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ikodai02
Staff
ikodai02
Staff
June 8, 2026

Technical Tip: Behavior when configuring maximum-prefix and maximum-prefix-warning-only for BGP neighbors

  • June 8, 2026
  • 0 replies
  • 98 views

Description

This article describes the behavior when configuring maximum-prefix and maximum-prefix-warning-only for BGP neighbors.

Scope

 FortiGate.

Solution

  1. Configuration example.

When configured as follows:


config router bgp
    config neighbor
        edit "10.0.0.3"
            set maximum-prefix 5
            set maximum-prefix-warning-only enable/disable
            ...
        next
    end
end


  1. Set maximum-prefix 5 and maximum-prefix-warning-only enable.

When a neighbor advertises prefixes exceeding the maximum, a warning is logged, but the prefixes are still accepted.

Messages are output both before and after the limit is reached.


System event log:


date=2026-06-02 time=15:29:13 eventtime=1780381753391719966 tz="+0900" logid="0103020304" type="event" subtype="router" level="warning" vd="root" logdesc="Routing log warning" msg="BGP: %BGP-3-MAXPFXEXCEED: No. of prefix received from 10.0.0.3 (afi-safi 1-1): 6 exceed limit 5"
date=2026-06-02 time=15:29:13 eventtime=1780381753391709026 tz="+0900" logid="0103020304" type="event" subtype="router" level="warning" vd="root" logdesc="Routing log warning" msg="BGP: %BGP-4-MAXPFX: No. of prefix received from 10.0.0.3 (afi-safi 1-1): reaches 4, max 5"


# get router info routing-table bgp
Routing table for VRF=0
B       192.168.10.0/24 [200/0] via 10.0.0.3 (recursive is directly connected, port7), 00:00:35, [1/0]
B       192.168.11.0/24 [200/0] via 10.0.0.3 (recursive is directly connected, port7), 00:00:35, [1/0]
B       192.168.12.0/24 [200/0] via 10.0.0.3 (recursive is directly connected, port7), 00:00:35, [1/0]
B       192.168.13.0/24 [200/0] via 10.0.0.3 (recursive is directly connected, port7), 00:00:35, [1/0]
B       192.168.14.0/24 [200/0] via 10.0.0.3 (recursive is directly connected, port7), 00:00:35, [1/0]
B       192.168.15.0/24 [200/0] via 10.0.0.3 (recursive is directly connected, port7), 00:00:35, [1/0]
B       192.168.16.0/24 [200/0] via 10.0.0.3 (recursive is directly connected, port7), 00:00:35, [1/0]
B       192.168.17.0/24 [200/0] via 10.0.0.3 (recursive is directly connected, port7), 00:00:35, [1/0]
B       192.168.18.0/24 [200/0] via 10.0.0.3 (recursive is directly connected, port7), 00:00:35, [1/0]
B       192.168.19.0/24 [200/0] via 10.0.0.3 (recursive is directly connected, port7), 00:00:35, [1/0]
B       192.168.20.0/24 [200/0] via 10.0.0.3 (recursive is directly connected, port7), 00:00:35, [1/0]

# get router info bgp summary

VRF 0 BGP router identifier 2.2.2.2, local AS number 64511
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
10.0.0.3 4      64511      55      54        0    0    0 00:45:23       11

Total number of neighbors 1


  1. Set maximum-prefix 5 and maximum-prefix-warning-only disable.

Routes exceeding the configured maximum are not accepted from the neighbor advertising them.


System event log:


date=2026-06-02 time=15:42:39 eventtime=1780382559323941397 tz="+0900" logid="0103020300" type="event" subtype="router" level="warning" vd="root" logdesc="BGP neighbor status changed" msg="BGP: %BGP-5-ADJCHANGE: VRF 0 neighbor 10.0.0.3 Down BGP Notification FSM-ERR"
date=2026-06-02 time=15:42:39 eventtime=1780382559323892124 tz="+0900" logid="0103020304" type="event" subtype="router" level="warning" vd="root" logdesc="Routing log warning" msg="BGP: %BGP-3-NOTIFICATION: sending to 10.0.0.3 6/1 (Cease/Maximum Number of Prefixes Reached.) 7 data-bytes [00 01 01 00 00 00 05]"
date=2026-06-02 time=15:42:39 eventtime=1780382559323881641 tz="+0900" logid="0103020304" type="event" subtype="router" level="warning" vd="root" logdesc="Routing log warning" msg="BGP: %BGP-3-MAXPFXEXCEED: No. of prefix received from 10.0.0.3 (afi-safi 1-1): 6 exceed limit 5"
date=2026-06-02 time=15:42:39 eventtime=1780382559323871181 tz="+0900" logid="0103020304" type="event" subtype="router" level="warning" vd="root" logdesc="Routing log warning" msg="BGP: %BGP-4-MAXPFX: No. of prefix received from 10.0.0.3 (afi-safi 1-1): reaches 4, max 5"
date=2026-06-02 time=15:42:14 eventtime=1780382534074410589 tz="+0900" logid="0103020300" type="event" subtype="router" level="warning" vd="root" logdesc="BGP neighbor status changed" msg="BGP: %BGP-5-ADJCHANGE: VRF 0 neighbor 10.0.0.3 Up "


# get router info routing-table bgp
No route available

# get router info bgp summary

VRF 0 BGP router identifier 2.2.2.2, local AS number 64511
BGP table version is 1
0 BGP AS-PATH entries
0 BGP community entries

Neighbor V         AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
10.0.0.3 4      64511      20      23        0    0    0    never Idle (PfxCt)

Total number of neighbors

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