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BGP 001 – Route Advertisement from Indirect Sources

• 7 June 202617 June 2026
• 0

We have a topology like below. R1 and R2 have BGP neighborship. R1 have 1 static route to R3 and 1 OSPF route to R4. We want to see the prefixes on R2.

Configurations on R1 like below;

IOU1# sh run | inc "some lines of I want to show"

ip route 192.168.0.3 255.255.255.255 13.13.13.3
!
ip access-list standard ACL_001
 10 permit 192.168.0.4
!
route-map MAP_001 permit 1 
 match ip address ACL_001
!
router ospf 1
 router-id 192.168.0.1
 network 14.14.14.1 0.0.0.0 area 0
 network 192.168.0.1 0.0.0.0 area 0
!
router bgp 65100
 bgp log-neighbor-changes
 neighbor 12.12.12.2 remote-as 65200
 !
 address-family ipv4
  network 192.168.0.1 mask 255.255.255.255
  redistribute static
  redistribute ospf 1 route-map MAP_001
  neighbor 12.12.12.2 activate
 exit-address-family
!

IOU1#sh ip route | inc 192.168.0
      192.168.0.0/32 is subnetted, 3 subnets
C        192.168.0.1 is directly connected, Loopback0
S        192.168.0.3 [1/0] via 13.13.13.3
O        192.168.0.4 [110/11] via 14.14.14.4, 00:00:37, Ethernet0/1

I want to talk about ALC_001 and MAP_001. In the neighborship between R1 and R4, I also have the OSPF prefix 14.14.14.0/24, which is used between R1 and R4. However, I do not want to advertise it to R2, so I filtered it out.

Configurations on R2;

IOU2#sh run | sec bgp

router bgp 65200
 bgp log-neighbor-changes
 neighbor 12.12.12.1 remote-as 65100
 !
 address-family ipv4
  neighbor 12.12.12.1 activate
 exit-address-family
!

IOU2#sh  ip bgp ipv4 unicast | begin Network
     Network          Next Hop            Metric LocPrf Weight Path
 *>   192.168.0.1/32   12.12.12.1               0             0 65100 i
 *>   192.168.0.3/32   12.12.12.1               0             0 65100 ?
 *>   192.168.0.4/32   12.12.12.1              11             0 65100 ?

On R2, as shown above, when I receive a route from a directly connected neighbor, its origin is ‘i’, which means IGP. Other networks that I receive through redistribution have an origin of ‘?’, which means incomplete. Another point I want to highlight is that the prefix I receive from OSPF has a metric of 11.

In a later post, I will address how to redistribute only the specific neighbor I want. With the method I used here, I believe it will redistribute to all neighbors.

There are two ways to do that; First one is just using route-map permit and deny. Second one is tagging OSPF routes. Let’s try first one. To do it I added some lines to connect R5. All BGP config and new topology are below.

router bgp 65100
 bgp log-neighbor-changes
 neighbor 12.12.12.2 remote-as 65200
 neighbor 15.15.15.5 remote-as 65300
 !
 address-family ipv4
  network 192.168.0.1 mask 255.255.255.255
  redistribute static
  neighbor 12.12.12.2 activate
  neighbor 15.15.15.5 activate
 exit-address-family

Now let’s try to check if R5 received all redistributed prefixes.

IOU5#sh bgp ipv4 unicast | begin Network
     Network          Next Hop            Metric LocPrf Weight Path
 *>   192.168.0.1/32   15.15.15.1               0             0 65100 i
 *>   192.168.0.3/32   15.15.15.1               0             0 65100 ?
 *>   192.168.0.4/32   15.15.15.1              11             0 65100 ?

Yes, we see all. Now on R1 let’s add the lines below.

ip prefix-list PREFIX_TO_R5 seq 5 permit 192.168.0.1/32
!
route-map FILTER_R5 permit 10 
 match ip address prefix-list PREFIX_TO_R5
!
router bgp 65100
 neighbor 15.15.15.5 remote-as 65300
 !
 address-family ipv4
  neighbor 15.15.15.5 route-map FILTER_R5 out

Now let’s try to check if R5 received just the R1 loopback address.

IOU5#sh bgp ipv4 unicast | begin Network
     Network          Next Hop            Metric LocPrf Weight Path
 *>   192.168.0.1/32   15.15.15.1               0             0 65100 i

That’s it.

Now, let’s try to tag OSPF prefix and redistribute tagged prefixes. On my device , it couldn’t be supported tagging in route-map. Maybe I will try later on another platform.