BGP有条件通告的配置和验证

2007-08-12 13:59:35

由ITAA zmjjtu 作

BGP有条件通告的配置和验证

BGP有条件通告特性对所通告的路由提供了额外的控制，其依赖于BGP表中存在的其他前缀。

一般地，路由是否通告与存在的不同路径没有必然的联系。BGP有条件通告特性使用了命令neighbor advertise-map中non-exist-map和advertise-map两个关键字依赖路由前缀来追踪路由。如果路由前缀没有出现在命令non-exist-map所指定的外出路由中那么在命令advertise-map中被指定的路由就会通告出去。此特性在多宿主(multihomed)网络中很有作用，如果没有接收到来自其他服务提供商的某些信息（可能是对等体会话失败或只存在局部可达性）那么一些指定的前缀将被通告给其中一个服务提供商。

top图如下：

在上图中，R103的loopback口用来将192.168.50.0/24通告给R102。默认情况下R102向其两个eBGP对等体R101和R103通告网络128.16.16.0/24。

通过BGP有条件通告特性可以在R102上完成以下两个任务：

① 如果192.168.50.0/24存在于R102的BGP表中，那么不将128.16.16.0/24通告给R101。

② 如果192.168.50.0/24不存在于R102的BGP表中，那么将128.16.16.0/24通告给R101。

配置如下：

R101

R101(config)#int s1/2

R101(config-if)#ip add 10.10.10.1 255.255.255.0

R101(config-if)#no sh

R101(config-if)#int lo0

R101(config-if)#ip add 200.200.200.1 255.255.255.0

R101(config-if)#router bgp 1

R101(config-router)#bgp log-neighbor-changes

R101(config-router)#network 200.200.200.0

R101(config-router)#neighbor 10.10.10.2 remote-as 2

R101(config-router)#end

R102

R102(config)#int s1/0

R102(config-if)#ip add 10.10.10.2 255.255.255.0

R102(config-if)#no sh

R102(config-if)#int s1/2

R102(config-if)#ip add 10.10.20.2 255.255.255.0

R102(config-if)#no sh

R102(config)#int lo0

R102(config-if)#ip add 128.16.16.1 255.255.255.0

R102(config)#router bgp 2

R102(config-router)#bgp log-neighbor-changes

R102(config-router)#network 128.16.16.0 mask 255.255.255.0

R102(config-router)#network 130.130.0.0

R102(config-router)#neighbor 10.10.10.1 remote-as 1

R102(config-router)#neighbor 10.10.10.1 advertise-map ADVERTISE non-exist-map NON-EXIST

如果BGP表中不存在route-map NON-EXIST所指定的路由(192.168.50.0/24)那么就通告route-map ADVERTISE中指定的路由 (128.16.16.0/24)

R102(config-router)#neighbor 10.10.20.3 remote-as 3

R102(config-router)#exit

R102(config)#ip route 130.130.0.0 255.255.0.0 null 0

R102(config)#access-list 60 permit 128.16.16.0 0.0.0.255

R102(config)#access-list 65 permit 192.168.50.0 0.0.0.255

R102(config)#route-map NON-EXIST permit 10

R102(config-route-map)#match ip address 65

R102(config-route-map)#route-map ADVERTISE permit 10

R102(config-route-map)#match ip address 60

R102(config-route-map)#end

R103

R103(config)#int s1/2

R103(config-if)#ip add 10.10.20.3 255.255.255.0

R103(config-if)#no sh

R103(config-if)#int lo0

R103(config-if)#ip add 192.168.50.1 255.255.255.0

R103(config-if)#router bgp 3

R103(config-router)#bgp log-neighbor-changes

R103(config-router)#network 192.168.50.0

R103(config-router)#neighbor 10.10.20.2 remote-as 2

1.验证当192.168.50.0/24在R102的BGP表中存在时的情况：

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查看R102的BGP表

R102# sh 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

只要192.168.50.0/24在R102的BGP表中存在那么R102就一定不会向R101通告128.16.16.0/24

R102#sh 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

注意没有向邻居10.10.10.1通告128.16.16.0/24

R102#sh ip bgp 128.16.16.0

BGP routing table entry for 128.16.16.0/24, version 5

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Advertised to non peer-group peers:

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

没有通告给R101

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检查R102上有条件通告的情况：

R102#sh 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 00:29:26

Last read 00:00:26, hold time is 180, keepalive interval is 60 seconds

Neighbor capabilities:

Route refresh: advertised and received(old & new)

Address family IPv4 Unicast: advertised and received

Message statistics:

InQ depth is 0

OutQ depth is 0

Sent Rcvd

Opens: 1 1

Notifications: 0 0

Updates: 2 1

Keepalives: 32 32

Route Refresh: 0 0

Total: 35 34

Default minimum time between advertisement runs is 30 seconds

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

Sent Rcvd

Prefix activity: ---- ----

Prefixes Current: 2 1 (Consumes 36 bytes)

Prefixes Total: 2 1

Implicit Withdraw: 0 0

Explicit Withdraw: 0 0

Used as bestpath: n/a 1

Used as multipath: n/a 0

Outbound Inbound

Local Policy Denied Prefixes: -------- -------

advertise-map: 1 n/a

Bestpath from this peer: 1 n/a

Total: 2 0

Number of NLRIs in the update sent: max 1, min 0

（以下输出省略）

以上输出显示了有条件通告状态为“withdrawn”，与路由图ADVERTISE相匹配的网络没有向对等体10.10.10.1通告

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为了证实和route-map“ADVERTISE”相匹配的路由没有向R101通告，现在来检查R101的BGP表

R101#sh ip bgp 128.16.16.0

% Network not in table

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2.接下来验证当192.168.50.0/24在R102的BGP表中不存在时的情况：

首先关闭R103上的loopback0使其不再向R102通告192.168.50.0/24

R103(config)#int lo0

R103(config-if)#sh

R103(config-if)#

01:05:13: %LINK-5-CHANGED: Interface Loopback0, changed state to administratively down

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查看R102的BGP表验证其没有学到192.168.50.0/24

R102# sh 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

BGP表中不存在192.168.50.0/24

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查看有条件通告要多长时间才能生效：

R102#debug ip bgp updates

BGP updates debugging is on

01:19:49: BGP(0): 10.10.20.3 rcv UPDATE about 192.168.50.0/24 -- withdrawn

01:19:49: BGP(0): no valid path for 192.168.50.0/24

R102#

01:19:49: BGP(0): nettable_walker 192.168.50.0/24 no best path

R102#

01:20:15: BGP(0): 10.10.20.3 computing updates, afi 0, neighbor version 12, table version 13, starting at 0.0.0.0

01:20:15: BGP(0): 10.10.20.3 update run completed, afi 0, ran for 4ms, neighbor version 12, start version 13, throttled to 13

R102#

01:20:18: BGP(0): 10.10.10.1 computing updates, afi 0, neighbor version 12, table version 13, starting at 0.0.0.0

01:20:18: BGP(0): 10.10.10.1 send unreachable 192.168.50.0/24

01:20:18: BGP(0): 10.10.10.1 send UPDATE 192.168.50.0/24 -- unreachable

01:20:18: BGP(0): 10.10.10.1 1 updates enqueued (average=27, maximum=27)

01:20:18: BGP(0): 10.10.10.1 update run completed, afi 0, ran for 0ms, neighbor version 12, start version 13, throttled to 13

R102#

01:20:28: BPG(0): Condition NON-EXIST changes to Advertise

01:20:28: BGP(0): net 128.16.16.0/24 matches ADV MAP ADVERTISE: bump version to 14

01:20:29: BGP(0): nettable_walker 128.16.16.0/24 route sourced locally

R102#

01:20:42: BGP(0): 10.10.20.3 computing updates, afi 0, neighbor version 13, table version 14, starting at 0.0.0.0

01:20:42: BGP(0): 10.10.20.3 skip UPDATE 128.16.16.0/24 (chgflags: 0x0), next 0.0.0.0, path

01:20:42: BGP(0): 10.10.20.3 update run completed, afi 0, ran for 0ms, neighbor version 13, start version 14, throttled to 14

R102#

01:20:47: BGP(0): 10.10.10.1 computing updates, afi 0, neighbor version 13, table version 14, starting at 0.0.0.0

01:20:47: BGP(0): 10.10.10.1 128.16.16.0/24 matches advertise map ADVERTISE, state: Advertise

01:20:47: BGP(0): 10.10.10.1 send UPDATE (format) 128.16.16.0/24, next 10.10.10.2, metric 0, path

01:20:47: BGP(0): 10.10.10.1 1 updates enqueued (average=52, maximum=52)

01:20:47: BGP(0): 10.10.10.1 update run completed, afi 0, ran for 8ms, neighbor version 13, start version 14, throttled to 14

有条件通告由BGP扫描进程触发，其每60s运行一次。这意味着有条件通告的最大生效时间为60s。有条件通告能很快的起到作用就依赖于追踪路由是否从BGP表中移除。

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下面验证R102对邻居10.10.10.1有条件通告的情况：

R102#sh 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 01:00:43

Last read 00:00:43, hold time is 180, keepalive interval is 60 seconds

Neighbor capabilities:

Route refresh: advertised and received(old & new)

Address family IPv4 Unicast: advertised and received

Message statistics:

InQ depth is 0

OutQ depth is 0

Sent Rcvd

Opens: 1 1

Notifications: 0 0

Updates: 6 1

Keepalives: 63 63

Route Refresh: 0 0

Total: 70 65

Default minimum time between advertisement runs is 30 seconds

For address family: IPv4 Unicast

BGP table version 10, neighbor version 10

Index 1, Offset 0, Mask 0x2

Condition-map NON-EXIST, Advertise-map ADVERTISE, status: Advertise

Sent Rcvd

Prefix activity: ---- ----

Prefixes Current: 2 1 (Consumes 36 bytes)

Prefixes Total: 4 1

Implicit Withdraw: 0 0

Explicit Withdraw: 2 0

Used as bestpath: n/a 1

Used as multipath: n/a 0

Outbound Inbound

Local Policy Denied Prefixes: -------- -------

advertise-map: 1 n/a

Bestpath from this peer: 1 n/a

Total: 2 0

Number of NLRIs in the update sent: max 1, min 0

（以下输出省略）

现在R101的BGP表中有128.16.16.0/24的信息

R101#sh ip bgp

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BGP有条件通告的配置和验证