简单说明
BGP nexthop属性是一个公认必选属性，它是去往目的路由下一跳路由器的IP地址。该地址并不一定是邻居路由器的地址。该属性需要遵守如下规则：
-
从EBGP邻居学习到的路由会传递给我的EBGP邻居，下一跳改变，变成自己的IP地址。
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从EBGP邻居学习到的路由会传递给我的IBGP邻居，下一跳不变，还是EBGP邻居，需要使用next-hop-self改变。
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从IBGP邻居学习到的路由会传递给我的EBGP邻居，下一跳改变，变成自己的IP。
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特殊协议特殊对待，比如对于evpn协议强制不能修改该属性。因为该属性表示的是源vtep的IP地址，如果修改该属性将会导致vxlan隧道建立错误。
今天我们重点套路第二条：从EBGP邻居学习到的路由会传递给我的IBGP邻居，下一跳不变，还是EBGP邻居，需要使用next-hop-self改变。
实验TOPO
实验说明：如上图所示三个路由器，RTA和RTB在自治区AS65001中，两者运行IBGP协议。RTC在自治区AS65002中，与RTB之间运行EBGP协议，建立EBGP邻居。RTC发布路由6.6.6.0/24，希望RTB在收到该路由后将下一跳改为自己。
实验配置
RTA
RTA# show running-config
Building configuration...
Current configuration:
!
frr version 7.1
frr defaults traditional
hostname 97c64887c9b7
log syslog informational
no ipv6 forwarding
hostname RTA
service integrated-vtysh-config
!
router bgp 65001
neighbor 10.1.1.2 remote-as internal
!
line vty
!
end
RTA#
RTB
RTB# show running-config
Building configuration...
Current configuration:
!
frr version 7.1
frr defaults traditional
hostname 7e9362cc0fca
log syslog informational
no ipv6 forwarding
hostname RTB
service integrated-vtysh-config
!
router bgp 65001
neighbor 10.1.1.1 remote-as internal
neighbor 10.1.2.2 remote-as external
!
line vty
!
end
RTB#
RTC
RTC# show running-config
Building configuration...
Current configuration:
!
frr version 7.1
frr defaults traditional
hostname d4996181649f
log syslog informational
no ipv6 forwarding
hostname RTC
service integrated-vtysh-config
!
interface lo
ip address 6.6.6.6/24
!
router bgp 65002
neighbor 10.1.2.1 remote-as external
!
address-family ipv4 unicast
network 6.6.6.0/24
exit-address-family
!
line vty
!
end
RTC#
查看RTA路由
RTA# show ip bgp
BGP table version is 0, local router ID is 172.17.0.2, vrf id 0
Default local pref 100, local AS 65001
Status codes: s suppressed, d damped, h history, * valid, > best, = multipath,
i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
i6.6.6.0/24 10.1.2.2 0 100 0 65002 i
Displayed 1 routes and 1 total paths
RTA#
RTA# show ip route bgp
RTA#
从上面可以看出bgp路由表中已经存在了6.6.6.0/24的路由，其下一跳为10.1.2.2。但是在路由表中却没有该路由，这是因为在RTA上下一跳10.1.2.2不可达，对于下一跳不可达的路由，路由器时不会安装的。所以需要在RTA上解决下一跳10.1.2.2的可达问题，可以设置静态路由。
设置静态路由
RTA(config)# ip route 10.1.2.0/24 10.1.1.2 eth1
RTA(config)#
RTA# show ip bgp
BGP table version is 1, local router ID is 172.17.0.2, vrf id 0
Default local pref 100, local AS 65001
Status codes: s suppressed, d damped, h history, * valid, > best, = multipath,
i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>i6.6.6.0/24 10.1.2.2 0 100 0 65002 i
Displayed 1 routes and 1 total paths
RTA# show ip route bgp
Codes: K - kernel route, C - connected, S - static, R - RIP,
O - OSPF, I - IS-IS, B - BGP, E - EIGRP, N - NHRP,
T - Table, v - VNC, V - VNC-Direct, A - Babel, D - SHARP,
F - PBR, f - OpenFabric,
> - selected route, * - FIB route, q - queued route, r - rejected route
B> 6.6.6.0/24 [200/0] via 10.1.2.2 (recursive), 00:00:23
* via 10.1.1.2, eth1, 00:00:23
RTA#
从上面可以看出通过添加静态路由后，RTA安装了6.6.6.0/24路由，且有10.1.2.2 (recursive)字段，表示递归解决下一跳。
那有没有其它办法呢？从递归路由来看，对于RTA来说，其最近的下一跳应该是"via 10.1.1.2, eth1",即与RTB连接的RTB接口IP地址，如果让RTB在发布路由的时候将其下一跳改为自己，那岂不就可以啦。是的，BGP提供这个功能。
配置RTB的next hop self属性
RTA# configure terminal
RTA(config)# no ip route 10.1.2.0/24 10.1.1.2 eth1
RTA(config)# exit
RTA#
RTB# configure terminal
RTB(config)# router bgp 65001
RTB(config-router)# address-family ipv4 unicast
RTB(config-router-af)# neighbor 10.1.1.1 next-hop-self
RTB(config-router-af)#
RTA# show ip bgp
BGP table version is 3, local router ID is 172.17.0.2, vrf id 0
Default local pref 100, local AS 65001
Status codes: s suppressed, d damped, h history, * valid, > best, = multipath,
i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>i6.6.6.0/24 10.1.1.2 0 100 0 65002 i
Displayed 1 routes and 1 total paths
RTA# show ip route bgp
Codes: K - kernel route, C - connected, S - static, R - RIP,
O - OSPF, I - IS-IS, B - BGP, E - EIGRP, N - NHRP,
T - Table, v - VNC, V - VNC-Direct, A - Babel, D - SHARP,
F - PBR, f - OpenFabric,
> - selected route, * - FIB route, q - queued route, r - rejected route
B>* 6.6.6.0/24 [200/0] via 10.1.1.2, eth1, 00:00:43
RTA#
从上面的信息可以看出,在RTA的bgp路由表中，6.6.6.0/24的下一跳被改为了10.1.1.2。