王巧雷
作者王巧雷2020-03-01 01:19
系统工程师, sino-bridge

BGP协议的测试实验

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  1. 基本概念

描述一些前置知识:

1. BGP的全称是border gateway protocol,是用在AS之间的协议。
2. AS是Autonomous system的简称,可以理解为运行同一种选路策略、统一管理的一组路由器。
3. 每个AS都有一个AS number,取值范围为 1-65535 ,其中1-64511是公有的,需要向IANA申请 , 64512-65535是私有的可以自由使用,类似192.168的IP地址。
4. 跨AS的是EBGP,AS内的是IBGP
5. BGP是基于tcp做的封包,可以理解为bgp的包头外面会由tcp封包,tcp外面由IP封包,然后然后是二层的包头。在网络中传输。

关于路由黑洞:
以2.1的拓扑图为例:
pc1的192.168.1.10对pc2的172.16.30.10做ping测试,就需要有路由信息。路由信息封装在bgp中, 从R4到R5的链路中,R1没有运行bgp,当bgp的数据包传输的时候,因为是基于tcp封装的,bgp信息依次被tcp、ip、以太网等包头封装,和普通的网络包一样,只要相邻的两台路由器可以互通,就可以把数据包从R4传送到R5; 但是数据包从pc2返回pc1的时候,封包格式为icmp+ip,IP包头中会有src和dest的地址,这个dest ip就是pc1的192.168.1.10地址,这个数据包到了R3的后,R1是没有bgp协议的,也就学不到192.168.1.0的路由信息,所以传输会失败。这就是路由黑洞。

解决路由黑洞的方法有好几种,比如:

  • 物理线路的full mesh,也就是说将as内的边缘路由器连线
  • 将BGP重分布进OSPF
  • AS内所有路由器都运行bgp(本次实验采用的)
  • 使用MPLS做tunnel

2. 测试过程

本次实验是采用GNS3模拟器做的,采用了IOU设备的方式。
模拟了bgp的实现过程,顺便验证了一下BGP中的路由黑洞。

2.1  测试拓扑

拓扑解释:

  • R1、R2、R3为一个自治区域,号码为123,为ibgp,3台路由器之间运行ospf,R2和R3运行bgp,R1仅运行ospf

  • R4为一个自治区域,号码为4,和R123之间为ebgp

  • R5为一个自治区域,号码为5,和R123之间为ebgp

  • 各自路由器的router id为路由器编号地址,比如R1的为1.1.1.1

2.2  R1、R2、R3配置IP,配置ospf

1  配置IP地址

R1配置IP:

R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#int s2/0
R1(config-if)#ip add 12.1.1.2 255.255.255.0
R1(config-if)#no shu
R1(config-if)#exit
R1(config)#int s2/1
R1(config-if)#ip add 11.1.1.1 255.255.255.0
R1(config-if)#no shu

R2配置IP:

R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#int s2/0
R2(config-if)#ip add 14.1.1.2 255.255.255.0
R2(config-if)#no shu
R2(config-if)#exit
R2(config)#int s2/1
R2(config-if)#ip add 12.1.1.1 255.255.255.0
R2(config-if)#no shu

R3配置IP:

R3#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R3(config)#int s2/0
R3(config-if)#ip add 11.1.1.2 255.255.255.0
R3(config-if)#no shu
R3(config-if)#exit
R3(config)#int s2/1
R3(config-if)#ip add 13.1.1.1 255.255.255.0
R3(config-if)#no shu

 2 配置ospf,对于R2和R3来说,只配置AS内的接口,外联ebgp的口不配置到ospf中

R1配置

R1(config)#router ospf 110
R1(config-router)#router-id 1.1.1.1
R1(config-router)#net 12.1.1.2 0.0.0.0 area 0
R1(config-router)#net 11.1.1.1 0.0.0.0 area 0

R2配置

R2(config)#router ospf 110
R2(config-router)#router-id 2.2.2.2
R2(config-router)#net 12.1.1.1 0.0.0.0 area 0

R3配置

R3(config)#router ospf 110 
R3(config-router)#router-id 3.3.3.3
R3(config-router)#network 11.1.1.2 0.0.0.0 area 0

3 查看路由表信息

R2#show ip route 
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override

Gateway of last resort is not set

      11.0.0.0/24 is subnetted, 1 subnets
O        11.1.1.0 [110/128] via 12.1.1.2, 00:01:46, Serial2/1
      12.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        12.1.1.0/24 is directly connected, Serial2/1
L        12.1.1.1/32 is directly connected, Serial2/1

查看ospf

R2#show ip ospf database 

            OSPF Router with ID (2.2.2.2) (Process ID 110)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         178         0x80000005 0x00A804 4
2.2.2.2         2.2.2.2         205         0x80000004 0x0083EC 2
3.3.3.3         3.3.3.3         179         0x80000004 0x002742 2

2.3 配置bgp

这部分分为ibgp和ebgp的

2.3.1 为as123配置ibgp

1 在R2配置bgp协议

R2(config)#router bgp 123
R2(config-router)#bgp router-id 2.2.2.2
R2(config-router)#neighbor 11.1.1.2 remote-as 123   //为bgp指定邻居,这里指定的是本as内R3的s2/0
R2(config-router)#
*Feb 29 15:43:36.408: %BGP-5-ADJCHANGE: neighbor 11.1.1.2 Up //等待片刻,可以看到邻居建立

R2(config-router)#neighbor 14.1.1.1 remote-as 4     //这里是指定ebgp对端as4的邻居关系

2  在R3配置bgp协议

R3(config)#router bgp 123
R3(config-router)#bgp router-id 3.3.3.3
R3(config-router)#neighbor 12.1.1.1 remote-as 123
R3(config-router)#neighbor 13.1.1.2 remote-as 5

3 查看bgp信息,因为as4和5的IP和bgp还没配置,所以邻居部分的显示不正常

R2执行查看

R2#show ip bgp summary 
BGP router identifier 2.2.2.2, local AS number 123
BGP table version is 1, main routing table version 1

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
11.1.1.2        4          123       9       9        1    0    0 00:04:47        0
14.1.1.1        4            4       0       0        1    0    0 never    Idle

R3执行查看

R3#show ip bgp summary 
BGP router identifier 3.3.3.3, local AS number 123
BGP table version is 1, main routing table version 1

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
12.1.1.1        4          123       9      10        1    0    0 00:05:28        0
13.1.1.2        4            5       0       0        1    0    0 never    Idle

2.32 为AS4和5配置ebgp

1 首先配置R4和5的IP地址

R4的地址

R4#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R4(config)#int s2/0 
R4(config-if)#ip add 14.1.1.1 255.255.255.0
R4(config-if)#no shu
R4(config-if)#exit
R4(config)#int e0/0
R4(config-if)#ip add 192.168.1.1 255.255.255.0 
R4(config-if)#no shu

R5的地址

R5#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R5(config)#int s2/0
R5(config-if)#ip add 13.1.1.2 255.255.255.0
R5(config-if)#no shu
R5(config-if)#exit
R5(config)#int e0/0
R5(config-if)#ip add 172.16.30.1 255.255.255.0
R5(config-if)#no shu

2 为R4配置bgp

R4(config)#router bgp 4
R4(config-router)#bgp router-id 4.4.4.4
R4(config-router)#neighbor 14.1.1.2 remote-as 123    //指定as123中R2的s2/0为邻居
R4(config-router)#
*Feb 29 15:54:02.133: %BGP-5-ADJCHANGE: neighbor 14.1.1.2 Up  //邻居建立提示

3 为R5配置bgp

R5(config-router)#bgp router
R5(config-router)#bgp router-id 5.5.5.5
R5(config-router)#neighbor 13.1.1.1 remote-as 123

4 查看bgp信息

R4#show ip bgp summary 
BGP router identifier 4.4.4.4, local AS number 4
BGP table version is 1, main routing table version 1

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
14.1.1.2        4          123       6       6        1    0    0 00:02:38        0

此时再看R2或者R3的bgp信息,邻居关系就都正常了

R2#show ip bgp summary 
BGP router identifier 2.2.2.2, local AS number 123
BGP table version is 1, main routing table version 1

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
11.1.1.2        4          123      19      18        1    0    0 00:13:40        0
14.1.1.1        4            4       7       7        1    0    0 00:03:14        0

2.3.3 在as4的路由R4 宣告路由192.168.1.0

这里是要将pc1的192.168.1.0/24这个网络宣告到bgp中,步骤如下

1 在R4上宣告路由

R4(config)#router bgp 4
R4(config-router)#network 192.168.1.0 mask 255.255.255.0  //只能宣告网段,指定掩码

查看bgp 路由信息

R4#show ip bgp 
BGP table version is 2, local router ID is 4.4.4.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>  192.168.1.0      0.0.0.0                  0         32768 i

本地产生,下一跳为0.0.0.0

2 此时到R2上查看bgp信息,发现路由信息已经过来了,且可以看到192.168.1.0网段的下一跳为14.1.1.1(R4的S2/0,出口接口)

R2#show ip bgp
BGP table version is 2, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>  192.168.1.0      14.1.1.1                 0             0 4 i

R2上也能看到路由信息,可以看到192.168.1.0是通过bgp学习到的,metric是20,通过ebgp学习到的路由条目metric都是20,如果是ibgp则为200

R2#show ip route 
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override

Gateway of last resort is not set

      11.0.0.0/24 is subnetted, 1 subnets
O        11.1.1.0 [110/128] via 12.1.1.2, 00:31:18, Serial2/1
      12.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        12.1.1.0/24 is directly connected, Serial2/1
L        12.1.1.1/32 is directly connected, Serial2/1
      14.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        14.1.1.0/24 is directly connected, Serial2/0
L        14.1.1.2/32 is directly connected, Serial2/0
B     192.168.1.0/24 [20/0] via 14.1.1.1, 00:05:35

3  再看R3上的bgp信息,发现下一跳还是R4上的s2/0,而且路由条目前面没有>,不是最优路由,说明没有把信息传过来,ibgp传输路由信息的时候不会传输下一跳的地址

R3#show ip bgp
BGP table version is 1, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 * i 192.168.1.0      14.1.1.1                 0    100      0 4 i

需要在R2路由上添加next-hop-self,才会把下一跳的信息传递过来。 这个参数只用在as边缘路由器向内部传ibgp信息时。

R2(config)#router bgp 123
R2(config-router)#neighbor 11.1.1.2 next-hop-self

此时再看,发现下一跳的地址已经更新过来了,12.1.1.1正是R2的s2/1出口

R3#show ip bgp
BGP table version is 2, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>i 192.168.1.0      12.1.1.1                 0    100      0 4 i

再看R3的路由信息,也就有bgp学习到的条目了,当然因为是ibgp传输,所有metric值为200

R3#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override

Gateway of last resort is not set

      11.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        11.1.1.0/24 is directly connected, Serial2/0
L        11.1.1.2/32 is directly connected, Serial2/0
      12.0.0.0/24 is subnetted, 1 subnets
O        12.1.1.0 [110/128] via 11.1.1.1, 00:40:23, Serial2/0
      13.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        13.1.1.0/24 is directly connected, Serial2/1
L        13.1.1.1/32 is directly connected, Serial2/1
B     192.168.1.0/24 [200/0] via 12.1.1.1, 00:04:57

同理,R3上也需要为R2来设置next-hop-self,否则R5向R4方向的路由也会出现同样的问题

R3(config)#router bgp 123
R3(config-router)#neighbor 12.1.1.1 next-hop-self 

2.3.4 在R5上宣告路由172.16.30.0

过程和上一章节类似

1 直接在R5上宣告

R5(config)#router bgp 5                   
R5(config-router)#network 172.16.30.0 mask 255.255.255.0 

2  在R4上已经可以看到正常的路由条目了

R4#show bgp
BGP table version is 3, local router ID is 4.4.4.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>  172.16.30.0/24   14.1.1.2                               0 123 5 i
 *>  192.168.1.0      0.0.0.0                  0         32768 i

2.3.5 路由黑洞的测试

1.pc机配置IP地址

在pc1上配置192.168.1.10

在pc2上配置172.16.30.10

2.在路由器R1上打开debug

R1#debug ip packet 
IP packet debugging is on

3  执行ping测试,失败

wangql> ping 172.16.30.10
172.16.30.10 icmp_seq=1 timeout
172.16.30.10 icmp_seq=2 timeout
172.16.30.10 icmp_seq=3 timeout
172.16.30.10 icmp_seq=4 timeout
172.16.30.10 icmp_seq=5 timeout

4  观察R1的输出,提示源192.168.1.10到目的172.16.30.10不可达

*Feb 29 16:22:51.401: IP: s=192.168.1.10 (Serial2/0), d=172.16.30.10, len 84, input feature, MCI Check(99), rtype 0, forus FALSE, sendself FALSE, mtu 0, fwdchk FALSE
*Feb 29 16:22:51.401: IP: s=192.168.1.10 (Serial2/0), d=172.16.30.10, len 84, unroutable

因为R1没开启bgp,只有ospf,所有没有这两个网段的路由

R1#show ip route 
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override

Gateway of last resort is not set

      11.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        11.1.1.0/24 is directly connected, Serial2/1
L        11.1.1.1/32 is directly connected, Serial2/1
      12.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        12.1.1.0/24 is directly connected, Serial2/0
L        12.1.1.2/32 is directly connected, Serial2/0

2.3.6 在R1启用BGP来解决路由黑洞

1  R1启用bgp,并添加对端端口的邻居

R1(config)#router bgp 123
R1(config-router)#bgp router-id 1.1.1.1
R1(config-router)#neighbor 12.1.1.1 remote-as 123
R1(config-router)#neighbor 11.1.1.2 remote-as 123

2 R2和R3两天边缘路由器需要对R1添加邻居信息和next-hop-self,否则无法传递bgp路由的下一跳参数

R2

R2(config)#router bgp 123
R2(config-router)#neighbor 12.1.1.2 remote-as 123
R2(config-router)#neighbor 12.1.1.2 next-hop-self 

R3

R3(config)#router bgp 123
R3(config-router)#neighbor 11.1.1.1 remote-as 123
R3(config-router)#neighbor 11.1.1.1 next-hop-self 

3 再次测试pc1和pc2,网络通畅了

wangql> ping 172.16.30.10
84 bytes from 172.16.30.10 icmp_seq=1 ttl=59 time=36.557 ms
84 bytes from 172.16.30.10 icmp_seq=2 ttl=59 time=36.735 ms
84 bytes from 172.16.30.10 icmp_seq=3 ttl=59 time=36.005 ms
84 bytes from 172.16.30.10 icmp_seq=4 ttl=59 time=33.740 ms


wangql> trace 172.16.30.10
trace to 172.16.30.10, 8 hops max, press Ctrl+C to stop
 1   192.168.1.1   0.290 ms  0.141 ms  0.163 ms
 2   14.1.1.2   9.157 ms  9.058 ms  8.496 ms
 3   12.1.1.2   18.042 ms  17.537 ms  17.808 ms
 4   11.1.1.2   26.682 ms  26.471 ms  26.210 ms
 5   13.1.1.2   36.360 ms  35.922 ms  34.192 ms
 6   *172.16.30.10   37.326 ms (ICMP type:3, code:3, Destination port unreachable)

3. 小结

通过上面的实验,总结如下:

1. bgp不同于那些igp的协议,配置的时候需要手工指定邻居关系。
2. 在ibgp的配置过程中,除了显式指定邻居关系,还需要手工配置 next-hop-self,否则传递的bgp路由信息中下一跳的信息不会发送改变,导致路由信息无效
3. 在做网络规划的时候,要注意拓扑的合理性,避免出现路由黑洞。

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