OSPF in Junos#

lab@vSRX> show configuration interfaces ge-0/0/2 
unit 0 {
    family inet {
        address 192.168.11.2/24;
    }
}

lab@vSRX> show configuration protocols ospf 
area 0.0.0.0 {
    interface ge-0/0/2.0;
}

lab@vSRX> 

lab@vSRX> show ospf interface 
Interface           State   Area            DR ID           BDR ID          Nbrs
ge-0/0/2.0          BDR     0.0.0.0         192.168.1.1     192.168.31.1       1

lab@vSRX> 

lab@vSRX> show ospf neighbor 
Address          Interface              State           ID               Pri  Dead
192.168.11.1     ge-0/0/2.0             Full            192.168.1.1      128    39

lab@vSRX>

lab@vSRX> show route protocol ospf  

inet.0: 24 destinations, 38 routes (24 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

192.168.1.1/32     *[OSPF/10] 01:58:32, metric 1
                    >  to 192.168.11.1 via ge-0/0/2.0
224.0.0.5/32       *[OSPF/10] 01:58:42, metric 1
                       MultiRecv

inet6.0: 1 destinations, 1 routes (1 active, 0 holddown, 0 hidden)

lab@vSRX>

MTU mismatch#

lab@vSRX> show ospf neighbor 
Address          Interface              State           ID               Pri  Dead
192.168.11.1     ge-0/0/2.0             ExStart         192.168.1.1      128    35

lab@vSRX>

lab@vSRX> show interfaces ge-0/0/2 | match mtu 
  Link-level type: Ethernet, MTU: 1400, LAN-PHY mode, Link-mode: Full-duplex, Speed: 10Gbps,
    Protocol inet, MTU: 1386

lab@vSRX> show configuration interfaces ge-0/0/2 | display inheritance 
##
## '1400' was inherited from group 'group1'
##
mtu 1400;
unit 0 {
    family inet {
        ##
        ## 'no-neighbor-learn' was inherited from group 'group1'
        ##
        no-neighbor-learn;
        address 192.168.11.2/24 {
            arp 192.168.11.1 mac 00:50:56:a9:50:92;
        }
    }
}

lab@vSRX> 

lab@vMX-2> show interfaces ge-0/0/2 | match mtu 
  Link-level type: Ethernet, MTU: 1514, MRU: 1522, LAN-PHY mode, Speed: 1000mbps, BPDU Error: None,
    Protocol inet, MTU: 1500
    Protocol multiservice, MTU: Unlimited

lab@vMX-2> 

lab@vMX-2> show configuration interfaces ge-0/0/2 | display inheritance 
description "vr1 to vsrx1";
unit 0 {
    family inet {
        address 192.168.11.1/24;
    }
}

lab@vMX-2> 

The solution in this case is to change the MTU value of the physical interface on either side of the link:

lab@vSRX> edit 
Entering configuration mode
Users currently editing the configuration:
  lab terminal pts/1 (pid 81293) on since 2026-08-28 15:05:05 UTC, idle 01:23:35
      [edit]

[edit]
lab@vSRX# 

[edit]
lab@vSRX# set interfaces ge-0/0/2 mtu 1514 

[edit]
lab@vSRX# commit and-quit 
show ccommit complete
oExiting configuration mode

lab@vSRX> show configuration interfaces ge-0/0/2 | display inheritance 
mtu 1514;
unit 0 {
    family inet {
        ##
        ## 'no-neighbor-learn' was inherited from group 'group1'
        ##
        no-neighbor-learn;
        address 192.168.11.2/24 {
            arp 192.168.11.1 mac 00:50:56:a9:50:92;
        }
    }
}

lab@vSRX> show ospf neighbor 
Address          Interface              State           ID               Pri  Dead
192.168.11.1     ge-0/0/2.0             Full            192.168.1.1      128    39

lab@vSRX>