Skip to main content
subramanis
Staff
subramanis
Staff
October 16, 2025

Technical Tip: Understanding FortiGate Behavior with set attribute-unchanged as-path in eBGP

  • October 16, 2025
  • 0 replies
  • 672 views
Description This article describes the behavior of a FortiGate during route advertisement to an external BGP (eBGP) peer when the set attribute-unchanged as-path option is configured.
Scope FortiGate.
Solution
By default, when advertising a route to an eBGP peer, the FortiGate automatically prepends its own AS number to the AS-PATH attribute. This behavior is part of the standard BGP loop-prevention mechanism, ensuring that a router’s AS is visible in the path and avoiding routing loops.
 
When the set attribute-unchanged as-path option is configured, the FortiGate preserves the original AS-PATH attribute and advertises the route to the eBGP peer without adding its own AS number.
 
This feature is typically used in special scenarios such as:
  • Route server deployments at Internet Exchange Points (IXPs).
  • Carrier or service-provider environments.
 
Diagram:
2.2.2.2/32-----FGT1 (192.168.1.1)-------(192.168.1.2 ) ISP (192.168.100.1)-------(192.168.100.2) FGT2
 
FGT1 # show  router bgp 
config router bgp
    set as 65000
    set router-id 1.1.1.1
        config neighbor
            edit "192.168.1.2"
                set remote-as 65001
            next
        end
        config redistribute "static"
            set status enable
        end
 
FGT1 # show router static
config router static
    edit 1
        set dst 2.2.2.2 255.255.255.255
        set gateway 10.254.1.100
        set device "port1"
    next
end
 
ISP # show  router bgp 
config router bgp
    set as 65001
    set router-id 2.2.2.2
        config neighbor
            edit "192.168.1.1"
                set remote-as 65000
            next
            edit "192.168.100.2"
                set soft-reconfiguration enable
                set remote-as 65002
            next
        end
 
FGT2 # show  router bgp 
config router bgp
    set as 65002
    set router-id 3.3.3.3
        config neighbor
            edit "192.168.100.1"
                set soft-reconfiguration enable
                set remote-as 65001
            next
        end
 
Initial Behavior (Default eBGP):
 
  1. FGT1 advertises route 2.2.2.2/32 to its eBGP peer, the ISP (AS 65001).
  2. ISP forwards the route to FGT2 with prepending its own AS.
  3. By default, the AS-PATH is modified, so FGT2 sees the route as coming from 65001 65000, where 65001 (ISP) is prepended to the original AS-PATH.
 
FGT2 # get router info bgp  summary 
VRF 0 BGP router identifier 3.3.3.3, local AS number 65002
BGP table version is 1
1 BGP AS-PATH entries
0 BGP community entries
 
Neighbor      V         AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
192.168.100.1 4      65001      25      24        0    0    0 00:04:47        1
Total number of neighbors 1
 
FGT2 # get router info bgp  neighbors 192.168.100.1 received-routes 
VRF 0 BGP table version is 1, local router ID is 3.3.3.3
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
*> 2.2.2.2/32       192.168.100.1                          0        0 65001 65000 ? <-/->
Total number of prefixes 1
 
Behavior after configuration:
  1. FGT1 advertises route 2.2.2.2/32 -> AS-PATH {65000}.
  2. ISP forwards the route to FGT2 without prepending its own AS.
  3. FGT2 sees the route as coming directly from AS 65000.
 
ISP # show  router bgp 
config router bgp
    set as 65001
    set router-id 2.2.2.2
        config neighbor
            edit "192.168.1.1"
                set remote-as 65000
            next
            edit "192.168.100.2"
                set attribute-unchanged as-path <-----
                set soft-reconfiguration enable
                set remote-as 65002
            next
        end
 
FGT2 #  get router info bgp  neighbors 192.168.100.1 received-routes
VRF 0 BGP table version is 1, local router ID is 3.3.3.3
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
*> 2.2.2.2/32       192.168.100.1                          0        0 65000 ? <-/->
Total number of prefixes 1
      Terms & ConditionsAccessibility statement