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HS08
HS08
Visitor III
January 23, 2026
Solved

BGP Prefix List

  • January 23, 2026
  • 4 replies
  • 841 views

I have prefix list to filter which prefix will be send to azure.

Previously ID no 4 is not exist and i can see in the advertise route only showig 10.103.0.0/16 and 172.30.30.0/24

Screenshot 2026-01-23 132653.png

 

FW01 (Internet) # get router info bgp neighbors 10.201.0.4 advertised-routes
VRF 0 BGP table version is 43362, local router ID is 10.103.103.103
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight RouteTag Path
*> 10.103.0.0/16 10.210.0.103 100 32768 0 i <-/->
*> 172.30.30.0/24 10.210.0.103 100 32768 0 i <-/->

Total number of prefixes 2

 

But when i add ID number 4 above why the fgt still advertise 2 prefix and not advertise 10.105.0.0/16?

Best answer by HS08

Hi..

I just realized that prefix list will be execute from lower id to higher id. The picture i post earlier should be sort by ID. 

So after bgp to that peer cleared then only prefix 10.103.0.0/16 will be advertised.

4 replies

Toshi_Esumi
SuperUser
Toshi_Esumi
SuperUser
January 23, 2026

Do you have "set soft-reconfituration enable" in the neighbor configuration? If so, try "exe router clear bgp ip 10.201.0.4 soft out" a couple of times. Otherwise, you might need to reset the session (take the neighbor down and bring it back up).

Toshi 

funkylicious
SuperUser
funkylicious
SuperUser
January 23, 2026

do you have a route in RIB on the FGT for that route? if not that may be a reason why it isnt advertised.

"jack of all trades, master of none"
tayi
tayi
New Member
January 23, 2026

check;

get router info routing-table details 10.105.0.0

or

get router info routing-table all | grep 10.105

 

check whether the subnet exists as 16 or 24

 

if no route exists for 10.105, BGP cannot advertise it

 

test by adding a blackhole static route;

config router static

edit 100
set dst 10.105.0.0 255.255.0.0
set blackhole enable

end

 

re-check;

get router info bgp neighbors 10.201.0.4 advertised-routes

HS08
HS08AuthorAnswer
Visitor III
January 23, 2026

Hi..

I just realized that prefix list will be execute from lower id to higher id. The picture i post earlier should be sort by ID. 

So after bgp to that peer cleared then only prefix 10.103.0.0/16 will be advertised.

Toshi_Esumi
SuperUser
Toshi_Esumi
SuperUser
January 23, 2026

No. They're examined from the top of the list regardless of the ID. If you go into CLI then delete the ID#1 prefix list, the GUI should show only #3, #4 and #2. Then if you go back and configure,

config router prefix-list
  edit [prefix-list_name]
    config rule
      edit 1 
        set prefix 10.103.0.0 255.255.0.0
        unset ge
        unset le
      next
    end
  next
end

the ID#1 prefix would be added to the end of the list. So now GUI would show in the order  #3, #4, #2, and #1. Then #3 would be examined first and #1 last.

Toshi

HS08
HS08Author
Visitor III
January 24, 2026

But if we look at https://community.fortinet.com/t5/FortiGate/Technical-Tip-How-FortiGate-Match-Routes-on-the-Prefix-List/ta-p/316238 then "the prefix matching is performed based on Sequence ID". This same with what i facing.

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