菱形组网之BGP MED、负载分担及GR篇
菱形组网之BGP MED、负载分担及GR篇BGP选路规则1. MED、RouterID（<）和负载分担到达同一目的地的多条路由，选择最优路由，下发到IP路由表在PE1查看路由表dis ip routing-table Route Flags: R - relay, D - download to fib ------------------------------------------------------------------------------ Routing Tables: Public Destinations : 22 Routes : 22
Destination/Mask Proto PreCost 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路由选择的是PE3为下一跳继续查看详细原因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, active, 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, not preferred for MED Not advertised to any peer yet
MED（Multi-exit Discriminator），多出口辨别符，主要就是给来自AS外的两个目的IP相同，下一跳不同的路由选择用，本例中，PE2和PE3都给了PE1 4.4.4.4路由，但是为啥没有选择PE3的？可以看看PE1的BGP路由表，选择MED值小的。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上发布100.2.3.0路由 在PE1上看到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, active, 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 preferred for router ID Not advertised to any peer yet 11条，路由器ID小的被选中。 看红色字体部分，选中了一条路由，如果想要选中两条路由做负载分担，可以在PE1配置maximum load-balancing ebgp 2
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
如果想让PE3的被选中可以在PE3上bgp进程里应用路由策略，路由策略route-policy med permit node 10 apply cost 99 （该值是MED值）peer 100.1.3.1 route-policy med exportreset BGP进程然后在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 preferred for MED Not advertised to any peer yet
2. BGP GR没使能GR之前，在PE2 reset bgp进程--- 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 msPE1上会出现丢包前面OSPFv3已经说过，此处不再重复。一个restarter一个helper。
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