BGP選路規則
1. MED、RouterID（<）和負載分擔
到達同一目的地的多條路由，選擇最優路由，下發到IP路由表
在PE1查看路由表
[PE1]dis ip routing-table
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Tables: Public
Destinations : 22 Routes : 22
Destination/Mask Proto Pre Cost Flags NextHop Interface
4.4.4.4/32 EBGP 255 0 D 100.1.3.2 GigabitEthernet0/2/2
可以
可以看到PE4的loopback0路由選擇的是PE3爲下一跳
繼續查看詳細原因
[PE1]dis bgp routing-table 4.4.4.4 32
BGP local router ID : 1.1.1.1
Local AS number : 200
Paths: 2 available, 1 best, 1 select
BGP routing table entry information of 4.4.4.4/32:
From: 100.1.3.2 (3.3.3.3)
Route Duration: 00h19m35s
Direct Out-interface: GigabitEthernet0/2/2
Original nexthop: 100.1.3.2
Qos information : 0x0
AS-path 100, origin incomplete, pref-val 0, valid, external, best, select, acti
ve, pre 255
Advertised to such 2 peers:
100.1.3.2
100.1.2.2
BGP routing table entry information of 4.4.4.4/32:
From: 100.1.2.2 (2.2.2.2)
Route Duration: 00h17m34s
Direct Out-interface: GigabitEthernet0/2/5
Original nexthop: 100.1.2.2
Qos information : 0x0
AS-path 100, origin incomplete, MED 1, pref-val 0, valid, external, pre 255, no
t preferred for MED
Not advertised to any peer yet
MED（Multi-exit Discriminator），多出口辨別符，主要就是給來自AS外的兩個目的IP相同，下一跳不同的路由選擇用，本例中，PE2和PE3都給了PE1 4.4.4.4路由，但是爲啥沒有選擇PE3的？可以看看PE1的BGP路由表，選擇MED值小的。
[PE1]dis bgp routing-table
BGP Local router ID is 1.1.1.1
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Total Number of Routes: 12
Network NextHop MED LocPrf PrefVal Path/Ogn
*> 4.4.4.4/32 100.1.3.2 0 100?
* 100.1.2.2 1 0 100?
路由器ID
在PE2上發佈100.2.3.0路由
在PE1上看到
[PE1]dis bgp routing-table 100.2.3.0
BGP local router ID : 1.1.1.1
Local AS number : 200
Paths: 2 available, 1 best, 1 select
BGP routing table entry information of 100.2.3.0/24:
From: 100.1.2.2 (2.2.2.2)
Route Duration: 00h06m07s
Direct Out-interface: GigabitEthernet0/2/5
Original nexthop: 100.1.2.2
Qos information : 0x0
AS-path 100, origin igp, MED 0, pref-val 0, valid, external, best, select, acti
ve, pre 255
Advertised to such 2 peers:
100.1.3.2
100.1.2.2
BGP routing table entry information of 100.2.3.0/24:
From: 100.1.3.2 (3.3.3.3)
Route Duration: 00h02m05s
Direct Out-interface: GigabitEthernet0/2/2
Original nexthop: 100.1.3.2
Qos information : 0x0
AS-path 100, origin igp, MED 0, pref-val 0, valid, external, pre 255, not prefe
rred for router ID
Not advertised to any peer yet
11條，路由器ID小的被選中。
看紅色字體部分，選中了一條路由，如果想要選中兩條路由做負載分擔，可以在PE1配置
maximum load-balancing ebgp 2
[PE1]dis bgp routing-table 100.2.3.0
BGP local router ID : 1.1.1.1
Local AS number : 200
Paths: 2 available, 1 best, 2 select
如果想讓PE3的被選中
可以在PE3上bgp進程裏應用路由策略，路由策略
route-policy med permit node 10
apply cost 99 （該值是MED值）
peer 100.1.3.1 route-policy med export
reset BGP進程
然後在PE1上看效果
BGP routing table entry information of 100.2.3.0/24:
From: 100.1.3.2 (3.3.3.3)
Route Duration: 00h00m23s
Direct Out-interface: GigabitEthernet0/2/2
Original nexthop: 100.1.3.2
Qos information : 0x0
AS-path 100, origin igp, MED 99, pref-val 0, valid, external, pre 255, not pref
erred for MED
Not advertised to any peer yet
2. BGP GR
沒使能GR之前，在PE2 reset bgp進程
--- 100.2.3.1 ping statistics ---
143 packet(s) transmitted
127 packet(s) received
11.18% packet loss
round-trip min/avg/max = 1/1/1 ms
PE1上會出現丟包
前面OSPFv3已經說過，此處不再重複。一個restarter一個helper。