Description

This KB describes the behavior of route-suppression for DHCP function.

Symptoms

As a default behavior, DHCP function will install routes from the DHCP sessions. For example, route information will be added by DHCP client binding operation.
If you want to suppress routes by this operation, you can use route-suppression option. However, even if you enabled this option, there is a case that route will be installed.
Here is the sample topology and output when route information will be added with route-suppression destination option.

<Topology>
DHCP client ------ (ge-0/0/1)Switch(ge-0/0/2) ------ DHCP server

*Switch is working as DHCP relay with route-suppression destination option. 
user@Switch# run show dhcp relay binding
IP address    Session Id Hardware address  Expires   State   Interface
192.168.3.113   17     54:4b:8c:XX:XX:XX 14049    BOUND   ge-0/0/1.0

user@Switch# run show route forwarding-table | match 192.168.3.113
192.168.3.113/32   dest     0 54:4b:8c:XX:XX:XX  ucst     1735     1 ge-0/0/1.0  <<<<=== There is a destination route

Solution

With route-suppression destination option, ARP mapping won't be done automatically and so you can suppress installing route entry from forwarding table.
Here is the output comparison between with and without route-suppression.

Without route-suppression
--------------------

user@Switch# run show dhcp relay binding
IP address    Session Id Hardware address  Expires   State   Interface
192.168.3.113   17     54:4b:8c:XX:XX:XX 14049    BOUND   ge-0/0/1.0

user@Switch# run show arp no-resolve | match 192.168.3.113
54:4b:8c:XX:XX:XX 192.168.3.113  ge-0/0/1.0        permanent  <<<=========== Here, permanent ARP mapping done by DHCP binding.
user@Switch# run show route forwarding-table | match 192.168.3.113
192.168.3.113/32  dest   0 54:4b:8c:XX:XX:XX ucst   1736   1 ge-0/0/1.0  <<<=========== Destination route
--------------------

And if you clear DHCP relay binding, permanent ARP entry will also be removed.
-------------------- 
user@Switch# run clear dhcp relay binding all

{master:0}[edit]
user@Switch# run show arp no-resolve | match 192.168.3.113

--------------------

With route-suppression destination option, permanent ARP entry won't be created and there is no destination route in forwarding table.
With route-suppression destination
-------------------- 
user@Switch# run show dhcp relay binding
IP address    Session Id Hardware address  Expires   State   Interface
192.168.3.113   2      54:4b:8c:XX:XX:XX 85496    BOUND   ge-0/0/1.0

user@Switch# run show arp no-resolve | match 192.168.3.113 <<<=========== No permanent ARP mapping


user@Switch# run show route forwarding-table | match 192.168.3.113 <<<=========== No destination route


user@Switch# run show route 192.168.3.113

inet.0: 10 destinations, 10 routes (10 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
192.168.3.113/32  *[Access-internal/12] 00:00:23
          > to 192.168.3.10 via ge-0/0/1.0
--------------------

And with route-suppression destination enabled, ARP entry will be learnt apart from DHCP relay function. So if you ping to DHCP client from DHCP relay switch, it will be learnt with 'none' flag and you can see destination route.
-------------------- 
user@Switch# run ping 192.168.3.113
PING 192.168.3.113 (192.168.3.113): 56 data bytes
64 bytes from 192.168.3.113: icmp_seq=0 ttl=64 time=29.788 ms


user@Switch# run show arp no-resolve | match 192.168.3.113
54:4b:8c:17:41:c9 192.168.3.113  ge-0/0/1.0        none  <<<=========== ARP mapping with none flag.

{master:0}[edit]
user@Switch# run show route forwarding-table | match 192.168.3.113

192.168.3.113/32  dest   0 54:4b:8c:XX:XX:XX ucst   1735   1 ge-0/0/1.0  <<<=========== Destination route

--------------------

Please note, if DHCP client has a route of DHCP relay switch as a next hop, ARP resolution will be done automatically so you will see ARP entry of DHCP client on DHCP relay server with 'none' flag.
For example, with below config, ARP resolution will be done automatically from DHCP client so you will see ARP entry with 'none' flag on DHCP relay device.
--------------------
DHCP client 
set routing-options static route 192.168.1.0/24 next-hop 192.168.3.10

DHCP relay device 
set interfaces ge-0/0/1 unit 0 family inet address 192.168.3.10/24
--------------------
*If you want to verify the simple behavior with route suppression for test purpose, it's recommend to remove above static route from DHCP client.

Modification History

2024-05-07 : Article Created