带OSPF配置问题的BGP

Question

ISP -通过default route将eBGP发送到sw1/2

sw1 -配置

router ospf 100
 network 0.0.0.0 255.255.255.255 area 0
 default-information originate always
!         
router bgp 100
 bgp log-neighbor-changes
 network 70.70.70.0 mask 255.255.255.0
 timers bgp 10 30
 redistribute ospf 100
 neighbor 74.74.74.1 remote-as 200
 neighbor 192.168.1.19 remote-as 100
 neighbor 192.168.1.19 update-source Loopback0
 neighbor 192.168.1.19 next-hop-self

sw2 -配置

router ospf 100
 network 0.0.0.0 255.255.255.255 area 0
 default-information originate always
!
router bgp 100
 bgp log-neighbor-changes
 network 70.70.70.0 mask 255.255.255.0
 timers bgp 10 30
 redistribute ospf 100
 neighbor 64.64.64.1 remote-as 200
 neighbor 192.168.1.20 remote-as 100
 neighbor 192.168.1.20 update-source Loopback0
 neighbor 192.168.1.20 next-hop-self
!

sw3 -配置

router ospf 100
 network 0.0.0.0 255.255.255.255 area 0

问题：

在这一点上一切正常，我的路由表在sw3上如下所示

sw1 - 10.0.0.5 
sw2 - 10.0.0.10

你可以看到它的负载平衡流量。

Gateway of last resort is 10.0.0.10 to network 0.0.0.0

O*E2  0.0.0.0/0 [110/1] via 10.0.0.10, 00:35:37, GigabitEthernet0/3
                [110/1] via 10.0.0.5, 00:35:47, GigabitEthernet0/1

现在我继续关闭sw1接口A (ISP侧接口)。

然后我在sw3上看到下面的路由表，如果您注意到我的默认网关现在是10.0.0.5，10.0.0.10消失了，那么现在我的流量就像这样

sw3-->sw1-->sw2-->ISP

Gateway of last resort is 10.0.0.5 to network 0.0.0.0

O*E2  0.0.0.0/0 [110/1] via 10.0.0.5, 00:39:04, GigabitEthernet0/1
      10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks

有人能解释为什么OSPF在这里选择长路径而不是向10.0.0.10 (sw2)发送流量(sw1)吗？

编辑

OSPF数据库输出：

sw1#sh ip ospf database 

            OSPF Router with ID (200.200.200.1) (Process ID 100)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
70.70.70.1      70.70.70.1     219         0x80000004 0x00756F 3
192.168.1.19    192.168.1.19    1320        0x80000005 0x007C09 4
200.200.200.1   200.200.200.1   22          0x80000007 0x006B44 5

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
10.0.0.1        200.200.200.1   240         0x80000002 0x004BB1
10.0.0.5        200.200.200.1   240         0x80000002 0x00EE3B
10.0.0.10       192.168.1.19    198         0x80000002 0x00855B

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
0.0.0.0         192.168.1.19    1317        0x80000001 0x00C946 100
0.0.0.0         200.200.200.1   13          0x80000001 0x001369 100

BGP命令输出：192.168.1.19和192.168.1.20是iBGP的回送接口

sw1 - BGP

sw1#sh ip bgp 
BGP table version is 43, local router ID is 200.200.200.1
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
 *>  0.0.0.0          74.201.98.1              0             0 200 ?
 * i                  192.168.1.19             0    100      0 200 ?
 * i 10.0.0.0/30      192.168.1.19             0    100      0 ?
 *>                   0.0.0.0                  0         32768 ?
 * i 10.0.0.4/30      192.168.1.19             2    100      0 ?
 *>                   0.0.0.0                  0         32768 ?
 *>  10.0.0.8/30      10.0.0.6                 2         32768 ?
 * i                  192.168.1.19             0    100      0 ?
 *>  64.64.64.0/30    10.0.0.2                 2         32768 ?
 * i                  192.168.1.19             0    100      0 ?
 * i 70.64.196.0/24   192.168.1.19             2    100      0 ?
 *>                   10.0.0.6                 2         32768 ?
 * i 74.74.74.0/30   192.168.1.19             2    100      0 ?
 *>                   0.0.0.0                  0         32768 ?
     Network          Next Hop            Metric LocPrf Weight Path
 r>i 192.168.1.19/32  192.168.1.19             0    100      0 ?
 *>  192.168.1.20/32  0.0.0.0                  0         32768 ?
 *>  200.200.200.0    0.0.0.0                  0         32768 ?
 r>i 200.200.200.1/32 192.168.1.19             2    100      0 ?
 *>  202.1.1.0        74.74.74.1              0             0 200 i
 * i                  192.168.1.19             0    100      0 200 i
 *>  202.202.202.0    74.74.74.1              0             0 200 i
 * i                  192.168.1.19             0    100      0 200 i

sw2 - BGP

sw2#sh ip bgp 
BGP table version is 37, local router ID is 192.168.1.19
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 0.0.0.0          192.168.1.20             0    100      0 200 ?
 *>                   64.64.64.1               0             0 200 ?
 * i 10.0.0.0/30      192.168.1.20             0    100      0 ?
 *>                   0.0.0.0                  0         32768 ?
 *>  10.0.0.4/30      10.0.0.9                 2         32768 ?
 * i                  192.168.1.20             0    100      0 ?
 * i 10.0.0.8/30      192.168.1.20             2    100      0 ?
 *>                   0.0.0.0                  0         32768 ?
 * i 64.64.64.0/30    192.168.1.20             2    100      0 ?
 *>                   0.0.0.0                  0         32768 ?
 * i 70.70.70.0/24   192.168.1.20             2    100      0 ?
 *>                   10.0.0.9                 2         32768 ?
 *>  74.74.74.0/30   10.0.0.1                 2         32768 ?
 * i                  192.168.1.20             0    100      0 ?
     Network          Next Hop            Metric LocPrf Weight Path
 *>  192.168.1.19/32  0.0.0.0                  0         32768 ?
 r>i 192.168.1.20/32  192.168.1.20             0    100      0 ?
 *>i 200.200.200.0    192.168.1.20             0    100      0 ?
 *>  200.200.200.1/32 10.0.0.1                 2         32768 ?
 * i 202.1.1.0        192.168.1.20             0    100      0 200 i
 *>                   64.64.64.1               0             0 200 i
 * i 202.202.202.0    192.168.1.20             0    100      0 200 i
 *>                   64.64.64.1               0             0 200 i

Answer 1

我认为您的问题在于always关键字：

default-information originate always

这将始终从该路由器注入一个默认路由到OSPF。由于我从以前的问题/聊天中了解到您的情况，您将收到ISP的默认路由。如果不使用always关键字，则路由器只有在路由表中已经存在时才会将默认路由注入OSPF：

default-information originate

如果您正在从ISP接收默认路由，那么当连接到ISP时，您的路由器只会将来自ISP的默认路由注入OSPF。