Description
This article describes that he BGP route with better AD 20 is not installed into the routing table while the OSPF route with AD 110 is preferred (if any BGP link fails the BGP routes will not come back as long as OSPF is online).
The route received from OSPF is redistributed into BGP.
Scope
FortiGate.
Solution
Diagram.
FGT1 receives 10.0.0.0/24 network via FGT2 over BGP. <----- Primary Route.
FGT1 receives 10.0.0.0/24 network via FGT3 over OSPF. <----- Backup path.
FGT1 # show router ospf
# config router ospf
set router-id 172.16.16.1
# config area
edit 0.0.0.0
next
end
# config ospf-interface
edit "1"
set interface "port2"
next
end
# config network
edit 1
set prefix 172.16.16.0 255.255.255.0
next
end
Hub # show router bgp
# config router bgp
set as 64512
set router-id 172.17.17.1
# config neighbor
edit "172.17.17.2"
set remote-as 64513
next
end
# config redistribute "ospf" <----- Redistributed OSPF into BGP.
set status enable
end
Before the BGP link failure/manually clear the BGP neighbourship.
FGT1 # get router info bgp neighbors 172.17.17.2 received-routes | grep 10.0.0.0/24
*> 10.0.0.0/24 172.17.17.2 0 0 64513 i <-/->
FGT1 # get router info routing-table details | grep 10.0.0.0/24
B 10.0.0.0/24 [20/0] via 172.17.17.2, port3, 00:00:22
The route received from BGP is installed in the routing table and the OSPF route kept in the database table.
FGT1 # get router info ospf route | grep 10.0.0.0/24
O 10.0.0.0/24 [101] via 172.16.16.2, port2, Area 0.0.0.0
FGT1# get router info routing-table database | grep 10.0.0.0/24
O 10.0.0.0/24 [110/101] via 172.16.16.2, port2, 00:01:05 <----- OSPF in the database.
B *> 10.0.0.0/24 [20/0] via 172.17.17.2, port3, 00:02:55
After the BGP link failure/manually cleared the BGP neighbourship.
FGT1 # get router info routing-table details | grep 10.0.0.0/24
O 10.0.0.0/24 [110/101] via 172.16.16.2, port2, 00:09:44
The OSPF route in the database table installed into the routing table once the BGP link down.
the BGP link is restored but it’s not installing into the routing table, still the OSPF route with higher AD 110 is preferred over BGP AD 20.
FGT1 # get router info bgp neighbors 172.17.17.2 received-routes | grep 10.0.0.0/24
*> 10.0.0.0/24 172.17.17.2 0 0 64513 i <-/->
FGT1 # get router info routing-table details | grep 10.0.0.0/24
O 10.0.0.0/24 [110/101] via 172.16.16.2, port2, 00:17:38
The issue is redistributing the OSPF route into BGP.
Hub # get router info bgp network
VRF 0 BGP table version is 2, local router ID is 172.17.17.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight RouteTag Path
* 10.0.0.0/24 172.17.17.2 0 0 0 64513 i <-/->
*> 172.16.16.2 101 32768 0 ? <-/1>
Total number of prefixes 1
The network 10.0.0.0/24 leaned via its peer 172.17.17.2 is having default weight 0 but the redistributed network from OSPF (next hop 172.16.16.2) to BGP has 32768.
When the eBGP link went down, the OSPF route was installed into the routing table and redistributed into BGP routing table.
The redistributed route in BGP is considered locally originated so it has a default weight of 32768.
When the eBGP link comes back up, the BGP routing table will have 2 entries for the same network so, considering the best path selection, BGP chooses the one with the higher weight so it will remain with the route learned from OSPF.
It is possible to avoid this by configuring the BGP weight for the route to be higher than the locally injected routes (OSPF).
Example.
config router prefix-list
edit "Test-Network"
# config rule
edit 1
set prefix 10.0.0.0/24
end
end
config router route-map
edit "test-map"
# config rule
edit 1
set match-ip-address "Test-Network"
set set-weight 32769
end
config router bgp
config neighbor
edit 172.17.17.2
set route-map-in "test-map"
next
end
Hub # get router info bgp network
VRF 0 BGP table version is 2, local router ID is 172.17.17.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight RouteTag Path
*> 10.0.0.0/24 172.17.17.2 0 32769 0 64513 i <-/1>
* 172.16.16.2 101 32768 0 ? <-/->
Total number of prefixes 1
Now the weight has changed to 32769 which is higher than the redistributed network.
