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配置和验证BGP条件通告特性

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已更新: 2005 年 8 月 10 日
文档 ID:16137
关于翻译

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Introduction

边界网关协议 (BGP) 条件通告功能可以根据 BGP 表中是否存在其他前缀，提供对路由通告的更多控制。

Prerequisites

Requirements

在尝试此配置前，请保证您熟悉：

Components Used

在本文描述的BGP条件通告特性在Cisco IOS软件版本11.1cc和11.2被介绍了并且是可用的在最新版本。

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(13)T13

  • Cisco 2500 系列路由器

The information in this document was created from the devices in a specific lab environment.All of the devices used in this document started with a cleared (default) configuration.If your network is live, make sure that you understand the potential impact of any command.

背景理论

通常，会传播路由，而不管是否存在不同的路径。BGP条件通告特性使用non-exist-map和advertise-map关键字neighbor advertise-map命令为了由路由前缀跟踪路由。如果路由前缀不是存在输出的non-exist-map命令中，则advertise-map命令指定的路由宣布。此功能为多址网络是有用的，一些前缀做通告到其中一个供应商，只有当从另一个供应商的信息不存在(这在对等体会话或部分可到达性中指示一个故障)。

有条件的BGP公告被发送除BGP路由器发送到其对等体的正常公告之外。

Conventions

有关文档规则的详细信息，请参阅 Cisco 技术提示规则。

Configure

本部分提供有关如何配置本文档所述功能的信息。

Note: 要寻找关于用于本文的命令的其他信息，请使用ios命令查找工具

Network Diagram

本文档使用此图所示的网络设置。

这里， R103环回接口用于宣布192.168.50.0/24到R102。R102有一个BGP网络， 128.16.16.0/24，默认情况下被通知对两个其外部BGP (EBGP)对等体， R101和R103。

cond_adv_01.gif

使用BGP条件通告特性，您能当前完成在R102的这些任务：

  • 如果192.168.50.0/24在R102's BGP表里存在，则请勿通告128.16.16.0/24网络对R101。

  • 如果192.168.50.0/24在R102's BGP表里不存在，则请通告128.16.16.0/24网络对R101。

使用Cisco IOS 12.2T或以上版本，已存在映射关键字可以用于完成这些任务：

  • 如果192.168.50.0/24在R102's BGP表里存在，则请通告128.16.16.0/24网络对R101。

  • 如果192.168.50.0/24在R102's BGP表里不存在，则请勿通告128.16.16.0/24网络对R101。

配置有条件通告功能

本文档使用以下配置：

Note: 这里示例讨论non-exist-map关键字。已存在映射关键字的使用方法类似于此。

R102 
hostname R102 
! 
interface Loopback0 
 ip address 128.16.16.1 255.255.255.0 
! 
interface Serial8/0 
 ip address 10.10.10.2 255.255.255.0 
! 
interface Serial9/0 
 ip address 10.10.20.2 255.255.255.0 
! 
router bgp 2 
 bgp log-neighbor-changes 
 network 128.16.16.0 mask 255.255.255.0 
 network 130.130.0.0 
 neighbor 10.10.10.1 remote-as 1 
 neighbor 10.10.10.1 advertise-map ADVERTISE non-exist-map NON-EXIST

!--- Advertises the routes matched in the route-map ADVERTISE (128.16.16.0/24) !--- only if the routes matched in route-map NON-EXIST (192.168.50.0/24) !--- do not exist in the BGP table.
 
 neighbor 10.10.20.3 remote-as 3 
! 
ip route 130.130.0.0 255.255.0.0 Null0 
! 
access-list 60 permit 128.16.16.0 0.0.0.255 
access-list 65 permit 192.168.50.0 0.0.0.255 
! 
route-map NON-EXIST permit 10 
 match ip address 65 
!  
route-map ADVERTISE permit 10 
 match ip address 60 
!

R103 
hostname R103 
! 
interface Loopback0 
 ip address 192.168.50.1 255.255.255.0 
! 
interface Serial9/0 
 ip address 10.10.20.3 255.255.255.0 
!  
router bgp 3 
 bgp log-neighbor-changes 
 network 192.168.50.0 
 neighbor 10.10.20.2 remote-as 2 
!

R101 
hostname R101 
! 
interface Loopback0 
 ip address 200.200.200.1 255.255.255.0 
! 
interface Serial8/0 
 ip address 10.10.10.1 255.255.255.0 
!  
router bgp 1 
 bgp log-neighbor-changes 
 network 200.200.200.0 
 neighbor 10.10.10.2 remote-as 2 
!

验证配置

示例 1

此示例验证什么BGP看起来，当192.168.50.0/24在R102's BGP表里时：

cond_adv_01.gif

首先请确认192.168.50.0/24是否在R102's BGP表里存在：

R102# 
show ip bgp

BGP table version is 6, local router ID is 128.16.16.1
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
*> 128.16.16.0/24   0.0.0.0                  0         32768 i
*> 130.130.0.0      0.0.0.0                  0         32768 i
*> 192.168.50.0     10.10.20.3               0             0 3 i
*> 200.200.200.0    10.10.10.1               0             0 1 i

因为192.168.50.0/24在R102's BGP表里，然后R102不能通告128.16.16/24到R101。

R102# show ip bgp neighbors 10.10.10.1 advertised-routes 
BGP table version is 6, local router ID is 128.16.16.1
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
*> 130.130.0.0      0.0.0.0                  0         32768 i
*> 192.168.50.0     10.10.20.3               0             0 3 i

!--- Note 128.16.16.0/24 is not advertised to neighbor 10.10.10.1.


R102# show ip bgp 128.16.16.0
BGP routing table entry for 128.16.16.0/24, version 6
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:

!--- This is not advertised to R101.
  
10.10.20.3     
  Local
    0.0.0.0 from 0.0.0.0 (128.16.16.1)
      Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, local, best

然后请检查条件通告的状态在R102的：

R102# show ip bgp neighbors 10.10.10.1
BGP neighbor is 10.10.10.1,  remote AS 1, external link
  BGP version 4, remote router ID 200.200.200.1
  BGP state = Established, up for 02:27:07
  Last read 00:00:07, hold time is 180, keepalive interval is 60 seconds

 
!--- Output suppressed. 


 For address family: IPv4 Unicast
  BGP table version 6, neighbor version 6
  Index 1, Offset 0, Mask 0x2
  Condition-map NON-EXIST, Advertise-map ADVERTISE, status: Withdraw
  1 accepted prefixes consume 36 bytes
  Prefix advertised 3, suppressed 0, withdrawn 1
  Number of NLRIs in the update sent: max 1, min 0


!--- Output suppressed.

输出表示“提取条件通告”，并且，匹配路由映射“的网络做通告”没有宣布对对等体10.10.10.1。

为了确认匹配路由映射“的路由请做通告”没有宣布对R101，检查在R101的BGP表： 

R101# show ip bgp 128.16.16.0
% Network not in table

示例 2

此示例显示什么BGP看起来，当网络192.168.50.0/24在R102 BGP表里时不存在：

cond_adv_02.gif

首先，请关闭在R103的环回接口0，以便R103不再宣布192.168.50.0/24对R102。 

R103(config)# interface loopback 0
R103(config-if)# shutdown 
R103(config-if)#
03:29:36: %LINK-5-CHANGED: Interface Loopback0, changed state to administratively down

然后请验证R102不了解192.168.50.0/24，并且网络在R102 BGP表里没有包括。 

R102# show ip bgp 
BGP table version is 8, local router ID is 128.16.16.1
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
*> 128.16.16.0/24   0.0.0.0                  0         32768 i
*> 130.130.0.0      0.0.0.0                  0         32768 i
*> 200.200.200.0    10.10.10.1               0             0 1 i

!--- Note 192.168.50.0/24 is not present.

现在，请参阅多长时间为条件通告需要插入在：

R102# 
debug ip bgp updates

*Mar  1 02:39:18.059: BGP(0): 10.10.20.3 rcv UPDATE about 192.168.50.0/24
-- withdrawn
*Mar  1 02:39:18.059: BGP(0): no valid path for 192.168.50.0/24
*Mar  1 02:39:18.079: BGP(0): nettable_walker 192.168.50.0/24 no best path
*Mar  1 02:39:18.219: BGP(0): 10.10.10.1 computing updates, afi 0, neighbor 
version 10, table version 11, starting at 0.0.0.0
*Mar  1 02:39:18.219: BGP(0): 10.10.10.1 send unreachable 192.168.50.0/24
*Mar  1 02:39:18.219: BGP(0): 10.10.10.1 send UPDATE 192.168.50.0/24 
-- unreachable
*Mar  1 02:39:18.219: BGP(0): 10.10.10.1 1 updates enqueued (average=27, maximum=27)
*Mar  1 02:39:18.219: BGP(0): 10.10.10.1 update run completed, afi 0, ran for 0ms, 
neighbor version 10, start version 11, throttled to 11

*Mar  1 02:40:04.747: BPG(0): Condition NON-EXIST changes to Advertise
*Mar  1 02:40:04.747: BGP(0): net 128.16.16.0/24 matches ADV MAP ADVERTISE: 
bump version to 12
*Mar  1 02:40:05.187: BGP(0): nettable_walker 128.16.16.0/24 route sourced 
locally
*Mar  1 02:40:05.187: BGP(0): 10.10.10.1 computing updates, afi 0, neighbor 
version 11, table version 12, starting at 0.0.0.0
*Mar  1 02:40:05.187: BGP(0): 10.10.10.1 128.16.16.0/24 matches advertise map 
ADVERTISE, state: Advertise
*Mar  1 02:40:05.187: BGP(0): 10.10.10.1 send UPDATE (format) 128.16.16.0/24, 
next 10.10.10.2, metric 0, path 
*Mar  1 02:40:05.187: BGP(0): 10.10.10.1 1 updates enqueued (average=52, maximum=52)
*Mar  1 02:40:05.187: BGP(0): 10.10.10.1 update run completed, afi 0, ran for 0ms, 
neighbor version 11, start version 12, throttled to 12

条件通告进程由BGP扫描程序进程触发，运行每60秒。这意味着条件通告的最大时间生效是60秒。条件通告能快生效，根据，当被跟踪的路由从BGP表时被去除，并且，当BGP扫描程序的下个实例发生时。发出这些on命令R102为了验证在R102的条件通告状况邻接10.10.10.1的：

R102# show ip bgp neighbors 10.10.10.1
BGP neighbor is 10.10.10.1,  remote AS 1, external link
  BGP version 4, remote router ID 200.200.200.1
  BGP state = Established, up for 02:45:27
  Last read 00:00:27, hold time is 180, keepalive interval is 60 seconds
  
!--- Output suppressed. 


 For address family: IPv4 Unicast
  BGP table version 12, neighbor version 12
  Index 1, Offset 0, Mask 0x2
  Condition-map NON-EXIST, Advertise-map ADVERTISE, status: Advertise
  1 accepted prefixes consume 36 bytes
  Prefix advertised 6, suppressed 0, withdrawn 4
  Number of NLRIs in the update sent: max 1, min 0
 
!--- Output suppressed.

R101's BGP表和路由表有128.16.16.0/24，如显示这里：

Note: 在此输出示例:中，有条件的BGP公告(网络128.16.16.0/24)被发送，除BGP路由器发送到其对等体的正常BGP公告(网络130.130.0.0/16)之外。

R101# show ip bgp
BGP table version is 18, local router ID is 200.200.200.1
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
*> 128.16.16.0/24   10.10.10.2               0             0 2 i
*> 130.130.0.0      10.10.10.2               0             0 2 i
*> 200.200.200.0    0.0.0.0                  0         32768 i


R101# show ip route bgp
     128.16.0.0/24 is subnetted, 1 subnets
B       128.16.16.0 [20/0] via 10.10.10.2, 00:09:32
B    130.130.0.0/16 [20/0] via 10.10.10.2, 02:48:46

示例 3

此示例在R102介入192.168.50.0/24网络的重新安装为了发现BGP更改从“如何做通告” “让步”。

cond_adv_01.gif

为了重新安装192.168.50.0/24，发出no shutdown命令带来在R103的interface loopback 0对“”。 

R103(config)# interface loopback 0
R103(config-if)# no shutdown
R103(config-if)#
03:49:06: %LINK-3-UPDOWN: Interface Loopback0, 
changed state to up
03:49:07: %LINEPROTO-5-UPDOWN: Line protocol on 
Interface Loopback0, changed state to up

!--- R102 kicks in conditional advertisement the moment the !--- conditional network is received again.

*Mar  1 02:51:42.227: BGP(0): 10.10.20.3 rcvd UPDATE w/ attr: 
nexthop 10.10.20.3, origin i, metric 0, path 3
*Mar  1 02:51:42.227: BGP(0): 10.10.20.3 rcvd 192.168.50.0/24
*Mar  1 02:51:42.247: BGP(0): Revise route installing 192.168.50.0/24 -> 
10.10.20.3 to main IP table
*Mar  1 02:51:42.379: BGP(0): 10.10.10.1 computing updates, afi 0, 
neighbor version 12, table version 13, starting at 0.0.0.0
*Mar  1 02:51:42.379: BGP(0): 10.10.10.1 send UPDATE (format) 
192.168.50.0/24, next 10.10.10.2, metric 0, path 3
*Mar  1 02:51:42.379: BGP(0): 10.10.10.1 1 updates enqueued 
(average=47, maximum=47)
*Mar  1 02:51:42.379: BGP(0): 10.10.10.1 update run completed, afi 0, 
ran for 0ms, neighbor version 12, start version 13, throttled to 13

*Mar  1 02:52:09.159: BPG(0): Condition NON-EXIST changes to Withdraw
*Mar  1 02:52:09.159: BGP(0): net 128.16.16.0/24 matches ADV MAP 
ADVERTISE: bump version to 14
*Mar  1 02:52:09.499: BGP(0): nettable_walker 128.16.16.0/24 route 
sourced locally
*Mar  1 02:52:11.559: BGP(0): 10.10.10.1 computing updates, afi 0, 
neighbor version 13, table version 14, starting at 0.0.0.0
*Mar  1 02:52:11.559: BGP(0): 10.10.10.1 128.16.16.0/24 matches advertise 
map ADVERTISE, state: Withdraw
*Mar  1 02:52:11.559: BGP(0): 10.10.10.1 send unreachable 128.16.16.0/24
*Mar  1 02:52:11.559: BGP(0): 10.10.10.1 send UPDATE 128.16.16.0/24 -- unreachable
*Mar  1 02:52:11.559: BGP(0): 10.10.10.1 1 updates enqueued (average=27, maximum=27)
*Mar  1 02:52:11.559: BGP(0): 10.10.10.1 update run completed, afi 0, ran for 0ms, 
neighbor version 13, start version 14, throttled to 14

R102不再通告128.16.16.0/24对R101。

R102# show ip bgp neighbors 10.10.10.1 advertised-routes 
BGP table version is 14, local router ID is 128.16.16.1
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
*> 130.130.0.0      0.0.0.0                  0         32768 i
*> 192.168.50.0     10.10.20.3               0             0 3 i

!--- Note 128.16.16.0/24 is not advertised.

R102# show ip bgp neighbors 10.10.10.1
BGP neighbor is 10.10.10.1,  remote AS 1, external link
  BGP version 4, remote router ID 200.200.200.1
  BGP state = Established, up for 03:01:32
  Last read 00:00:31, hold time is 180, keepalive interval is 60 seconds
  Neighbor capabilities:
!--- Output supressed.

 For address family: IPv4 Unicast
  BGP table version 14, neighbor version 14
  Index 1, Offset 0, Mask 0x2
  Condition-map NON-EXIST, Advertise-map ADVERTISE, status: Withdraw
  1 accepted prefixes consume 36 bytes
  Prefix advertised 7, suppressed 0, withdrawn 5
  Number of NLRIs in the update sent: max 1, min 0
  
!--- Output supressed.

Note: 在真实世界方案中， AS1和AS3被连接到互联网(全局BGP)。因此网络192.168.50.0/24可能通过从AS3的一个全球BGP网被传播到AS1 (R101) BGP表。AS1，反过来，可能传播前缀192.168.50.0到R102 (根据在AS1和AS2之间的策略协议)。如果R101传播AS3从全球BGP网的获知的路由到R102，则条件通告可以发生故障，如果另外的检查在不存在的映射没有安置。

为了更好知道条件通告为什么发生故障，请考虑此方案。AS1了解192.168.50.0/24从全球BGP网并且宣布192.168.50.0/24对AS2 (R102)。R102也了解从其直接同位体的前缀192.168.50.0/24到R103 (在对R103链路的R102)。当R102和R103之间的直接链接发生故障时，您预计前缀192.168.50.0/24在R102's BGP表里停止存在，并且条件通告起动并且宣布前缀128.16.16.0/24对R101。然而，因为前缀192.168.50.0/24在R102 BGP表里继续存在(了解从R101)，条件通告中断，因为在不存在的映射的前缀在R102 BGP表里仍然存在。要保证前缀192.168.50.0/24从与AS3 (对R103的R102的直接连接仅了解)为了条件通告能起动，请添加匹配as_path语句在匹配AS_PATH前缀192.168.50.0/24获知从直接连接(在这种情况下是AS 3)的不存在的映射下。此案件的常规表示是^3。使用在BGP的常规表示关于常规表示的更多信息，请参见。当心您不能匹配AS路径和不匹配前缀。match as-path能只补充被匹配的前缀的匹配标准。换句话说，如果前缀不从确定存在AS #，您不能通告一些前缀对相邻。

在R102的新的配置显示得这里。添加在粗体。

R101 
hostname R102  
!  
interface Loopback0  
 ip address 128.16.16.1 255.255.255.0  
!  
interface Serial8/0  
 ip address 10.10.10.2 255.255.255.0  
!  
interface Serial9/0  
 ip address 10.10.20.2 255.255.255.0  
!  
router bgp 2  
 bgp log-neighbor-changes  
 network 128.16.16.0 mask 255.255.255.0  
 network 130.130.0.0  
 neighbor 10.10.10.1 remote-as 1  
 neighbor 10.10.10.1 advertise-map ADVERTISE non-exist-map NON-EXIST  
 neighbor 10.10.20.3 remote-as 3  
!  
ip route 130.130.0.0 255.255.0.0 Null0 
! 
ip as-path access-list 1 permit ^3 
!  
access-list 60 permit 128.16.16.0 0.0.0.255  
access-list 65 permit 192.168.50.0 0.0.0.255  
!  
route-map NON-EXIST permit 10  
 match ip address 65  
  match as-path 1 
!  
route-map ADVERTISE permit 10  
 match ip address 60  
!

已知问题

有与BGP和条件通告关连的两三个已知Bug。欲知更多信息，请参见Bug CSCdp18563 (仅限注册用户)和CSCdp20320 (仅限注册用户)在Bug Toolkit。

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