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  3. BGP选路

BGP选路

mac2022-06-30  38

实验拓扑

实验需求

现有三个自治系统，需要对R1访问R4的loopback-X数据走向进行精确控制： R1访问R4的loopback0走R2，通过在R1上修改本地优先级实现 R1访问R4的loopback1走R3，通过在R2上修改MED实现 R1访问R4的loopback2走R2，通过在R4上修改AS-path实现 R1访问R4的loopback3走R3，通过在R3上修改origi属性实现

实验步骤

1.配置IP地址及环回口地址

2.配置OSPF 使全网互通(略)

3.R1访问R4的loopback0走R2，通过在R1上修改本地优先级实现

R1

[R1]ip ip-prefix 2 index 10 permit 4.4.4.1 32 [R1]route-policy c permit node 10 [R1-route-policy]if-match ip-prefix 2 [R1-route-policy]apply local-preference 200 [R1]route-policy c permit node 20 [R1]bgp 100 [R1-bgp]peer 192.168.1.2 route-policy c import

4. R1查看BGP协议路由表

[R1]dis bgp routing-table BGP Local router ID is 192.168.1.1 Status codes: * - valid, > - best, d - damped, h - history, i - internal, s - suppressed, S - Stale Origin : i - IGP, e - EGP, ? - incomplete Total Number of Routes: 6 Network NextHop MED LocPrf PrefVal Path/Ogn *> 4.4.4.1/32 192.168.1.2 200 0 200 300i * 192.168.2.2 0 200 300i *> 4.4.4.2/32 192.168.1.2 0 200 300i * 192.168.2.2 0 200 300i *> 4.4.4.3/32 192.168.1.2 0 200 300i * 192.168.2.2 0 200 300 30 0 300 300i

5. R1访问R4的loopback1走R3，通过在R2上修改MED实现

R2

[Huawei]ip ip-prefix 3 index 10 permit 4.4.4.2 32 [Huawei]route-policy h permit node 10 [Huawei-route-policy]if-match ip-prefix 3 [Huawei-route-policy]apply cost 100 [Huawei]route-policy h permit node 20 [Huawei]bgp 200 [Huawei-bgp]peer 192.168.1.1 route-policy h export

分析：当路由器学习到多条去往相同目的网段的BGP路由时，在其他条件都相同的情况下，BGP将比较这些路由的MED 属性，越小越优。其实可理解为就是改开销 

6. 在R1上查看BGP协议路由表

[R1]dis bgp routing-table BGP Local router ID is 192.168.1.1 Status codes: * - valid, > - best, d - damped, h - history, i - internal, s - suppressed, S - Stale Origin : i - IGP, e - EGP, ? - incomplete Total Number of Routes: 6 Network NextHop MED LocPrf PrefVal Path/Ogn *> 4.4.4.1/32 192.168.1.2 200 0 200 300i * 192.168.2.2 0 200 300i *> 4.4.4.2/32 192.168.2.2 0 200 300i * 192.168.1.2 100 0 200 300i *> 4.4.4.3/32 192.168.1.2 0 200 300i * 192.168.2.2 0 200 300 30 0 300 300i

7. 在R1上检查一下BGP的详细信息表

[R1]dis bgp routing-table 4.4.4.2 BGP local router ID : 192.168.1.1 Local AS number : 100 Paths: 2 available, 1 best, 1 select BGP routing table entry information of 4.4.4.2/32: From: 192.168.2.2 (192.168.2.2) Route Duration: 00h21m07s Direct Out-interface: GigabitEthernet0/0/1 Original nexthop: 192.168.2.2 Qos information : 0x0 AS-path 200 300, origin igp, pref-val 0, valid, external, best, select, active, pre 255 Advertised to such 2 peers: 192.168.1.2 192.168.2.2 BGP routing table entry information of 4.4.4.2/32: From: 192.168.1.2 (192.168.1.2) Route Duration: 00h09m37s Direct Out-interface: GigabitEthernet0/0/0 Original nexthop: 192.168.1.2 Qos information : 0x0 AS-path 200 300, origin igp, MED 100, pref-val 0, valid, external, pre 255, not preferred for MED Not advertised to any peer yet //发现我们修改的MED 值 已生效，在这里可以检查核对。

8. R1访问R4的loopback2走R2，通过在R4上修改AS-path实现

R4

[Huawei]ip ip-prefix s index 10 permit 4.4.4.3 32 [Huawei]route-policy s permit node 10 [Huawei-route-policy]if-match ip-prefix s [Huawei-route-policy]apply as-path 300 300 300 additive [Huawei]route-policy s permit node 20 [Huawei]bgp 300 [Huawei-bgp]peer 192.168.200.2 route-policy s export

分析：BGP 优选AS-path 属性最短的路由，我把AS-path 属性变长（增加了AS号）,达到实验需求。

9. R1上查看BGP路由表，发现在表中可以观察到已生效.

[R1]dis bgp routing-table BGP Local router ID is 192.168.1.1 Status codes: * - valid, > - best, d - damped, h - history, i - internal, s - suppressed, S - Stale Origin : i - IGP, e - EGP, ? - incomplete Total Number of Routes: 6 Network NextHop MED LocPrf PrefVal Path/Ogn *> 4.4.4.1/32 192.168.1.2 200 0 200 300i * 192.168.2.2 0 200 300i *> 4.4.4.2/32 192.168.2.2 0 200 300i * 192.168.1.2 100 0 200 300i *> 4.4.4.3/32 192.168.1.2 0 200 300i * 192.168.2.2 0 200 300 30 0 300 300i

10. R1访问R4的loopback3走R3，通过在R3上修改origi属性实现

做这道题，首先要搞清楚BGP的Origin属性的优先级

BGP属性 - Origin ；3种Origin属性的优先级为：i>e>Incomplete（？）

Origin的3种属性： i 表明BGP路由通过network命令注入； e 表明BGP路由是从EGP学来的，EGP协议在现网中很难见到，但可以通过路由策略将路由的Origin属性修改为e； ？ 即Incomplete表明BGP路由通过其它方式学到路由信息，如使用import命令引入的路由。
把loopback3引入BGP 300中，只引入4.4.4.4 的路由，不想产生其他路由条目。
[Huawei]ip ip-prefix huawei permit 4.4.4.4 32 [Huawei]route-policy huawei permit node 10 [Huawei-route-policy]if-match ip-prefix huawei [Huawei]route-policy huawei permit node 10 [Huawei]bgp 300 [Huawei-bgp]import-route direct route-policy huawei

11. 在AR1上查看BGP路由表，发现学习到的4.4.4.4路由为 ? ,此时只需要到R3上更改Origin属性为IGP 即可

R3

[R3] ip ip-prefix 321 permit 4.4.4.4 32 [R3] route-policy 321 permit node 10 [R3-route-policy]if-match ip-prefix 321 [R3-route-policy]apply origin igp //修改属性 [R3]route-policy 321 permit node 20 [R3]bgp 200 [R3-bgp]peer 192.168.2.1 route-policy 321 export //出方向

12. R1上查看BGP路由表

[R1]dis bgp routing-table BGP Local router ID is 192.168.1.1 Status codes: * - valid, > - best, d - damped, h - history, i - internal, s - suppressed, S - Stale Origin : i - IGP, e - EGP, ? - incomplete Total Number of Routes: 8 Network NextHop MED LocPrf PrefVal Path/Ogn *> 4.4.4.1/32 192.168.1.2 200 0 200 300i * 192.168.2.2 0 200 300i *> 4.4.4.2/32 192.168.2.2 0 200 300i * 192.168.1.2 100 0 200 300i *> 4.4.4.3/32 192.168.1.2 0 200 300i * 192.168.2.2 0 200 300 30 0 300 300i *> 4.4.4.4/32 192.168.2.2 0 200 300i * 192.168.1.2 0 200 300?

转载于:https://www.cnblogs.com/yu15/p/11429085.html

相关资源：华为BGP选路原则.txt
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