Technical Tip: FortiGate 8xF-DSL advertises an unexpected connected route via DSL interface

FortiGate 80F-DSL/81F-DSL when connected to the ISP modem using a DSL connection installs a /8 connected route, even if the IP address assigned on the DSL interface is /24.

The below output shows the unexpected entry for 169.0.0.0/8:

diagnose ip address list
IP=169.254.2.2->169.254.2.2/255.255.255.0 index=15 devname=dsl
IP=169.254.2.2->169.254.2.2/255.0.0.0 index=15 devname=dsl
IP=180.150.119.250->180.150.119.250/255.255.252.0 index=15 devname=dsl

Routing table for VRF=0
S* 0.0.0.0/0 [5/0] via 192.168.13.1, wan2, [1/0]
S 8.8.4.4/32 [10/0] is a summary, Null, [1/0]
S 8.8.8.8/32 [10/0] is a summary, Null, [1/0]
C 169.0.0.0/8 is directly connected, dsl <<<<<<<<< Unexpected route
C 169.254.2.0/24 is directly connected, dsl

diagnose ip route list
tab=254 vf=0 scope=253 type=1 proto=2 prio=0 0.0.0.0/0.0.0.0/0->169.0.0.0/8 pref=169.254.2.2 gwy=0.0.0.0 dev=15(dsl)
tab=254 vf=0 scope=253 type=1 proto=2 prio=0 0.0.0.0/0.0.0.0/0->169.254.2.0/24 pref=169.254.2.2 gwy=0.0.0.0 dev=15(dsl)

This is a known issue#1065553 in FortiGate 8xF-DSL in vv7.6.0 and earlier. It has been fixed in v7.6.1, v7.4.8 and v7.2.11:

Resolved issues | FortiGate / FortiOS 7.4.8

FortiOS 7.6.1 Resolved issues