这是一个典型的不同目的地负载均衡。可使双链路中其中一条发布汇总，另一条发布汇总及特定的具体路由；针对多ISP，使用as-path prepend而非MED影响入流量；使用local-pref属性影响出流量。 案例分析：
一、针对出流量。由于BGP与其他路由协议之间的默认路由重发布比较复杂，这里我只实现了完全重发布的方法。R1、R2、R3之间运行OSPF，使用redistribute bgp 123 subnets进行所有外部路由重发布。实际工作肯定不会使用这种方法的，这也是本实验委曲求全的无奈之举，敬请见谅。
二、针对入流量，由于是多ISP，可以采用as-path影响所有ISP的路由选择。这形成了多链路的负载均衡：若ISP转发到达具体路由为目的地的数据包，由于路由表中存在该详细路由，因此将选择发布该详细路由的BGP peer作为下一跳；若转发目的地不是具体路由，则由于另一peer的汇总路由拥有较短的as-path，而选择该peer作为下一跳；
三、可以使用aggregation+specific route的方式。从两条链路发送as-path长短不同的网络汇总到ISP，并在其中一条链路上发布详细路由(R3本地的)。可对返回流量进行控制； 普通配置： R1#show run
interface Loopback0
ip address 1.1.1.1 255.255.255.255
!
interface Loopback1
ip address 1.0.0.1 255.255.255.0
ip ospf network point-to-point
!
interface FastEthernet0/0
description To R2's F0/0
ip address 172.16.12.1 255.255.255.0
!
interface FastEthernet0/1
description To R3's F0/0
ip address 172.16.13.1 255.255.255.0
!
router ospf 100
router-id 1.1.1.1
passive-interface Loopback0
passive-interface Loopback1
network 1.0.0.1 0.0.0.0 area 0
network 1.1.1.1 0.0.0.0 area 0
network 172.16.12.1 0.0.0.0 area 0
network 172.16.13.1 0.0.0.0 area 0
end R2#show run
interface Loopback0
ip address 2.2.2.2 255.255.255.255
!
interface Loopback1
ip address 172.16.2.2 255.255.255.0
ip ospf network point-to-point
!
interface FastEthernet0/0
description To R1's F0/0
ip address 172.16.12.2 255.255.255.0
!
interface FastEthernet0/1
description To R3's F0/1
ip address 172.16.23.2 255.255.255.0
!
interface FastEthernet1/0
description To R4's F1/0
ip address 24.0.0.2 255.255.255.0
!
router ospf 100
router-id 2.2.2.2
redistribute bgp 123 subnets
passive-interface Loopback0
passive-interface Loopback1
network 2.2.2.2 0.0.0.0 area 0
network 172.16.2.2 0.0.0.0 area 0
network 172.16.12.2 0.0.0.0 area 0
network 172.16.23.2 0.0.0.0 area 0
!
router bgp 123
no synchronization
bgp router-id 2.2.2.2
network 2.2.2.2 mask 255.255.255.255
network 172.16.1.0 mask 255.255.255.0
network 172.16.2.0 mask 255.255.255.0
neighbor LOCAL_AS peer-group
neighbor LOCAL_AS remote-as 123
neighbor LOCAL_AS update-source Loopback0
neighbor LOCAL_AS next-hop-self
neighbor 3.3.3.3 peer-group LOCAL_AS
neighbor 24.0.0.4 remote-as 4
neighbor 24.0.0.4 filter-list 10 out
no auto-summary
!
ip as-path access-list 10 permit ^$
!
end R3#show run
interface Loopback0
ip address 3.3.3.3 255.255.255.255
!
interface Loopback1
ip address 172.16.3.3 255.255.255.0
ip ospf network point-to-point
!
interface FastEthernet0/0
description To R1's F0/1
ip address 172.16.13.3 255.255.255.0
!
interface Serial0/0
description To R5's S0/0
ip address 35.0.0.3 255.255.255.0
!
interface FastEthernet0/1
description To R2's F0/1
ip address 172.16.23.3 255.255.255.0
!
router ospf 100
router-id 3.3.3.3
redistribute bgp 123 subnets
passive-interface Loopback0
passive-interface Loopback1
network 3.3.3.3 0.0.0.0 area 0
network 172.16.3.3 0.0.0.0 area 0
network 172.16.13.3 0.0.0.0 area 0
network 172.16.23.3 0.0.0.0 area 0
!
router bgp 123
no synchronization
bgp router-id 3.3.3.3
network 3.3.3.3 mask 255.255.255.255
network 172.16.1.0 mask 255.255.255.0
network 172.16.3.0 mask 255.255.255.0
neighbor LOCAL_AS peer-group
neighbor LOCAL_AS remote-as 123
neighbor LOCAL_AS update-source Loopback0
neighbor LOCAL_AS next-hop-self
neighbor 2.2.2.2 peer-group LOCAL_AS
neighbor 35.0.0.5 remote-as 5
neighbor 35.0.0.5 filter-list 10 out
no auto-summary
!
ip as-path access-list 10 permit ^$
!
end R4#show run
interface Loopback0
ip address 4.4.4.4 255.255.255.255
!
interface FastEthernet0/0
ip address 45.0.0.4 255.255.255.0
!
interface FastEthernet1/0
ip address 24.0.0.4 255.255.255.0
!
router bgp 4
no synchronization
bgp router-id 4.4.4.4
network 4.4.4.4 mask 255.255.255.255
neighbor 24.0.0.2 remote-as 123
neighbor 45.0.0.5 remote-as 5
no auto-summary
!
end R5#show run
interface Loopback0
ip address 5.5.5.5 255.255.255.255
!
interface FastEthernet0/0
description To R4's F0/0
ip address 45.0.0.5 255.255.255.0
!
interface Serial0/0
ip address 35.0.0.5 255.255.255.0
!
interface Serial0/2
description To R6's S0/0
ip address 56.0.0.5 255.255.255.0
!
router bgp 5
no synchronization
bgp router-id 5.5.5.5
network 5.5.5.5 mask 255.255.255.255
neighbor 35.0.0.3 remote-as 123
neighbor 45.0.0.4 remote-as 4
neighbor 56.0.0.6 remote-as 6
no auto-summary
!
end R6#show run
interface Loopback0
ip address 6.6.6.6 255.255.255.255
!
interface Loopback1
ip address 6.0.0.6 255.255.255.0
!
interface Serial0/0
description To R5's S0/2
ip address 56.0.0.6 255.255.255.0
!
router bgp 6
no synchronization
bgp router-id 6.6.6.6
network 6.0.0.0 mask 255.255.255.0
network 6.6.6.6 mask 255.255.255.255
neighbor 56.0.0.5 remote-as 5
no auto-summary
!
end 在没有策略和负载均衡的配置下，很容易看出上述配置与我们所制定的目标并不一致： R1#show ip route
1.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 1.1.1.1/32 is directly connected, Loopback0
C 1.0.0.0/24 is directly connected, Loopback1
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/11] via 172.16.12.2, 00:00:48, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/11] via 172.16.13.3, 00:00:48, FastEthernet0/1
4.0.0.0/32 is subnetted, 1 subnets
O E2 4.4.4.4 [110/1] via 172.16.12.2, 00:00:48, FastEthernet0/0
5.0.0.0/32 is subnetted, 1 subnets
O E2 5.5.5.5 [110/1] via 172.16.13.3, 00:00:48, FastEthernet0/1
6.0.0.0/8 is variably subnetted, 2 subnets, 2 masks O E2 6.6.6.6/32 [110/1] via 172.16.13.3, 00:00:48, FastEthernet0/1
O E2 6.0.0.0/24 [110/1] via 172.16.13.3, 00:00:49, FastEthernet0/1
172.16.0.0/24 is subnetted, 3 subnets
O 172.16.23.0 [110/20] via 172.16.13.3, 00:00:49, FastEthernet0/1
[110/20] via 172.16.12.2, 00:00:49, FastEthernet0/0
C 172.16.12.0 is directly connected, FastEthernet0/0
C 172.16.13.0 is directly connected, FastEthernet0/1 R5#show ip bgp
Network Next Hop Metric LocPrf Weight Path *> 2.2.2.2/32 35.0.0.3 0 123 i
* 45.0.0.4 0 4 123 i
* 3.3.3.3/32 45.0.0.4 0 4 123 i
*> 35.0.0.3 0 0 123 i
*> 4.4.4.4/32 45.0.0.4 0 0 4 i
*> 5.5.5.5/32 0.0.0.0 0 32768 i
*> 6.0.0.0/24 56.0.0.6 0 0 6 i
*> 6.6.6.6/32 56.0.0.6 0 0 6 i * 172.16.1.0/24 45.0.0.4 0 4 123 i
*> 35.0.0.3 11 0 123 i
* 172.16.2.0/24 45.0.0.4 0 4 123 i
*> 35.0.0.3 0 123 i
* 172.16.3.0/24 45.0.0.4 0 4 123 i
*> 35.0.0.3 0 0 123 i R5#show ip route
35.0.0.0/24 is subnetted, 1 subnets
C 35.0.0.0 is directly connected, Serial0/0
2.0.0.0/32 is subnetted, 1 subnets B 2.2.2.2 [20/0] via 35.0.0.3, 00:43:42 3.0.0.0/32 is subnetted, 1 subnets
B 3.3.3.3 [20/0] via 35.0.0.3, 00:43:42
4.0.0.0/32 is subnetted, 1 subnets
B 4.4.4.4 [20/0] via 45.0.0.4, 00:43:42
5.0.0.0/32 is subnetted, 1 subnets
C 5.5.5.5 is directly connected, Loopback0
6.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
B 6.6.6.6/32 [20/0] via 56.0.0.6, 00:43:42
B 6.0.0.0/24 [20/0] via 56.0.0.6, 00:43:42
172.16.0.0/24 is subnetted, 3 subnets B 172.16.1.0 [20/11] via 35.0.0.3, 00:06:35
B 172.16.2.0 [20/0] via 35.0.0.3, 00:11:07
B 172.16.3.0 [20/0] via 35.0.0.3, 00:08:30
56.0.0.0/24 is subnetted, 1 subnets
C 56.0.0.0 is directly connected, Serial0/2
45.0.0.0/24 is subnetted, 1 subnets
C 45.0.0.0 is directly connected, FastEthernet0/0 可以看到，R1中使用R3作为AS 6中6.0.0.0/24的下一跳；同样，R5使用R3作为R1、R2网络的下一跳。这都是我们之前制定策略不允许的。 为满足需要，做了以下修改： R2(config)# router bgp 123 aggregate-address 172.16.0.0 255.255.252.0 summary-only R3(config)# router ospf 100 redistribute bgp 123 metric 20 subnets router bgp 123 aggregate-address 172.16.0.0 255.255.252.0 suppress-map ADVERTISE_ROUTE neighbor 35.0.0.5 route-map SET_LOCAL_PREF_RM in neighbor 35.0.0.5 route-map SET_AS_PATH_RM out
! ip as-path access-list 20 permit 5$
! ip prefix-list SET_AS_PATH_PL description PERMIT LOCAL PREFIX-LIST
ip prefix-list SET_AS_PATH_PL seq 5 permit 172.16.3.0/24
ip prefix-list SET_AS_PATH_PL seq 10 permit 3.3.3.3/32
! route-map SET_AS_PATH_RM permit 10
match ip address prefix-list SET_AS_PATH_PL
!
route-map SET_AS_PATH_RM permit 20
set origin incomplete
set as-path prepend 123
! route-map ADVERTISE_ROUTE deny 10
match ip address prefix-list SET_AS_PATH_PL
!
route-map ADVERTISE_ROUTE permit 20
! route-map SET_LOCAL_PREF_RM permit 10
match as-path 20
set local-preference 200
!
route-map SET_LOCAL_PREF_RM permit 20
set local-preference 50 首先，绿色字体代表了入流量的负载均衡。由于R2仅仅通告了汇总路由；而R3中做了两项工作：一、通告次优的汇总路由(通过route-map增加了as-path长度和修改了origin值)；二、通告了唯一一条本地具体路由(红色字体部分)。注意suppress-map的作用是抑制策略中所有前缀在aggregate-address后的通告，这里仅允许R3本地前缀172.16.3.0/24向外通告。对于AS 5的路由器R5而言，R3本地的路由在路由表中是唯一的详细条目，因此到达R3本地网络采用R3的链路，其余的由于R2策略更优，因此采用R2的链路： R5#show ip bgp
Network Next Hop Metric LocPrf Weight Path * 2.2.2.2/32 35.0.0.3 0 123 123 ?
*> 45.0.0.4 0 4 123 i
* 3.3.3.3/32 45.0.0.4 0 4 123 i
*> 35.0.0.3 0 0 123 i
*> 4.4.4.4/32 45.0.0.4 0 0 4 i
*> 5.5.5.5/32 0.0.0.0 0 32768 i
*> 6.0.0.0/24 56.0.0.6 0 0 6 i
*> 6.6.6.6/32 56.0.0.6 0 0 6 i *> 172.16.0.0/22 45.0.0.4 0 4 123 i
* 35.0.0.3 0 0 123 123 ?
*> 172.16.3.0/24 35.0.0.3 0 0 123 i R5#show ip route 35.0.0.0/24 is subnetted, 1 subnets
C 35.0.0.0 is directly connected, Serial0/0
2.0.0.0/32 is subnetted, 1 subnets B 2.2.2.2 [20/0] via 45.0.0.4, 01:21:42
3.0.0.0/32 is subnetted, 1 subnets
B 3.3.3.3 [20/0] via 35.0.0.3, 01:18:57
4.0.0.0/32 is subnetted, 1 subnets
B 4.4.4.4 [20/0] via 45.0.0.4, 01:21:42
5.0.0.0/32 is subnetted, 1 subnets
C 5.5.5.5 is directly connected, Loopback0
6.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
B 6.6.6.6/32 [20/0] via 56.0.0.6, 01:21:42
B 6.0.0.0/24 [20/0] via 56.0.0.6, 01:21:42
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks B 172.16.0.0/22 [20/0] via 45.0.0.4, 01:21:42
B 172.16.3.0/24 [20/0] via 35.0.0.3, 01:21:42
56.0.0.0/24 is subnetted, 1 subnets
C 56.0.0.0 is directly connected, Serial0/2
45.0.0.0/24 is subnetted, 1 subnets
C 45.0.0.0 is directly connected, FastEthernet0/0 其次，为解决R1路由表的问题，采用了Local-Pref + weight的方式。其中R3把最终AS为5的路由weight置为200，使其大于默认weight值0；然后把所有的路由Local-Pref置为50，小于默认值100。这样本AS内所有的iBGP peer都认为R3不是合适的网关。这点可以从R2的BGP表中看出： R2#show ip bgp *> 2.2.2.2/32 0.0.0.0 0 32768 i
r>i3.3.3.3/32 3.3.3.3 0 100 0 i
*> 4.4.4.4/32 24.0.0.4 0 0 4 i * i5.5.5.5/32 3.3.3.3 0 50 0 5 i
*> 24.0.0.4 0 4 5 i*> 6.0.0.0/24 24.0.0.4 0 4 5 6 i
*> 6.6.6.6/32 24.0.0.4 0 4 5 6 i
r> 172.16.0.0/22 0.0.0.0 32768 i
r i 3.3.3.3 0 100 0 i
s> 172.16.1.0/24 172.16.12.1 11 32768 i
s> 172.16.2.0/24 0.0.0.0 0 32768 i
r>i172.16.3.0/24 3.3.3.3 0 100 0 i 现在R1的路由表应该满足我们的要求了： R1#show ip route
1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/11] via 172.16.12.2, 02:10:47, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/11] via 172.16.13.3, 02:10:47, FastEthernet0/1
4.0.0.0/32 is subnetted, 1 subnets O E2 4.4.4.4 [110/1] via 172.16.12.2, 00:13:15, FastEthernet0/0
5.0.0.0/32 is subnetted, 1 subnets
O E2 5.5.5.5 [110/1] via 172.16.12.2, 00:00:02, FastEthernet0/0
6.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O E2 6.6.6.6/32 [110/1] via 172.16.12.2, 00:13:15, FastEthernet0/0
O E2 6.0.0.0/24 [110/1] via 172.16.12.2, 00:13:15, FastEthernet0/0
172.16.0.0/16 is variably subnetted, 7 subnets, 2 masks
O 172.16.23.0/24 [110/20] via 172.16.13.3, 02:10:48, FastEthernet0/1
[110/20] via 172.16.12.2, 02:10:48, FastEthernet0/0
C 172.16.12.0/24 is directly connected, FastEthernet0/0
C 172.16.13.0/24 is directly connected, FastEthernet0/1 O E2 172.16.0.0/22 [110/20] via 172.16.13.3, 00:00:02, FastEthernet0/1
C 172.16.1.0/24 is directly connected, Loopback1
O 172.16.2.0/24 [110/11] via 172.16.12.2, 02:10:48, FastEthernet0/0
O 172.16.3.0/24 [110/11] via 172.16.13.3, 02:10:48, FastEthernet0/1 ×××条目可以通过重分配的route-map去掉。当然不去掉也没什么关系，因为实验的汇总并不十分正确，正常情况下应该是汇总里所有具体条目都确确实实存在的。不过无论如何，本实验通过aggregate-address的策略及细化，实现了对外AS入流量的负载均衡。其中失误的地方敬请指教。
- 打赏
- 赞
- 收藏
- 评论
- 分享 微博 QQ 微信
上一篇：Internet路由结构学习心得一：通告汇聚和具体路由影响AS入流量1
下一篇：一名漂泊在外的蚁族写给农民工父亲的忏悔信
提问和评论都可以，用心的回复会被更多人看到 评论 发布评论 全部评论 ()邹盼盼
2010-04-01 23:05
向你学习！ 回复 点赞 删除
