BGP的同步规则分析

   BGP的同步规则：学习自IBGP邻居的路由在进入IGP路由表或者被宣告给BGP对等体以前，必须首先经过IGP来知晓该路由，下面经过实验进行分析。

   

如上图：

R1与R2之间是EBGP邻居关系，R2和R3之间是IBGP邻居关系，而R1通告1.1.1.1/32的路由，R3通告3.3.3.3/32的路由，基础配置略，当R2的同步开启的时候（高版本的IOS默认状况下同步是关闭的），查看R1、R2的路由状况；

R1#show ip bgp

BGP table version is 2, local router ID is 1.1.1.1

Status codes: s suppressed, d damped, h history, * valid, > best,i - internal,

              r RIB-failure, SStale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 1.1.1.1/32      0.0.0.0                  0         32768 i

R2#show ip bgp

BGP table version is 6, 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

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 1.1.1.1/32      12.1.1.1                 0             0 10 i

* i3.3.3.3/32      23.1.1.2                 0    100     0 i

R2#show ip route

Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

       D - EIGRP, EX - EIGRPexternal, O - OSPF, IA - OSPF inter area

       N1 - OSPF NSSA externaltype 1, N2 - OSPF NSSA external type 2

       E1 - OSPF external type1, E2 - OSPF external type 2

       i - IS-IS, su - IS-ISsummary, 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 - periodicdownloaded static route

Gateway of last resort is not set

     1.0.0.0/32 is subnetted,1 subnets

B       1.1.1.1 [20/0] via12.1.1.1, 00:05:15

     2.0.0.0/32 is subnetted,1 subnets

C       2.2.2.2 is directlyconnected, Loopback0

     23.0.0.0/30 is subnetted,1 subnets

C       23.1.1.0 is directlyconnected, Serial1/0

     12.0.0.0/30 is subnetted,1 subnets

C       12.1.1.0 is directlyconnected, FastEthernet0/0

    R1并无收到3.3.3.3/32的路由，而R2收到3.3.3.3/32的路由，可是不是最优的，只是可用，而且R2上的IGP表中没有3.3.3.3/32的路由，这是因为BGP的同步规则，在同步开启的状况下，从IBGP学到的路由是不会传递给他的EBGP邻居的，除非IGP路由信息中有该路由；

如今R2上配置一条静态路由3.3.3.3/32，下一跳指向R3，以下：

R2(config)#ip route3.3.3.3 255.255.255.255 23.1.1.2

如今再次查看R2及R1的BGP路由状况

R2#show ip bgp

BGP table version is 8, local router ID is 2.2.2.2

Status codes: s suppressed, d damped, h history, * valid, > best,i - internal,

              r RIB-failure, SStale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 1.1.1.1/32      12.1.1.1                 0             0 10 i

r>i3.3.3.3/32       23.1.1.2                 0    100     0 i

R1#show ip bgp

BGP table version is 3, local router ID is 1.1.1.1

Status codes: s suppressed, d damped, h history, * valid, > best,i - internal,

              r RIB-failure, SStale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 1.1.1.1/32      0.0.0.0                  0         32768 i

*> 3.3.3.3/32       12.1.1.2                               0 23 i

   R2上的3.3.3.3/32路由成为最优的，而且R1上收到了3.3.3.3/32的路由，而且也是最优的。

   所以验证了，从IBGP邻居学到的路由，除非也从IGP学习到该路由，不然不会把该路由装载到BGP路由表中，也不会传递给他的EBGP邻居。

 上述状况是在R2同步开启的状况下，现将R2的同步关闭，并删除配置的静态路，配置以下：

R2(config)#no ip route 3.3.3.3 255.255.255.255 23.1.1.2
R2(config)#router bgp 23
R2(config-router)#no synchronization

R2#clear ip bgp *

清除邻居以后，再次查看R2及R1的BGP路由状况：

R2#show ip bgp

BGP table version is 3, local router ID is 2.2.2.2

Status codes: s suppressed, d damped, h history, * valid, > best,i - internal,

              r RIB-failure, SStale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 1.1.1.1/32      12.1.1.1                 0             0 10 i

*>i3.3.3.3/32       23.1.1.2                 0    100     0 i

R1#show ip bgp

BGP table version is 5, local router ID is 1.1.1.1

Status codes: s suppressed, d damped, h history, * valid, > best,i - internal,

              r RIB-failure, SStale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 1.1.1.1/32       0.0.0.0                  0         32768 i

*> 3.3.3.3/32       12.1.1.2                               0 23 i

R3#show ip bgp

BGP table version is 9, local router ID is 3.3.3.3

Status codes: s suppressed, d damped, h history, * valid, > best,i - internal,

              r RIB-failure, SStale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

* i1.1.1.1/32      12.1.1.1                 0    100     0 10 i

*> 3.3.3.3/32      0.0.0.0                  0        32768 i

   R3收到1.1.1.1/32的路由不优，下一跳是12.1.1.1，在R3上该下一跳地址是不可达的。现采起下面两种方式，而后再次来查看BGP路由状况；

如今R3上配置静态路由：

R3(config)#ip route 12.1.1.1 255.255.255.255 23.1.1.1
或者
R2(config)#router bgp 23
R2(config-router)#neighbor 23.1.1.2 next-hop-self

再次查看R3的BGP路由状况：

R3#show ip bgp

BGP table version is 10, local router ID is 3.3.3.3

Status codes: s suppressed, d damped, h history, * valid, > best,i - internal,

              r RIB-failure, SStale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*>i1.1.1.1/32       12.1.1.1                 0    100     0 10 i

*> 3.3.3.3/32      0.0.0.0                  0         32768 i

    从上述输出可知，当BGP同步关闭，而且IGP表中没有3.3.3.3/32的路由状况下，从IBGP学到的路由，能够传递给EBGP邻居，可是在R3上看见1.1.1.1/32的路由并非最优的，其缘由是下一跳不可达，当配置静态路由或者针对R3修改IBGP的下一跳时，1.1.1.1/32就变成最优的路由。

   所以验证了，若是不是通告IGP学到的路由，或者下一跳不可达，从IBGP邻居学到的路由是不会装载至BGP路由表的。