实验指南：BGP路径选择

2007-05-06 17:14:11

实验指南

初始配置

Wills
conf t
host Wills
int l 0
ip ad 1.1.1.1 255.255.255.255
int s2/0
encap f
no arp f
no frame inver
no shut
int s2/0.1 p
ip ad 10.1.1.1 255.255.255.0
frame int 102
int s2/0.2 p
ip ad 10.1.2.1 255.255.255.0
frame inter 103

router rip
ver 2
no au
net 10.00.0.0
net 1.0.0.0
net 62.0.0.0
router bgp 2001
no au
no sy
bgp router-id 1.1.1.1
nei 2.2.2.2 remote 645
nei 2.2.2.2 up l 0
nei 2.2.2.2 e 255
nei 3.3.3.3 remote 645
nei 3.3.3.3 up l 0
nei 3.3.3.3 e 255
end

Amold
conf t
host Amold
int l 0
ip ad 2.2.2.2 255.255.255.255
int s2/0
encap f
no arp f
no frame inver
no shut
int s2/0.1 p
ip ad 10.1.1.2 255.255.255.0
frame int 201
int s2/1
ip ad 10.2.2.1 255.255.255.0
encap f
no arp f
no frame inver
frame map ip 10.2.2.3 216 b
frame map ip 10.2.2.2 216 b
no shut
router rip
ver 2
no au
net 2.0.0.0
net 10.0.0.0
router bgp 645
no au
no sy
bgp router-id 2.2.2.2
nei 1.1.1.1 remote 2001
nei 1.1.1.1 up l 0
nei 1.1.1.1 e 255
nei 6.6.6.6 remote 645
nei 6.6.6.6 up l 0
end

Kimberly
conf t
host Kimberly
int l 0
ip ad 3.3.3.3 255.255.255.255
int s 2/0
encap f
no arp f
no frame inver
no shut
int s2/0.1 p
ip ad 10.1.2.2 255.255.255.0
frame int 301
int s2/1
ip ad 10.2.2.2 255.255.255.0
encap f
no arp f
no frame inver
frame map ip 10.2.2.1 316 b
frame map ip 10.2.2.3 316 b
no shut
router rip
ver 2
no au
net 3.0.0.0
net 10.0.0.0
router bgp 645
no au
no sy
bgp router-id 3.3.3.3
nei 1.1.1.1 remote 2001
nei 1.1.1.1 up l 0
nei 1.1.1.1 e 255
nei 6.6.6.6 remote 645
nei 6.6.6.6 up l 0
end

Router6
conf t
host Router6
int l 0
ip ad 6.6.6.6 255.255.255.255
int 1

ip ad 189.168.82.1 255.255.254.0
int l2
ip ad 189.168.84.1 255.255.254.0
int l3
ip ad 189.168.86.1 255.255.254.0
int l4
ip ad 189.168.88.1 255.255.254.0
int s2/1
ip ad 10.2.2.3 255.255.255.0
encap f
no arp f
no frame inver
frame map ip 10.2.2.1 612 b
frame map ip 10.2.2.2 613 b
no shut
router rip
ver 2
net 6.0.0.0
net 10.0.0.0
router bgp 645
bgp router-id 6.6.6.6
no au
no sy
nei 2.2.2.2 remote 645
nei 2.2.2.2 up l 0
nei 3.3.3.3 remote 645
nei 3.3.3.3 up l 0
nei 2.2.2.2 route-reflector-client
nei 3.3.3.3 route-reflector-client
net 189.168.82.0 mask 255.255.254.0
net 189.168.84.0 mask 255.255.254.0
net 189.168.86.0 mask 255.255.254.0
net 189.168.88.0 mask 255.255.254.0
end

1，修改origin来影响BGP路径选择
Wills#sh ip route bgp | include 189
     189.168.0.0/23 is subnetted, 17 subnets
B       189.168.84.0 [20/0] via 2.2.2.2, 01:44:47
B       189.168.86.0 [20/0] via 2.2.2.2, 01:44:47
B       189.168.82.0 [20/0] via 2.2.2.2, 01:44:47
B       189.168.88.0 [20/0] via 2.2.2.2, 01:44:47
从Wills的路由表状态可以看出，到达189.168.0.0/16网段是从Amold学到，若想从Kimberly学到，有几种方法，先讲根据origin来影响

Wills
access-list 1 permit 189.168.0.0 0.0.255.255
route-map ORIGIN permit 10
match ip ad 1
set origin incomplete
router bgp 2001
nei 2.2.2.2 route-map ORIGIN incomplete
end

这时，Wills上路由表的状态
Wills(config-router)#do sh ip route bgp | include 189
     189.168.0.0/23 is subnetted, 17 subnets
B       189.168.84.0 [20/0] via 3.3.3.3, 00:00:07
B       189.168.86.0 [20/0] via 3.3.3.3, 00:00:07
B       189.168.82.0 [20/0] via 3.3.3.3, 00:00:07
B       189.168.88.0 [20/0] via 3.3.3.3, 00:00:07
很明显，从Wills到189.168.0.0/16网段，要走路由器Kimberly。

Wills上BGP表状态
Wills(config-router)#do sh ip bgp | include 189
BGP table version is 63, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 189.168.82.0/23 3.3.3.3                                0 645 i
*                   2.2.2.2                                0 645 ?
*> 189.168.84.0/23 3.3.3.3                                0 645 i
*                   2.2.2.2                                0 645 ?
*> 189.168.86.0/23 3.3.3.3                                0 645 i
*                   2.2.2.2                                0 645 ?
*> 189.168.88.0/23 3.3.3.3                                0 645 i
*                   2.2.2.2                                0 645 ?
从邻居2.2.2.2学到的BGP条目的origin为incomplete，所以Origin Codes 标识为“？”根据BGP路径优先的选择进程，Wills将从Kimberly到达189.168.0.0/16网段

2，使用AS path属性影响BGP路径选择
上例中，我们通过BGP的origin属性来影响BGP路径选择，本例讲到达189.168.0.0/16网段是从Amold学到，若想从Kimberly学到的第二种方法

Wills
access-list 2 permit 189.168.0.0 0.0.255.255
route-map ASPATH permit 10
match ip ad 2
set as-path prepend last-as 2
或者 set as-path prepend 123 1234
router bgp 2001
nei 2.2.2.2 route-map ASPATH in
end
第一个set命令是收到路由条目所经过的最后一个AS显示的次数，第二个set命令是加入预先设定的AS值。因为BGP路径选择时会比较AS path，BGP会选择经过AS path少的路径

Wills上BGP表的状态
Wills#sh ip bgp
BGP table version is 46, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
* 189.168.82.0/23 2.2.2.2                                0 123 1234 645 645 645 i
*>                  3.3.3.3                                0 645 i
* 189.168.84.0/23 2.2.2.2                                0 123 1234 645 645 645 i
*>                  3.3.3.3                                0 645 i
* 189.168.86.0/23 2.2.2.2                                0 123 1234 645 645 645 i
*>                  3.3.3.3                                0 645 i
* 189.168.88.0/23 2.2.2.2                                0 123 1234 645 645 645 i
*>                  3.3.3.3                                0 645 i

CISCO IOS提供了一条隐藏的命令，可以使得BGPpath选择时忽略AS path的长短
Wills
router bgp 2001
bgp bestpath as-path ignore
end

这时，在Wills上BGP表的状态
Wills#sh ip bgp
BGP table version is 46, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
* 189.168.82.0/23 3.3.3.3                                0 645 i
*>                  2.2.2.2                                0 123 1234 645 645 645 i
* 189.168.84.0/23 3.3.3.3                                0 645 i
*>                  2.2.2.2                                0 123 1234 645 645 645 i
* 189.168.86.0/23 3.3.3.3                                0 645 i
*>                  2.2.2.2                                0 123 1234 645 645 645 i
* 189.168.88.0/23 3.3.3.3                                0 645 i
*>                  2.2.2.2                                0 123 1234 645 645 645 i
这里可以看出，配置bgp bestpath as-path ignore后，将不再比较AS path的值。通过routerID去选择路径了。

3，使用AS path属性过滤BGP路由
使用AS path属性过滤掉从Amold上发送的经过AS645的路由条目，要使用正则表达式

Wills
ip as-path access-list 1 deny _645_
ip as-path access-list 1 permit .*
route-map FILTER permit 10
match as-path 1
router bgp 2001
nei 2.2.2.2 route-map FILTER in
end

这时Wills上BGP表的状态
Wills(config-router)#do sh ip bgp | include 189
*> 189.168.82.0/23 3.3.3.3                                0 645 i
*> 189.168.84.0/23 3.3.3.3                                0 645 i
*> 189.168.86.0/23 3.3.3.3                                0 645 i
*> 189.168.88.0/23 3.3.3.3                                0 645 i
说明从Amold上发送的路由条目已经被过滤了
还可以使用filter-list 进行AS path过滤,建立as-path列表的步骤一样，不过就是不用建立route-map了
Wills
router bgp 2001
nei 2.2.2.2 filter-list 1 in
end

用这个方法要比使用route-map简便许多，也同时能起到过滤AS path的作用

PS：正则表达式比较麻烦，需要多多练习

4，使用MED属性选择BGP最佳路径
默认情况下Wills要189.168.0.0/16网段要走Amold，下面通过修改Amold和Kimberly的mertic值来改变Wills到达189.168.0.0/16网段要走的路径
Amold
access-list 1 permit 189.168.0.0 0.0.255.255
route-map METRIC permit 10
match ip ad 1
set metric 100
route-map METRIC permit 20
router bgp 645
nei 1.1.1.1 route-map METRIC out
end
把从Amold发出的189.168.0.0/16网段的metric设为100（默认为0），这时Wills的BGP表状态
Wills(config-router)#do sh ip bgp
BGP table version is 63, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 189.168.82.0/23 3.3.3.3                                0 645 i
*                   2.2.2.2                100             0 645 i
*> 189.168.84.0/23 3.3.3.3                                0 645 i
*                   2.2.2.2                100             0 645 i
*> 189.168.86.0/23 3.3.3.3                                0 645 i
*                   2.2.2.2                100             0 645 i
*> 189.168.88.0/23 3.3.3.3                                0 645 i
*                   2.2.2.2                100             0 645 i
很明显，Wills将选择metric小的路径到达目的网段

<注>本例中没有提到的一种情况就是，BGP默认并不比较通过多个AS到达目的网段的metric值，也就是说，如果Amold和Kimberly不在同一个AS中时，BGP并不比较这两个AS的mertric值。这样就有可能到达目的网段并不是走最佳路径，CISCO IOS提供了一条命令可以比较多个AS的metric值，以达到最佳路径的目的
router bgp <AS>
bgp always-compare-med
这样可以比较多个AS的metric值了

5，通过修改local_pref来选择网络出口
Wills
acccess-list 1 permit 189.168.0.0 0.0.255.255
route-map LOCAL_PREF permit 10
match ip ad 1
set local-preference 1000
router bgp 2001
nei 3.3.3.3 route-map LOCAL_PREF in
end

Wills上BGP表的状态
Wills(config-router)#do sh ip bgp
BGP table version is 46, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
* 189.168.82.0/23 2.2.2.2                                0 645 i
*>                  3.3.3.3                      1000      0 645 i
* 189.168.84.0/23 2.2.2.2                                0 645 i
*>                  3.3.3.3                      1000      0 645 i
* 189.168.86.0/23 2.2.2.2                                0 645 i
*>                  3.3.3.3                      1000      0 645 i
* 189.168.88.0/23 2.2.2.2                                0 645 i
*>                  3.3.3.3                      1000      0 645 i
因为Wills到Kimberly的local_pref高，所以BGP选择Kimberly到达目的网段

本文出自 “穿过地狱去看海” 博客，谢绝转载！

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