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[Router R1 AS65000]---eBGP---[Router R2 AS65010]---iBGP/OSPF---[Router R3 AS65010]
| | |
[1.1.1.0/24] [2.2.2.0/24] [3.3.3.0/24]
1.1.1.0/24
2.2.2.0/24
3.3.3.0/24
### R1 ###
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# jan/ 5/2019 13:31:44 by RouterOS 6.43.8
# software id = 13FD-KY09
#
Flags: X - disabled, A - active, D - dynamic,
C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme,
B - blackhole, U - unreachable, P - prohibit
# DST-ADDRESS PREF-SRC GATEWAY DISTANCE
0 ADC 1.1.1.0/24 1.1.1.1 br1 0
1 ADb 2.2.2.0/24 10.1.2.2 20
2 ADb 3.3.3.0/24 10.1.2.2 20
3 ADC 10.1.2.0/24 10.1.2.1 ether1 0
4 ADC 192.168.99.1/32 192.168.99.1 brlo 0
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# jan/ 5/2019 13:32:16 by RouterOS 6.43.8
# software id = P955-524K
#
Flags: X - disabled, A - active, D - dynamic,
C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme,
B - blackhole, U - unreachable, P - prohibit
# DST-ADDRESS PREF-SRC GATEWAY DISTANCE
0 ADb 1.1.1.0/24 10.1.2.1 20
1 ADC 2.2.2.0/24 2.2.2.1 br2 0
2 ADb 3.3.3.0/24 192.168.99.3 200
3 ADC 10.1.2.0/24 10.1.2.2 ether1 0
4 ADC 10.2.3.0/24 10.2.3.1 ether2 0
5 ADC 192.168.99.2/32 192.168.99.2 brlo 0
6 ADo 192.168.99.3/32 10.2.3.2 110
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# jan/ 5/2019 13:33:23 by RouterOS 6.43.8
# software id = 6CCJ-2JSK
#
Flags: X - disabled, A - active, D - dynamic,
C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme,
B - blackhole, U - unreachable, P - prohibit
# DST-ADDRESS PREF-SRC GATEWAY DISTANCE
0 ADb 1.1.1.0/24 192.168.99.2 200
1 ADb 2.2.2.0/24 192.168.99.2 200
2 ADC 3.3.3.0/24 3.3.3.0 br3 0
3 ADC 10.2.3.0/24 10.2.3.2 ether1 0
4 ADo 192.168.99.2/32 10.2.3.1 110
5 ADC 192.168.99.3/32 192.168.99.3 brlo 0
...
Now, let's enter an evil magic spell on router R3:
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/routing ospf network add network=3.3.3.0/24 area=backbone
....
OK, simple stuff, that evil spell. We are just advertising 3.3.3.0/24 not only through BGP (as we already did before), but now additionally via OSPF. More is more, right? Two paths to the same destination to chose from - one advertised via BGP, one via OSPF. No problem with that, should have no influence on the reachability of any of the three networks, right?
1.1.1.0/24
2.2.2.0/24
3.3.3.0/24
Wrong! eBGP on R2 stops advertising 3.3.3.0/24! Apparently because the OSPF route is the active one (lower admin distance), which causes the iBGP route to become inactive. And R2 does not seem to like inactive iBGP routes - at least it stops advertising them, even though it is a perfectly valid path!
### R1 - missing 3.3.3.0/24 ####
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# jan/ 5/2019 13:48:17 by RouterOS 6.43.8
# software id = 13FD-KY09
#
Flags: X - disabled, A - active, D - dynamic,
C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme,
B - blackhole, U - unreachable, P - prohibit
# DST-ADDRESS PREF-SRC GATEWAY DISTANCE
0 ADC 1.1.1.0/24 1.1.1.1 br1 0
1 ADb 2.2.2.0/24 10.1.2.2 20
2 ADC 10.1.2.0/24 10.1.2.1 ether1 0
3 ADC 192.168.99.1/32 192.168.99.1 brlo 0
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# jan/ 5/2019 13:48:25 by RouterOS 6.43.8
# software id = P955-524K
#
Flags: X - disabled, A - active, D - dynamic,
C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme,
B - blackhole, U - unreachable, P - prohibit
# DST-ADDRESS PREF-SRC GATEWAY DISTANCE
0 ADb 1.1.1.0/24 10.1.2.1 20
1 ADC 2.2.2.0/24 2.2.2.1 br2 0
2 ADo 3.3.3.0/24 10.2.3.2 110
3 Db 3.3.3.0/24 192.168.99.3 200
4 ADC 10.1.2.0/24 10.1.2.2 ether1 0
5 ADC 10.2.3.0/24 10.2.3.1 ether2 0
6 ADC 192.168.99.2/32 192.168.99.2 brlo 0
7 ADo 192.168.99.3/32 10.2.3.2 110
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Flags: X - disabled, A - active, D - dynamic,
C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme,
B - blackhole, U - unreachable, P - prohibit
# DST-ADDRESS PREF-SRC GATEWAY DISTANCE
0 ADb 1.1.1.0/24 192.168.99.2 200
1 ADb 2.2.2.0/24 192.168.99.2 200
2 ADC 3.3.3.0/24 3.3.3.0 br3 0
3 ADC 10.2.3.0/24 10.2.3.2 ether1 0
4 ADo 192.168.99.2/32 10.2.3.1 110
5 ADC 192.168.99.3/32 192.168.99.3 brlo 0
Works as designed?