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When redistributing OSPF routes into BGP on FortiGate, users may encounter problems where route-tags set via a route-map do not appear as expected in the BGP Routing Information Base (RIB). This can impact route selection, particularly in deployments where route-tag values are used to distinguish and prefer routes.
The issue occurs when using route-tag values close to the maximum unsigned integer limit (4294967295), which leads to incorrect backend processing in route-maps.
A prefix, 10.123.0.0/16, learned via OSPF with route-tag 4294967295 is redistributed into BGP with an incorrect tag (for example, 4294967223).
FortiGate is receiving OSPF external route with route-tag value 4294967295.
get router info ospf database adv-router x.x.x.x
.....
AS External Link States
10.123.0.0 10.101.101.101 625 80010df0 72dd 0012 E2 10.123.0.0/16 4294967295
10.123.0.0 10.102.102.102 1289 80010dea 60f1 0012 E2 10.123.0.0/16 4294967295
By default, the OSPF route-tag is automatically redistributed into the BGP RIB. In this scenario, an inbound route-map was applied to explicitly set the tags.
Example: Route-map configuration
config router route-map
edit "Out_BGP"
set comments "redistribute routes into BGP using OSPF tags"
config rule
edit 1
set match-tag 4294967223
unset set-ip-prefsrc
set set-tag 4294967223
set set-route-tag 4294967223
next
edit 2
set match-tag 4294967295
unset set-ip-prefsrc
set set-tag 4294967295
set set-route-tag 4294967295
next
end
Example: Redistribution configuration.
config router bgp
config redistribute "ospf"
set route-map Out_BGP
set status enable
end
end
When checking the BGP RIB, the redistributed OSPF route for 10.123.0.0/16 incorrectly shows a route-tag of 4294967223 instead of 4294967295.
get router info bgp network 10.123.0.0/16
VRF 0 BGP routing table entry for 10.123.0.0/16
Local
10.145.61.3 from 0.0.0.0 (200.1.100.1)
Origin incomplete, distance 20, metric 20, route tag 4294967223, tag 4294967295, localpref 100, valid, sourced, best
Workaround:
Avoid using high values for set-tag in route-maps. Use values less than or equal to 2147483647.
This issue will be fixed in future versions of FortiOS (starting from 7.4.10 or higher).
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