07-23-2008 05:39 AM - edited 03-03-2019 10:51 PM
Hi all,
It is strange that I don't know why the 26 router will learn below network via ospf
method, I think it should be learned via 17A ebgp Router (192.168.1.2). Anyone know
what happen on it ? Thank you
2600#sh ip route
O E2 172.20.1.0 [110/1] via 10.1.1.3, 00:11:10, Ethernet0/1
O E2 172.20.1.4 [110/1] via 10.1.1.3, 00:11:10, Ethernet0/1
O E2 192.168.170.192 [110/1] via 10.1.1.3, 00:11:10, Ethernet0/1
O E2 192.168.170.128 [110/1] via 10.1.1.3, 00:11:10, Ethernet0/1
O E2 192.168.170.64 [110/1] via 10.1.1.3, 00:11:10, Ethernet0/1
O E2 192.168.170.0 [110/1] via 10.1.1.3, 00:11:10, Ethernet0/1
07-23-2008 06:02 AM
Hi,
Is BGP peering working on 2600 router ?
can you send output of "sh ip bgp " on 2600 router.
Regards,
07-23-2008 06:04 AM
sorry, the attached file already has "show ip bgp" output.
thanks
07-23-2008 06:17 AM
Hi,
AS I can understand.
AS 20 is advertising 192.168.169.0, which is learnt by 17A on 2600,
AS 30 is advertising 192.168.170.0 which is learnt by 17B on 3600.
NOW
2600 and 3600 router running ospf between them.so 3600 is sending 192.168.170.0 over ospf to 2600.
Regards,
07-23-2008 06:25 AM
But In the view of 2600, it will first to learn 192.168.170/24 from 192.168.1.2
Why only 202.122.65.0/24 and 192.168.169.0/24 can be learnt via 192.168.1.2. But 192.168.170.0/24 can't
07-23-2008 06:35 AM
subnet 192.168.170.0 is advertise by as 30 on 3600 where you redistributing into ospf , so 2600 is learning this by ospf over e0/1.if AS 20 advertise this subnet towards 2600, then it will be learnt by ebgp and will be in routing table as BGP route instead of ospf.
07-23-2008 06:49 AM
thanks, I still have question
At the same time, 192.168.170.0/26, 192.168.170.64/26 is advertised by 17B to downward direction (36->26 via ospf) and
upward direction (16C->14->16B->17A->26). Then downward direction will be chosen? The upward direction
whole path is ebgp
07-23-2008 07:03 AM
Apparently, you are not getting those networks via EBGP session with 192.168.1.2, but only via IBGP session.
Because IBGP has admin distance of 200, OSPF routes gets into the routing table with admin distance of 110.
Now we should start thinking why 17A does not send you the 192.168.170/26 networks. That's because you are advertising that route via redistribution from OSPF to BGP!
From 26 and 36 routers, you should advertise only networks which were originated in your AS.
07-23-2008 07:08 AM
Start filtering the redistribution from OSPF to BGP - create a route-map and match the OSFP tag 30.
07-23-2008 07:51 AM
Hi all,
It solve the problem after apply ip as-path access-list 10 permit ^$.
Thank you very much !
Sorry that two more questions.
(1)
You said Router 26 received 192.168.170/26
via ospf 10.1.1.3 and advertise back to Router 17A. So 17A stop advertise
192.168.170/26 to Router 26. So why 192.168.169/26 or 202.122.65.0/24 not follow above
process to produce this issue ?
(2)
Why this issue only occur in Router 26 and 36, why it not affect to other routers ?
Is it other router are fully configured ebgp, so that it can recieve the same route
from different direction and advertise to other direction.
07-23-2008 08:25 AM
Hi,
Good questions, indeed. You must realize that OPSF and BGP propagates their routes differently and speed of these propagations differ as well.
Also, you might configured OSPF before you established some EBGP peerings...
Main thing is that you overrided the split-horizon rule by sending routes back to BGP via OSPF redistribution. And this had to be corrected.
__
Please rate the post if you find it helpful.
07-23-2008 08:38 AM
Sorry lukacovsky,
The fault still occur, I don't know why the 26 router will learn below network via ospf
method, I think it should be learned via 17A ebgp Router (192.168.1.2).
I have check that this time 17A is able to advertise below route to Router 26. But from show ip bgp,
these route is in "r" status. Why ospf (10.1.1.3) win ebgp (192.168.1.2) ?
Attach config For your reference. Thank you
2600#sh ip route
O E2 172.20.1.0 [110/1] via 10.1.1.3, 00:11:10, Ethernet0/1
O E2 172.20.1.4 [110/1] via 10.1.1.3, 00:11:10, Ethernet0/1
O E2 192.168.170.192 [110/1] via 10.1.1.3, 00:11:10, Ethernet0/1
O E2 192.168.170.128 [110/1] via 10.1.1.3, 00:11:10, Ethernet0/1
O E2 192.168.170.64 [110/1] via 10.1.1.3, 00:11:10, Ethernet0/1
O E2 192.168.170.0 [110/1] via 10.1.1.3, 00:11:10, Ethernet0/1
07-23-2008 08:57 AM
do this:
router bgp 10
redistribute ospf 123 route-map noBGP
route-map noBGP deny 10
match tag 10
route-map noBGP permit 20
it will stop redistributing the routes learned from BGP back into BGP
OSPF routes are automatically tagged during the redistribution with the tag of your ASN.
07-23-2008 09:20 AM
Still fail, the route seems flapping
router bgp 10
no synchronization
bgp log-neighbor-changes
network 192.168.1.0 mask 255.255.255.252
network 192.168.168.0 mask 255.255.255.192
redistribute ospf 123 metric 100 route-map noBGP
neighbor 10.10.10.2 remote-as 10
neighbor 10.10.10.2 next-hop-self
neighbor 192.168.1.2 remote-as 20
neighbor 192.168.1.2 filter-list 1 out
no auto-summary
!
ip http server
no ip http secure-server
ip classless
!
ip as-path access-list 1 permit ^$
!
!
!
route-map noBGP deny 10
match tag 10
!
route-map noBGP permit 20
202.122.64.0/26 is subnetted, 4 subnets
O 202.122.64.192 [110/20] via 10.1.1.1, 00:12:01, Ethernet0/1
O 202.122.64.128 [110/20] via 10.1.1.1, 00:12:01, Ethernet0/1
O 202.122.64.64 [110/11] via 10.1.1.1, 00:12:01, Ethernet0/1
O 202.122.64.0 [110/11] via 10.1.1.1, 00:12:01, Ethernet0/1
202.122.65.0/26 is subnetted, 4 subnets
O E2 202.122.65.192 [110/1] via 10.1.1.3, 00:00:05, Ethernet0/1
O E2 202.122.65.128 [110/1] via 10.1.1.3, 00:00:05, Ethernet0/1
O E2 202.122.65.64
B 202.122.65.0 [20/0] via 192.168.1.2, 00:00:00
172.20.0.0/30 is subnetted, 2 subnets
O E2 172.20.1.0 [110/1] via 10.1.1.3, 00:09:54, Ethernet0/1
O E2 172.20.1.4 [110/1] via 10.1.1.3, 00:00:04, Ethernet0/1
10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
B 10.1.2.0/24 [200/100] via 10.10.10.2, 00:00:00
C 10.10.10.0/30 is directly connected, Ethernet0/0
C 10.1.1.0/24 is directly connected, Ethernet0/1
192.168.170.0/26 is subnetted, 4 subnets
O E2 192.168.170.192 [110/1] via 10.1.1.3, 00:00:04, Ethernet0/1
O E2 192.168.170.128 [110/1] via 10.1.1.3, 00:00:04, Ethernet0/1
O E2 192.168.170.64 [110/1] via 10.1.1.3, 00:00:04, Ethernet0/1
O E2 192.168.170.0 [110/1] via 10.1.1.3, 00:00:04, Ethernet0/1
192.168.1.0/30 is subnetted, 2 subnets
C 192.168.1.0 is directly connected, Serial0/1
B 192.168.1.4 [200/100] via 10.10.10.2, 00:00:00
192.168.168.0/26 is subnetted, 2 subnets
O E2 192.168.168.64 [110/1] via 10.1.1.3, 00:12:03, Ethernet0/1
C 192.168.168.0 is directly connected, Loopback0
192.168.169.0/26 is subnetted, 4 subnets
B 192.168.169.192 [200/100] via 10.10.10.2, 00:00:00
B 192.168.169.128 [200/100] via 10.10.10.2, 00:00:01
B 192.168.169.64 [20/0] via 192.168.1.2, 00:00:01
B 192.168.169.0 [20/0] via 192.168.1.2, 00:00:01
07-23-2008 09:38 AM
Do the same on the 36 router and clear bgp neighborship completly:
clear ip bgp *
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