2

I test the BGP in my PC, this below is the topology:

R1--(eBGP)--R2--(iBGP)--R3. and R2-R3-R4 are in OSPF(omit details).

this is my detail configuration of BGP:

[R1-bgp]dis this
#
bgp 100
 peer 10.1.12.2 as-number 200
 #
 ipv4-family unicast
  undo synchronization
  network 100.1.1.1 255.255.255.255
  network 100.1.2.1 255.255.255.255
  peer 10.1.12.2 enable
#
return
[R2-bgp]dis this
#
bgp 200
 peer 3.3.3.3 as-number 200
 peer 3.3.3.3 connect-interface LoopBack0
 peer 10.1.12.1 as-number 100
 #
 ipv4-family unicast
  undo synchronization
  network 2.2.2.2 255.255.255.0
  network 10.1.23.0 255.255.255.0
  peer 3.3.3.3 enable
  peer 10.1.12.1 enable
#
return
[R3-bgp]dis this
#
bgp 200
 peer 2.2.2.2 as-number 200
 peer 2.2.2.2 connect-interface LoopBack0
 peer 4.4.4.4 as-number 200
 peer 4.4.4.4 connect-interface LoopBack0
 #
 ipv4-family unicast
  undo synchronization
  network 3.3.3.3 255.255.255.255
  peer 2.2.2.2 enable
  peer 4.4.4.4 enable
#
return

I have a question, you see in R3, the bgp routing-table, the network 100.1.1.1/32 and 100.1.2.1/32 is not valid, so can not added into the ip routing-table. but why it is invalid? and the same thing, in R1 there is no R3's 3.3.3.3 in bgp routing-table.

0
2

Routes received via eBGP and advertised to iBGP neighbors preserve original next hop information. To change that use the next-hop-self command when peering iBGP boundary routers with other iBGP routers, or add 10.1.12.0/30 into IGP.

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