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如果路由从iBGP对等体学到，那么边界路由器在把这条路由通告给eBGP对等体之前，会清除它的MED。
R1#sh ip bgp 55.55.55.55
BGP routing table entry for 55.55.55.55/32, version 16
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x4800
  Advertised to update-groups:
     1        
  Local
    15.1.1.5 from 15.1.1.5 (5.5.5.5) <----- from iBGP nei
      Origin incomplete, metric 200, localpref 100, valid, internal, best

R2#sh ip bgp 55.55.55.55
BGP routing table entry for 55.55.55.55/32, version 25
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Not advertised to any peer
  1
    192.168.1.1 from 192.168.1.1 (11.11.11.11)
      Origin incomplete, localpref 100, valid, external, best

------------------------------------------------------------------------------------

如果通过network或redistribute命令注入到BGP中的路由是来自于直连路由的，那么BGP MED为O

R1#sh ip bgp
BGP table version is 16, local router ID is 11.11.11.11
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
*> 1.1.1.0/24       0.0.0.0                  0         32768 i  <-- R1 network
*> 2.2.2.0/24       192.168.1.2              0             0 2 i<-- R2 network
*> 4.4.4.4/32       14.1.1.4                 2         32768 ?
*>i5.5.5.0/24       15.1.1.5                 0    100      0 i  <-- R5 network
*> 11.11.11.11/32   0.0.0.0                  0         32768 ?  <-- R1 redistribute
*> 14.1.1.0/24      0.0.0.0                  0         32768 ?  <-- R1 redistribute
*>i55.55.55.55/32   15.1.1.5               200    100      0 ?
*> 172.16.0.0/22    0.0.0.0                            32768 i
*> 172.16.1.0/24    14.1.1.4                 2         32768 i
*> 172.16.2.0/24    14.1.1.4                 2         32768 ?
*> 172.16.3.0/24    14.1.1.4                 2         32768 ?

------------------------------------------------------------------------------------------

如果通过network或redistribute命令注入到BGP中的路由是来自于IGP的，那么BGP MED从IGP度量中导出：

R1#sh ip route ospf
     4.0.0.0/32 is subnetted, 1 subnets
O       4.4.4.4 [110/2] via 14.1.1.4, 01:58:39, FastEthernet1/0
     172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks
O       172.16.1.0/24 [110/2] via 14.1.1.4, 01:58:39, FastEthernet1/0
O       172.16.2.0/24 [110/2] via 14.1.1.4, 01:58:39, FastEthernet1/0
O       172.16.3.0/24 [110/2] via 14.1.1.4, 01:58:39, FastEthernet1/0

R1#sh ip bgp            
BGP table version is 16, local router ID is 11.11.11.11
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
*> 1.1.1.0/24       0.0.0.0                  0         32768 i
*> 2.2.2.0/24       192.168.1.2              0             0 2 i
*> 4.4.4.4/32       14.1.1.4                 2         32768 ?   <--redistribute form IGP
*>i5.5.5.0/24       15.1.1.5                 0    100      0 i
*> 11.11.11.11/32   0.0.0.0                  0         32768 ?
*> 14.1.1.0/24      0.0.0.0                  0         32768 ?
*>i55.55.55.55/32   15.1.1.5               200    100      0 ?
*> 172.16.0.0/22    0.0.0.0                            32768 i
*> 172.16.1.0/24    14.1.1.4                 2         32768 i  <--redistribute form IGP
*> 172.16.2.0/24    14.1.1.4                 2         32768 ?  <--redistribute form IGP
*> 172.16.3.0/24    14.1.1.4                 2         32768 ?  <--redistribute form IGP

------------------------------------------------------------------------------------------

如果通过aggregate-address命令注入到BGP中的，那么BGP MED不被设置：

R1#sh ip bgp            
BGP table version is 16, local router ID is 11.11.11.11
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
*> 1.1.1.0/24       0.0.0.0                  0         32768 i
*> 2.2.2.0/24       192.168.1.2              0             0 2 i
*> 4.4.4.4/32       14.1.1.4                 2         32768 ?
*>i5.5.5.0/24       15.1.1.5                 0    100      0 i
*> 11.11.11.11/32   0.0.0.0                  0         32768 ?
*> 14.1.1.0/24      0.0.0.0                  0         32768 ?
*>i55.55.55.55/32   15.1.1.5               200    100      0 ?
*> 172.16.0.0/22    0.0.0.0                            32768 i <---aggregate
*> 172.16.1.0/24    14.1.1.4                 2         32768 i
*> 172.16.2.0/24    14.1.1.4                 2         32768 ?
*> 172.16.3.0/24    14.1.1.4                 2         32768 ?