
  Advertised to peers (in unique update groups):
    10.1.46.6       10.100.1.8      10.100.1.7
  65002 65001
    10.1.24.2 from 10.1.24.2 (10.100.1.2)
      Received Label 24002
      Origin IGP, localpref 100, valid, external, best, group-best, import-candidate
      Received Path ID 0, Local Path ID 1, version 13
      Origin-AS validity: not-found


RP/0/0/CPU0:R4#show route 10.100.1.1/32 detail 
Routing entry for 10.100.1.1/32
  Known via "bgp 65003", distance 20, metric 0, [ei]-bgp, labeled unicast (3107)
  Tag 65002, type external
  Installed Aug 23 12:37:11.440 for 00:06:02
  Routing Descriptor Blocks
    10.1.24.2, from 10.1.24.2, BGP external
      Route metric is 0
      Label: 0x5dc2 (24002)
      Tunnel ID: None
      Extended communities count: 0
      NHID:0x0(Ref:0)
  Route version is 0x4 (4)
  Local Label: 0x3e82 (16002)
  IP Precedence: Not Set
  QoS Group ID: Not Set
  Route Priority: RIB_PRIORITY_NON_RECURSIVE_LOW (11) SVD Type RIB_SVD_TYPE_LOCAL
 Download Priority 4, Download Version 28
  No advertising protos.

当不需要本地标签时，调试显示以下消息：

RP/0/0/CPU0:Aug 23 13:01:15.801 : bgp[1048]: [default-lbl]: Prefix 10.100.1.1/32:()doesn't require label, releasing

RP/0/0/CPU0:Aug 23 13:01:15.801 : bgp[1048]: [default-lbl]: bgp_label_release_label: perform label release onnet 10.100.1.1/32net retain 0 label_retain 0

如果前缀在LFIB中，则取决于前缀是否被标记，以及allocate-label是否适用于该前缀。

收到的标签为24002，表示以下前缀。  由于BGP没有allocate-label命令，因此LFIB中未安装该命令。

RP/0/0/CPU0:R4#show bgp ipv4 unicast  10.100.1.1/32
BGP routing table entry for 10.100.1.1/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                  4           4
    Local Label: 24002
Last Modified: Aug  8 13:52:57.276 for 00:00:36
Paths: (1 available, best #1)
  Advertised to update-groups (with more than one peer):
    0.6
  Advertised to peers (in unique update groups):
    10.100.1.7
  Path #1: Received by speaker 0
  Advertised to update-groups (with more than one peer):
    0.6
  Advertised to peers (in unique update groups):
    10.100.1.7
  65002 65001
    10.1.24.2 from 10.1.24.2 (10.100.1.2)
      Received Label 24002
      Origin IGP, localpref 100, valid, external, best, group-best, labeled-unicast
      Received Path ID 0, Local Path ID 0, version 4
      Origin-AS validity: not-found

router bgp 65003
 bgp unsafe-ebgp-policy
 address-family ipv4 unicast
 !

RP/0/0/CPU0:R4# show mpls forwarding
Local  Outgoing    Prefix             Outgoing     Next Hop        Bytes
Label  Label       or ID              Interface                    Switched
------ ----------- ------------------ ------------ --------------- ------------
24000  Aggregate   10.1.24.0/24       default                      0
24001  Aggregate   10.1.45.0/24       default                      0

如果allocate-label命令存在，则本地标签在LFIB中存在：

router bgp 65003
 bgp unsafe-ebgp-policy
 address-family ipv4 unicast
  allocate-label all
 !

RP/0/0/CPU0:R4#show mpls forwarding
Local  Outgoing    Prefix             Outgoing     Next Hop        Bytes
Label  Label       or ID              Interface                    Switched
------ ----------- ------------------ ------------ --------------- ------------
24000  Aggregate   10.1.24.0/24       default                      0
24001  Aggregate   10.1.45.0/24       default                      0
24002  24002       10.100.1.1/32                   10.1.24.2       0

即使BGP前缀通过LU会话接收，但未分配本地标签，在NHS完成后，路由也不会通过另一个LU会话通告。表1中的情况为14。如果传出BGP会话为eBGP，则情况为14。

示例：

RP/0/0/CPU0:R2#show bgp ipv4 unicast 10.100.1.1/32 detail
BGP routing table entry for 10.100.1.1/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                  3           3
    Flags: 0x00001001+0x00000000;
Last Modified: Aug 22 09:00:20.646 for 00:10:56
Paths: (1 available, best #1)
  Not advertised to any peer
  Path #1: Received by speaker 0
  Flags: 0x4080000001060001, import: 0x20
  Not advertised to any peer
  65001
    10.1.12.1 from 10.1.12.1 (10.100.1.1)
      Received Label 3
      Origin IGP, metric 0, localpref 100, valid, external, best, group-best, labeled-unicast
      Received Path ID 0, Local Path ID 0, version 3
      Origin-AS validity: not-found

RP/0/0/CPU0:R2#show route 10.100.1.1/32 detail

Routing entry for 10.100.1.1/32
  Known via "bgp 65002", distance 20, metric 0, labeled unicast (3107)
  Tag 65001, type external
  Installed Aug 22 09:00:20.416 for 00:10:59
  Routing Descriptor Blocks
    10.1.12.1, from 10.1.12.1, BGP external
      Route metric is 0
      Label: 0x100004 (1048580)
      Tunnel ID: None
      Binding Label: None
      Extended communities count: 0
      NHID:0x0(Ref:0)
  Route version is 0x1 (1)
  No local label
  IP Precedence: Not Set
  …

这可能是因为路由器BGP下没有地址系列单播的命令“allocate-label”。

删除“allocate-label”命令时，您需要重新启动进程BGP，以便路由器删除BGP路由的本地标签。

表1中的新advertise local-labeled-route命令是一个新命令，用于指示具有本地标签的路由不应通过SAFI-1通告为无标签路由。

此命令如下：

   advertise local-labeled-route [disable]

此命令在neighbor address-family下配置。此命令的功能是指示是否应通过IPv4/v6单播(SAFI 1)将带有本地标签的IPv4/v6路由通告给BGP邻居。

默认行为是通告带有本地标签的路由。

新命令也可配置为：

   advertise local-labeled-route safi-unicast [disable]

此命令在BGP部分的af-group下配置。其功能与上述功能相同，适用于所有BGP邻居。

默认行为是通告带有本地标签的路由。

地址系列IPv4单播下的“Advertise routes with local-label via Unicast SAFI”或“Do not advertise routes with local-label via Unicast SAFI”一行显示在命令上，表示BGP扬声器允许通告带有本地标签的路由。

默认行为示例：

RP/0/0/CPU0:R4#show bgp neighbor 10.1.45.5
…
For Address Family: IPv4 Unicast
  BGP neighbor version 5
  Update group: 0.1 Filter-group: 0.5  No Refresh request being processed
    Extended Nexthop Encoding: advertised and received
  Route refresh request: received 0, sent 0
  0 accepted prefixes, 0 are bestpaths
  Exact no. of prefixes denied : 0.
  Cumulative no. of prefixes denied: 0.
  Prefix advertised 2, suppressed 0, withdrawn 0
  Maximum prefixes allowed 1048576
  Threshold for warning message 75%, restart interval 0 min
  An EoR was not received during read-only mode
  Last ack version 5, Last synced ack version 0
  Outstanding version objects: current 0, max 1, refresh 0
  Additional-paths operation: None
  Advertise routes with local-label via Unicast SAFI
…

或

RP/0/0/CPU0:R4# conf t
RP/0/0/CPU0:R4(config)#router bgp 65003
RP/0/0/CPU0:R4(config-bgp)# neighbor 10.1.45.5
RP/0/0/CPU0:R4(config-bgp-nbr)#  address-family ipv4 unicast
RP/0/0/CPU0:R4(config-bgp-nbr-af)#advertise local-labeled-route disable
RP/0/0/CPU0:R4(config-bgp-nbr-af)#commit


RP/0/0/CPU0:R4#show bgp neighbor 10.1.45.5

BGP neighbor is 10.1.45.5
…
For Address Family: IPv4 Unicast
  BGP neighbor version 5
  Update group: 0.1 Filter-group: 0.5 (Update-group Change
pending)
  No Refresh request being processed
    Extended Nexthop Encoding: advertised and received
  Route refresh request: received 0, sent 0
  0 accepted prefixes, 0 are bestpaths
  Exact no. of prefixes denied : 0.
  Cumulative no. of prefixes denied: 0.
  Prefix advertised 2, suppressed 0, withdrawn 0
  Maximum prefixes allowed 1048576
  Threshold for warning message 75%, restart interval 0 min
  An EoR was not received during read-only mode
  Last ack version 5, Last synced ack version 0
  Outstanding version objects: current 0, max 1, refresh 0
  Additional-paths operation: None
  Do not advertise routes with local-label via Unicast SAFI

最佳路径计算

最佳路径计算过程中没有更改。如果路径是SAFI 1或SAFI 4，或者路径是否带有标签，则最佳路径计算过程不会有任何差异。因此，SAFI 1或SAFI 4路径之间没有首选项。无论同一BGP会话或不同会话上是否存在SAFI 1/SAFI 4。因此，如果一个BGP会话是SAFI 1和4，并且在两个地址系列上都收到前缀，则最佳路径计算将选择一个作为最佳路径，因为所有属性都相同。如果U和LU路径之间的所有BGP属性相同，则最后收到的路径成为最佳路径。

如果从不同的BGP对等体接收SAFI 1和SAFI 4路径，则通向BGP的路径始终存在差异，始终从两条路径中选择相同的最佳路径。即使在这种情况下，所有属性都相同，邻居地址也不同。查看BGP最佳路径选择算法，从具有最低邻居地址的邻居选择路径（最后步骤13）作为最佳路径。

使用命令“show bgp <AFI> <SAFI> <prefix> bestpath-compare”验证最佳路径最佳的原因。

此首选项可由用户使用RPL进行。

下面是此类RPL的示例。

RP/0/0/CPU0:R7#show bgp ipv4 un 10.100.1.1/32 detail
BGP routing table entry for 10.100.1.1/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                682         682
    Flags: 0x00003001+0x00010000;
Last Modified: Aug 28 13:16:26.826 for 00:00:10
Paths: (2 available, best #2)
  Not advertised to any peer
  Path #1: Received by speaker 0
  Flags: 0x4000000000020005, import: 0x20
  Not advertised to any peer
  65001
    10.100.1.4 (metric 2) from 10.100.1.4 (10.100.1.10)
      Origin IGP, metric 0, localpref 100, valid, internal
      Received Path ID 1, Local Path ID 0, version 0
      Originator: 10.100.1.10, Cluster list: 10.100.1.4
  Path #2: Received by speaker 0
  Flags: 0x4080000001060005, import: 0x20
  Not advertised to any peer
  65001
    10.100.1.4 (metric 2) from 10.100.1.4 (10.100.1.10)
      Received Label 24003
      Origin IGP, metric 0, localpref 100, valid, internal, best, group-best, labeled-unicast
      Received Path ID 1, Local Path ID 0, version 682
      Originator: 10.100.1.10, Cluster list: 10.100.1.4

LU路径是最佳路径。

带权重的RPL用于首选U路径。

route-policy weight
  if destination in (10.100.1.1/32) then
    set weight 60000
  endif
end-policy

router bgp 65003
 address-family ipv4 unicast
  additional-paths receive
  additional-paths send
 !
 neighbor 10.100.1.4
  remote-as 65003
  update-source Loopback0
  address-family ipv4 unicast
   route-policy weight in
  !
  address-family ipv4 labeled-unicast
  !
 !

RP/0/0/CPU0:R7#show bgp ipv4 un 10.100.1.1/32 bestpath-compare
BGP routing table entry for 10.100.1.1/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                726         726
Last Modified: Aug 28 13:39:27.826 for 00:04:54
Paths: (2 available, best #1)
  Not advertised to any peer
  Path #1: Received by speaker 0
  Not advertised to any peer
  65001
    10.100.1.4 (metric 2) from 10.100.1.4 (10.100.1.10)
      Origin IGP, metric 0, localpref 100, weight 60000, valid, internal, best, group-best
      Received Path ID 1, Local Path ID 0, version 726
      Originator: 10.100.1.10, Cluster list: 10.100.1.4
      best of AS 65001, Overall best
  Path #2: Received by speaker 0
  Not advertised to any peer
  65001
    10.100.1.4 (metric 2) from 10.100.1.4 (10.100.1.10)
      Received Label 24003
      Origin IGP, metric 0, localpref 100, valid, internal, labeled-unicast
      Received Path ID 1, Local Path ID 0, version 0
      Originator: 10.100.1.10, Cluster list: 10.100.1.4
      Lower weight than best path (path #1)

U路径现在是最佳路径。

没有新命令可优先选择标记路径而非未标记路径。您只需在地址系列单播或BGP邻居下的标记单播下配置RPL。

检验BGP中的行为

要调试IOS-XR中的BGP更新传播，可以打开以下debug命令：debug bgp update <BGP neighbor> in |输出。

这将显示从该BGP扬声器传入或传出的BGP更新。对于未标记的IPv4未播(AFI 1/SAFI 1)，地址系列显示为(ip4u)；对于已标记的IPv4单播(AFI 1/SAFI 4)，显示为(ipv4lu)。 IPv6也会出现这种情况。

有一个新字段“labeled-unicast”，表示路径是通过SAFI 4获知的。

示例：

RP/0/0/CPU0:R1#show bgp ipv4 unicast 10.100.1.7/32
BGP routing table entry for 10.100.1.7/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                 26          26
Last Modified: Sep  4 10:45:44.551 for 00:29:11
Paths: (1 available, best #1)
  Not advertised to any peer
  Path #1: Received by speaker 0
  Not advertised to any peer
  Local
    10.100.1.4 (metric 3) from 10.100.1.102 (10.100.1.4)
      Received Label 24000
      Origin incomplete, metric 0, localpref 100, valid, internal, best, group-best, labeled-unicast
      Received Path ID 0, Local Path ID 1, version 26
      Originator: 10.100.1.4, Cluster list: 10.100.1.2

要验证前缀是否通告，可以在末尾使用“show bgp ... neighbors”命令和关键字“advertised-routes”。

示例：

R4两次向邻居10.100.1.7通告10.100.1.1/32，因为已启用add-path（两条路径不同）。

RP/0/0/CPU0:R4#show bgp ipv4 labeled-unicast neighbors 10.100.1.7 advertised-routes
Network            Next Hop        From            AS Path
10.1.24.0/24       10.100.1.4      Local           ?
10.1.34.0/24       10.100.1.4      Local           ?
10.1.45.0/24       10.100.1.4      Local           ?
10.1.46.0/24       10.100.1.4      Local           ?
10.1.47.0/24       10.100.1.4      Local           ?
10.1.48.0/24       10.100.1.4      Local           ?
10.1.49.0/24       10.100.1.4      Local           ?
10.1.104.0/24      10.100.1.4      Local           ?
10.1.114.0/24      10.100.1.4      Local           ?
10.100.1.1/32      10.100.1.4      10.100.1.9      65001i
                                   10.100.1.10     65001i
10.100.1.4/32      10.100.1.4      Local           ?

Processed 11 prefixes, 12 paths

iBGP和下一跳自身 — 统一MPLS

表1中的规则适用。使用统一MPLS或无缝MPLS时，区域边界路由器(ABR)既充当路由反射器，也是iBGP路由的下一跳。ABR位于标记流量的转发路径中。ABR必须具有next-hop-self的显式配置。

解决方法

• 如果在一个BGP会话上不支持U和LU，则解决方法是在一对路由器之间有两个BGP会话。每台路由器上必须使用两个（环回）接口。

IOS-XR的配置示例。

interface Loopback0
 ipv4 address 10.100.1.7 255.255.255.255
!
interface Loopback1
 ipv4 address 10.100.1.107 255.255.255.255
!
router bgp 65003
 address-family ipv4 unicast
!
 neighbor 10.100.1.4   -> towards loopback0 on peer
  remote-as 65003
  update-source Loopback0
  address-family ipv4 unicast
  !
 !
 neighbor 10.100.1.104  -> towards loopback1 on peer
  remote-as 65003
  update-source Loopback1
  address-family ipv4 labeled-unicast
  !

U和LU路径通过两个不同的BGP会话发送/接收。

RP/0/0/CPU0:R7#show bgp ipv4 unicast 10.100.1.1/32 detail
BGP routing table entry for 10.100.1.1/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                753         753
    Flags: 0x00001001+0x00010000;
Last Modified: Aug 28 14:06:40.826 for 00:22:10
Paths: (2 available, best #1)
  Not advertised to any peer
  Path #1: Received by speaker 0
  Flags: 0x4000000001060005, import: 0x20
  Not advertised to any peer
  65001
    10.100.1.4 (metric 2) from 10.100.1.4 (10.100.1.10)
      Origin IGP, metric 0, localpref 100, valid, internal, best, group-best
      Received Path ID 1, Local Path ID 0, version 753
      Originator: 10.100.1.10, Cluster list: 10.100.1.4
  Path #2: Received by speaker 0
  Flags: 0x4080000000020005, import: 0x20
  Not advertised to any peer
  65001
    10.100.1.104 (metric 2) from 10.100.1.104 (10.100.1.10)
      Received Label 24003
      Origin IGP, metric 0, localpref 100, valid, internal, labeled-unicast
      Received Path ID 1, Local Path ID 0, version 0
      Originator: 10.100.1.10, Cluster list: 10.100.1.4