使用BGP社区值配置和控制上游提供商网络

简介

本文档介绍如何使用BGP社区值控制上游提供商网络中的路由策略。

先决条件

要求

本文档要求对 BGP 路由协议及其操作有所了解。

使用的组件

本文档不限于特定的软件和硬件版本。但是，本文档中的信息基于以下软件和硬件版本：

• Cisco IOS® 软件版本 12.2(27)

• Cisco 2500 系列路由器

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。如果您的网络处于活动状态，请确保您了解所有命令的潜在影响。

背景信息

虽然社区本身不会更改BGP最佳路径进程，但社区可以用作标记，以标记一组路由。然后，上游服务提供商路由器可以使用这些标志在其网络内应用特定路由策略（例如，本地首选项）。

提供商在您的可配置社区值和提供商网络中相应的本地优先级值之间进行映射。您可以具有特定策略，这些策略要求在提供商网络中修改LOCAL_PREF，在其路由更新中设置相应的社区值。

社区是一组前缀，这些前缀共享某个公共属性，并且可以使用 BGP 社区属性进行配置。BGP 社区属性是一种长度可变的、可传递的可选属性。该属性由一组（四个）用于指定社区的八位组值组成。社区属性值在前两个二进制八位数中使用自治系统(AS)编号进行编码，另外两个二进制八位数由AS定义。前缀可具有多个社区属性。在前缀中看到多个社区属性的BGP发言者可以根据一个属性、部分属性或所有属性进行操作。在路由器将社区属性传递到其他对等体之前，路由器可以添加或修改该属性。要了解有关社区属性的详细信息，请参阅 BGP 案例分析。

本地优先级属性是AS首选到达特定网络的路径的指示。当有多条路径指向同一目标时，将选择具有较高优先级的路径（本地优先级属性的默认值为100）。 有关详细信息，请参阅案例研究。

规则

有关文件规则的更多信息请参见“ Cisco技术提示规则”。

配置和控制路由策略

注意：要寻找关于用于本文的指令的其他信息，请使用命令查找工具.

为了简化，假设在上游服务提供商(AS100)和您的设备(AS30)之间建立社区属性和本地优先级属性映射。

本地首选	社区值
130	100:300
125	100:250

如果前缀的社区属性等于100:300，则上游服务提供商将这些路由的本地优先级设置为130和125（如果社区属性等于100:250）。

如果更改通告给服务提供商的前缀的社区值，则这样您就可以控制服务提供商网络中的路由策略。

在网络图中，AS 30希望将此路由策略与社区属性一起使用。

• 从AS 100入站到网络10.0.10.0/24的流量通过R1-R3链路传输。如果R1-R3链路发生故障，所有流量都会通过R2-R3。

• 从AS 100发往网络10.1.0.0/24的入站流量通过R2-R3链路传输。如果R2-R3链路发生故障，所有流量都会通过R1-R3。

为了实现此路由策略，R3通告其前缀如下：

到 R1：

• 10.0.10.0/24，社区属性为 100:300
• 10.1.0.0/24，社区属性为 100:250

到 R2：

• 10.0.10.0/24，社区属性为 100:250

• 10.1.0.0/24，社区属性为 100:300

一旦BGP邻居R1和R2收到来自R3的前缀，R1和R2将根据社区和本地首选属性之间的映射应用配置的策略（如上表所示），从而实现您(AS 30)指定的路由策略。R1 在 BGP 表中安装以下前缀：

• 10.0.10.0/24，本地优先级为 130

• 10.1.0.0/24，本地优先级为 125

R2 在其 BGP 表中安装以下前缀：

• 10.0.10.0/24，本地优先级为 125

• 10.1.0.0/24，本地优先级为 130

由于BGP路径选择标准中首选较高的本地优先级，因此本地优先级为130（130大于125）的路径被选作AS 100中的最佳路径，并安装在R1和R2的IP路由表中。有关BGP路径选择标准的详细信息，请参阅BGP最佳路径选择算法

网络图

BGP网络

配置

本文档使用以下配置：

• R3 

• R1 

• R2 

R3

Current configuration : 2037 bytes
!
version 12.2
!
hostname R3
!
interface Loopback0
 ip address 10.0.10.0 255.255.255.0
!
interface Ethernet0/0
 ip address 10.1.0.0 255.255.255.1
!
interface Serial8/0
 ip address 10.10.13.3 255.255.255.0

!--- Interface connected to R1.

 ! 
interface Serial9/0
 ip address 10.10.23.3 255.255.255.0

!--- Interface connected to R2.

!
router bgp 30
 network 10.0.10.0 mask 255.255.255.0
 network 10.1.0.0 mask 255.255.255.1

!--- Network commands announce prefix 10.0.10.0/24


!--- and 10.1.0.0/24.

 neighbor 10.10.13.1 remote-as 100

!--- Establishes peering with R1.

 neighbor 10.10.13.1 send-community

- !--- Without this command, the community attributes !--- are not sent to the neighbor.

 neighbor 10.10.13.1 route-map Peer-R1 out

!--- Configures outbound policy as defined by


!--- route-map "Peer-R1" when peering with R1.

 neighbor 10.10.23.2 remote-as 100

!--- Establishes peering with R2.

 neighbor 10.10.23.2 send-community

!--- Configures to send community attribute to R2.

 neighbor 10.10.23.2 route-map Peer-R2 out

!--- Configures outbound policy as defined by


!--- route-map "Peer-R2" when peering with R2.

 no auto-summary
!
ip classless
ip bgp-community new-format

!--- Allows you to configure the BGP community


!--- attribute in AA:NN format.

!
access-list 101 permit ip host 10.0.10.0 host 255.255.255.0
access-list 102 permit ip host 10.1.0.0 host 255.255.255.1
!
!
route-map Peer-R1 permit 10
 match ip address 101
 set community 100:300

!--- Sets community 100:300 for routes matching access-list 101.

!
route-map Peer-R1 permit 20
 match ip address 102
 set community 100:250

!--- Sets community 100:250 for routes matching access-list 102.

!
route-map Peer-R2 permit 10
 match ip address 101
 set community 100:250

!--- Sets community 100:250 for routes matching access-list 101.

!
route-map Peer-R2 permit 20
 match ip address 102
 set community 100:300

!--- Sets community 100:300 for routes matching access-list 102.

!
end

R1

Version 12.2
!
hostname R1
!
interface Loopback0
 ip address 200.200.10.1 255.255.255.0
!         
interface Serial8/0
 ip address 10.10.13.1 255.255.255.1

!--- Connected to R3.
 
!         
interface Serial10/0
 ip address 10.10.12.1 255.255.255.0

!--- Connected to R2.

!         
router bgp 100
 no synchronization
 bgp log-neighbor-changes
 neighbor 10.10.12.2 remote-as 100

!--- Establishes peering with R2.

 neighbor 10.10.12.2 next-hop-self
 neighbor 10.10.13.3 remote-as 30

!--- Establishes peering with R3.

 neighbor 10.10.13.3 route-map Peer-R3 in

!--- Configures the inbound policy as defined by


!--- route-map "Peer-R3" when peering with R3.

 no auto-summary
!         
ip bgp-community new-format

!--- Allows you to configure the BGP community


!--- attribute in AA:NN format.

ip community-list 1 permit 100:300
ip community-list 2 permit 100:250

!--- Defines community list 1 and 2.

!         
route-map Peer-R3 permit 10
 match community 1
 set local-preference 130

!--- Sets local preference 130 for all routes


!--- matching community list 1.

!         
route-map Peer-R3 permit 20
 match community 2
 set local-preference 125

!--- Sets local preference 125 for all routes


!--- matching community list 2.

!         
route-map Peer-R3 permit 30

 !--- Without this permit 30 statement, updates that do not 


!--- match the permit 10 or permit 20 statements are dropped.


!
end

R2

Version 12.2
!
hostname R2
!
interface Loopback0
ip address 10.0.10.0 255.255.255.0
!
interface Serial9/0
ip address 10.10.23.2 255.255.255.1

!--- Connected to R3.

!
interface Serial10/0
ip address 10.10.12.2 255.255.255.0

!--- Connected to R1.

!
router bgp 100
 no synchronization
 bgp log-neighbor-changes
 neighbor 10.10.12.1 remote-as 100

!--- Establishes iBGP peering with R1.

 neighbor 10.10.12.1 next-hop-self
 neighbor 10.10.23.3 remote-as 30

!--- Establishes peering with R3.

 neighbor 10.10.23.3 route-map Peer-R3 in

!--- Configures inbound policy as defined by


!--- route-map "Peer-R3" when peering with R3.

 no auto-summary
!
ip bgp-community new-format

!--- Allows you to configure the BGP community


!--- attribute in AA:NN format.

!
ip community-list 1 permit 100:300
ip community-list 2 permit 100:250

!--- Defines community list 1 and 2.

!
route-map Peer-R3 permit 10
 match community 1
 set local-preference 130

!--- Sets local preference 130 for all routes


!--- matching community list 1.

! 
route-map Peer-R3 permit 20
 match community 2
 set local-preference 125

!--- Sets local preference 125 for all routes


!--- matching community list 2.

!
route-map Peer-R3 permit 30

 !--- Without this permit 30 statement, updates that do not 


!--- match the permit 10 or permit 20 statements are dropped.

!

end

确认

R1接收具有社区100:300和100:250的前缀10.0.10.0/24和10.1.0.0/24，如下一个show ip bgp命令输出结果所示。

注意：将这些路由基于配置的策略安装到 BGP 表中后，系统会对社区属性为 100:300 的前缀分配本地优先级 130，对社区属性为 100:250 的前缀分配本地优先级 125。

R1# show ip bgp 10.0.10.0
BGP routing table entry for 10.0.10.0/24, version 2
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  10.10.12.2 
  30
    10.10.13.3 from 10.10.13.3 (10.0.10.0)
      Origin IGP, metric 0, localpref 130, valid, external, best
      Community: 100:300

!--- Prefix 10.0.10.0/24 with community 100:300 received from


!--- 10.10.13.3 (R3) is assigned local preference 130.

R1# show ip bgp 10.1.0.0 
BGP routing table entry for 10.1.0.0/24, version 4
Paths: (2 available, best #1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  10.10.13.3 
  30
    10.10.12.2 from 10.10.12.2 (10.1.0.0)
      Origin IGP, metric 0, localpref 130, valid, internal, best
.0!--- Received prefix 10.1.0.0/24 over iBGP from 10.10.12.2


!--- (R2) with local preference 130.

!--- (R2) with local preference 130.

  30
    10.10.13.3 from 10.10.13.3 (198.50.100.0)
      Origin IGP, metric 0, localpref 125, valid, external
      Community: 100:250

!--- Prefix 10.1.0.0/24 with community 100:250 received from


!--- 10.10.13.3 (R3) is assigned local preference 125.

R1# show ip bgp
BGP table version is 4, local router ID is 200.200.10.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 10.0.10.0/24    10.10.13.3            0    130      0 30 i
*>i 10.1.0.0/24    10.10.12.2            0    130      0 30 i
*                   10.10.13.3            0    125      0 30 i

R1上的show ip bgp命令确认R1上选择的最佳路径为本地优先级(LoclPrf)= 130。同样，R2接收的前缀为10.0.10.0/24和10.1.0.0/24，社区为100:250和100:300，如此show ip中粗体所示bgp命令输出：

注意：将这些路由基于配置的策略安装到 BGP 表中后，系统会对社区属性为 100:300 的前缀分配本地优先级 130，对社区属性为 100:250 的前缀分配本地优先级 125。

R2# show ip bgp 10.0.10.0  
BGP routing table entry for 10.0.10.0/24, version 2
Paths: (2 available, best #2, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  10.10.23.3 
  30
    10.10.23.3 from 10.10.23.3 (10.0.10.0)
      Origin IGP, metric 0, localpref 125, valid, external
      Community: 100:250

!--- Prefix 10.0.10.0/24 with community 100:250 received from


!--- 10.10.23.3 (R3) is assigned local preference 125.

  30
    10.10.12.1 from 10.10.12.1 (200.200.10.1)
      Origin IGP, metric 0, localpref 130, valid, internal, best

!--- Received prefix 10.0.10.0/24 over iBGP from 10.10.12.1


!--- (R1) with local preference 130.

R2# show ip bgp 10.0.10.0 
BGP routing table entry for 10.0.10.0/24, version 3
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  10.10.12.1 
  30
    10.10.23.3 from 10.10.23.3 (10.0.10.0)
      Origin IGP, metric 0, localpref 130, valid, external, best
      Community: 100:300

!--- Prefix 10.1.0.0/24 with community 100:300 received from


!--- 10.10.23.3 (R3) is assigned local preference 130.

R2# show ip bgp
BGP table version is 3, local router ID is 192.168.50.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*  10.0.10.0/24      10.10.23.3           0    125      0 30 i
*>i                  10.10.12.1           0    130      0 30 i
*> 10.1.0.0/24       10.10.23.3           0    130      0 30 i

R2上的此show ip bgp命令输出确认R2上选择的最佳路径为本地优先级(loclPrf)= 130。到前缀10.0.10.0/24的IP路由首选R1-R3链路从AS 100向AS 30退出。R1和R3上的show ip route命令r2确认此首选项。

R1# show ip route 10.0.10.0 
Routing entry for 10.0.10.0/24
  Known via "bgp 100", distance 20, metric 0
  Tag 30, type external
  Last update from 10.10.13.3 3d21h ago
  Routing Descriptor Blocks:
  * 10.10.13.3, from 10.10.13.3, 3d21h ago
      Route metric is 0, traffic share count is 1
      AS Hops 1

!--- On R1, the IP route to prefix 10.0.10.0/24 points


!--- to next hop 10.10.13.3 which is R3 serial 8/0


!--- interface on the R1-R3 link.

R2# show ip route 10.1.0.0
Routing entry for 10.1.0.0/24
  Known via "bgp 100", distance 200, metric 0
  Tag 30, type internal
  Last update from 10.10.12.1 3d21h ago
  Routing Descriptor Blocks:
  * 10.10.12.1, from 10.10.12.1, 3d21h ago
      Route metric is 0, traffic share count is 1
      AS Hops 1

!--- On R2, IP route to prefix 10.1.0.0/24 points


!--- to next hop R1 (10.10.12.1) on its iBGP link.


!--- Thus traffic to network 10.1.0.0/24 from R2


!--- exits through R2-R1 and then R1-R3 link from


!--- AS 100 towards AS 30.

到前缀10.1.0.0/24的IP路由首选R2-R3链路从AS 100向AS 30退出。R1和R2上的show ip route 命令确认了此首选项。

R2# show ip route 10.1.0.0
Routing entry for 10.1.0.0/24
  Known via "bgp 100", distance 20, metric 0
  Tag 30, type external
  Last update from 10.10.23.3 3d22h ago
  Routing Descriptor Blocks:
  * 10.10.23.3, from 10.10.23.3, 3d22h ago
      Route metric is 0, traffic share count is 1
      AS Hops 1

!--- On R2, IP route to prefix 10.1.0.0/24 points


!--- to next hop 10.10.23.3 which is R3 serial 9/0


!--- interface on R2-R3 link.

R1# show ip route 10.1.0.0
Routing entry for 10.1.0.0/24
  Known via "bgp 100", distance 200, metric 0
  Tag 30, type internal
  Last update from 10.10.12.2 3d22h ago
  Routing Descriptor Blocks:
  * 10.10.12.2, from 10.10.12.2, 3d22h ago
      Route metric is 0, traffic share count is 1
      AS Hops 1


!--- On R1, IP route to prefix 10.1.0.0/24 points


!--- to next hop R2 (10.10.12.2) on its iBGP link.


!--- Thus traffic to network 10.1.0.0/24 from R1


!--- exits through R1-R2 and then R2-R3 link


!--- from AS 100 towards AS 30.

如果一条链路发生故障，例如R1-R3链路，所有流量都必须跟随R2-R3链路。如果关闭R1-R3之间的链路，可以模拟此流量。

R1# conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#int s8/0
R1(config-if)#shut
R1(config-if)#
3d22h: %BGP-5-ADJCHANGE: neighbor 10.10.13.3 Down Interface flap
3d22h: %LINK-5-CHANGED: Interface Serial8/0, changed state to
  administratively down
3d22h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial8/0,
  changed state to down

注意R1和R2上前缀10.0.10.0/24和10.1.0.0/24的IP路由表。使用R2-R3链路退出AS 100。

R1# show ip route 10.0.10.0 
Routing entry for 10.0.10.0/24
  Known via "bgp 100", distance 200, metric 0
  Tag 30, type internal
  Last update from 10.10.12.2 00:01:47 ago
  Routing Descriptor Blocks:
  * 10.10.12.2, from 10.10.12.2, 00:01:47 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1

R1# show ip route 10.1.0.0
Routing entry for 10.1.0.0/24
  Known via "bgp 100", distance 200, metric 0
  Tag 30, type internal
  Last update from 10.10.12.2 3d22h ago
  Routing Descriptor Blocks:
  * 10.10.12.2, from 10.10.12.2, 3d22h ago
      Route metric is 0, traffic share count is 1
      AS Hops 1

此show 命令输出显示到前缀10.0.10.0/24和10.1.0.0/24的路由指向下一跳10.10.12.2(R2)，这是预期的。现在，请查看R2上的IP路由表，检查前缀10.0.10.0/24和10.1.0.0/24的下一跳。要使配置的策略成功运行，下一跳必须是R3。

R2# show ip route 10.0.10.0 
Routing entry for 10.0.10.0/24
  Known via "bgp 100", distance 20, metric 0
  Tag 30, type external
  Last update from 10.10.23.3 00:04:10 ago
  Routing Descriptor Blocks:
  * 10.10.23.3, from 10.10.23.3, 00:04:10 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1

R2# show ip route 10.1.0.0 
Routing entry for 10.1.0.0/24
  Known via "bgp 100", distance 20, metric 0
  Tag 30, type external
  Last update from 10.10.23.3 3d22h ago
  Routing Descriptor Blocks:
  * 10.10.23.3, from 10.10.23.3, 3d22h ago
      Route metric is 0, traffic share count is 1
      AS Hops 1

下一跳 10.10.23.3 是 R2-R3 链路上的 R3 系列 9/0 接口。这样便可确认所配置的策略是按预期方式运行的。

相关信息

• RFC 1998
• BGP 故障排除
• 调试输出中显示“BGP:常见问题
• 与BGP的负载共享
• 技术支持和文档