Dynamic Routing (OSPF)¶

OSPF (Open Shortest Path First) is a link-state interior gateway protocol (IGP) standardised in RFC 2328. Unlike distance-vector protocols that only know next-hop direction, each OSPF router builds a complete map of the network topology — the link-state database — and uses Dijkstra's shortest path algorithm to independently compute optimal, loop-free routes. This makes OSPF fast to converge and well-suited to networks where topology changes frequently or where maintaining static routes across many routers becomes impractical.

OSPF is commonly used in RansNet deployments to exchange routing information between a branch router and on-premise infrastructure (core switches, firewalls, or data centre routers), or to propagate a default route from a gateway router to all connected internal routers.

Key concepts:

Term	Description
Area	A logical grouping of routers that share the same link-state database. All areas must connect to the backbone (area 0). Dividing a network into areas limits LSA flooding and reduces CPU load.
Router ID	A unique 32-bit identifier (in IPv4 address format) assigned to each OSPF router. Set this explicitly for stability — if not configured, OSPF auto-selects from an active interface IP, which can change.
Cost	OSPF selects paths by minimising cumulative interface cost. The default cost is `100 / interface_bandwidth_Mbps` but can be set manually per interface to influence path selection.
DR / BDR	On broadcast segments (Ethernet), OSPF elects a Designated Router (DR) and Backup DR (BDR) to reduce flooding. Higher interface priority wins the election; equal priority falls back to Router ID. Set priority to `0` to exclude a router from election.
Passive interface	An interface included in OSPF advertisements (the prefix is announced) but on which no hello packets are sent and no neighbours are formed. Use this on WAN or untrusted interfaces.
Redistribute	Imports routes from another source (connected, static, BGP) into OSPF as external routes (Type 5 LSA).

Typical Deployment¶

The following topology illustrates a two-router OSPF setup: A1 is an internet-facing gateway and A2 is an internal router. Both participate in OSPF area 0 to exchange routing information dynamically.

Device	Interface	Address	Role
A1	`eth0`	`202.63.156.1`	WAN uplink to internet
A1	`br0`	`172.16.1.254`	Internal segment, OSPF area 0
A2	`eth0`	`172.16.1.24`	Uplink to A1, OSPF area 0
A2	`eth1`	`10.10.10.10`	LAN segment advertised into OSPF

A1 originates the default route into OSPF so A2 learns how to reach the internet. A2 advertises its 10.10.10.0/24 LAN prefix so A1 (and any other OSPF peers) can route traffic back to it.

GUI Configuration¶

Navigate to Device Settings → Network → Dynamic Routing → OSPF.

Toggle Enable OSPF to activate the OSPF process on this device.

OSPF Options

Expand the following options as needed:

Option	Description
Router ID	Assign a fixed 32-bit router identity in IPv4 address format (e.g. `172.16.1.254`). If left unset, OSPF auto-selects from an active interface IP, which can change and cause adjacency resets. Setting an explicit Router ID is strongly recommended.
Redistribute	Inject routes from other sources into OSPF as external routes. Enables redistribution of connected prefixes, static routes, or BGP routes into the OSPF domain.

OSPF Neighbors

The Neighbors table lists manually configured OSPF neighbours. This is only needed on non-broadcast networks (e.g. NBMA or point-to-multipoint) where OSPF cannot discover neighbours via multicast. On standard Ethernet segments, neighbour discovery is automatic and this table can be left empty.

Field	Description
Neighbor IP	The unicast IP address of the OSPF neighbour to contact manually (e.g. `192.168.8.0`)
Priority	The neighbour's DR election priority as seen from this router (e.g. `100`)

OSPF Networks

The Networks table defines which interfaces participate in OSPF and which prefixes are advertised into the OSPF domain. Each entry maps a network prefix to an OSPF area.

Field	Description
Network/Prefix	The subnet to enable OSPF on, in CIDR notation (e.g. `192.168.8.0/24`). OSPF will activate on any interface whose IP falls within this prefix.
Area	The OSPF area number this prefix belongs to (e.g. `0` for the backbone area). All routers sharing a segment must agree on the area.

Click + Add in each table to add a new neighbour or network entry. Click Save when done.

CLI Configuration¶

Basic setup — Router A1 (gateway)¶

router ospf
  router-id 172.16.1.254
  network 172.16.1.0/24 area 0
  default-information originate
  passive-interface eth0

Key points:

router-id 172.16.1.254 — sets an explicit, stable router ID; using the internal interface IP is recommended
network 172.16.1.0/24 area 0 — enables OSPF on br0 and advertises the prefix into area 0
default-information originate — redistributes A1's default route into OSPF so downstream routers (A2) learn a default gateway automatically
passive-interface eth0 — suppresses hello packets on the WAN interface; the interface is not included in OSPF advertisements and no neighbour adjacency can form on it

Basic setup — Router A2 (internal router)¶

router ospf
  router-id 172.16.1.24
  network 172.16.1.0/24 area 0
  network 10.10.10.0/24 area 0

Key points:

network 172.16.1.0/24 area 0 — enables OSPF on eth0 and forms a neighbour adjacency with A1
network 10.10.10.0/24 area 0 — advertises the eth1 LAN prefix into OSPF, making it reachable by A1 and any other OSPF peers

Redistributing connected and static routes¶

router ospf
  router-id 10.0.0.1
  network 172.16.1.0/24 area 0
  redistribute connected route-map OSPF-REDIST
  redistribute static route-map OSPF-REDIST

Key points:

redistribute connected — injects all directly connected prefixes not already covered by a network statement as external OSPF routes (Type 5 LSA / ASBR)
redistribute static — injects manually configured static routes into OSPF; useful for summarising or injecting routes learned from non-OSPF sources
A route-map is required to control which routes are redistributed and optionally set the external metric

OSPF within a VRF¶

router ospf vrf 4
  router-id 10.0.0.1
  network 192.168.10.0/24 area 0

Interface cost and timers¶

interface eth0
  ip ospf cost 100
  ip ospf hello-interval 10
  ip ospf dead-interval 40
  ip ospf priority 1

Key points:

cost — lower cost = preferred path; range 1–65535. Set manually to influence traffic engineering when multiple paths exist.
hello-interval — how often hello packets are sent in seconds (default: 10 on broadcast networks). Must match between neighbours or the adjacency will not form.
dead-interval — how long before declaring a neighbour unreachable (default: 40 seconds; typically 4× hello). Must match between neighbours.
priority — DR/BDR election weight; higher = more likely to become DR. Set to 0 to prevent this router from ever becoming DR.

Point-to-point links (VPN tunnels, PPPoE)¶

interface tun0
  ip ospf network point-to-point
  ip ospf cost 50

Note

On point-to-point interfaces there is no DR/BDR election — the two endpoints form a direct adjacency immediately. Always set ip ospf network point-to-point on tunnel or PPPoE interfaces to avoid election delays and adjacency failures.

MD5 authentication¶

router ospf
  area 0 authentication message-digest
!
interface eth0
  ip ospf message-digest-key 1 md5 MySecretKey

Key points:

area 0 authentication message-digest — enables MD5 authentication for all interfaces in area 0
ip ospf message-digest-key <key-id> md5 <password> — sets the key on each participating interface; key ID and password must match on both sides of the adjacency

Verification¶

View OSPF process summary and area status:

show ip ospf

Example output:

 OSPF Routing Process, Router ID: 172.16.1.254
 ...
 Number of areas attached to this router: 1

 Area ID: 0.0.0.0 (Backbone)
   Number of interfaces in this area: 1
   Number of fully adjacent neighbors in this area: 1

Check neighbour adjacency state:

show ip ospf neighbor

Example output:

Neighbor ID     Pri State           Dead Time Address         Interface
172.16.1.24       1 Full/DR         00:00:35  172.16.1.24     br0:172.16.1.254

A neighbour in Full state has completed LSA exchange and the adjacency is fully operational. Any other state (Init, 2-Way, ExStart, Exchange, Loading) indicates the adjacency is still forming or has stalled — check that hello/dead timers and authentication match on both sides.

View OSPF-computed routes:

show ip ospf route

Example output:

============ OSPF network routing table ============
N    10.10.10.0/24         [10] area: 0.0.0.0
                           via 172.16.1.24, br0

============ OSPF router routing table =============
R    172.16.1.24           [10] area: 0.0.0.0, ASBR
                           via 172.16.1.24, br0

============ OSPF external routing table ===========
N E2 0.0.0.0/0             [10/1] tag: 0
                           via 172.16.1.254, eth0

Check OSPF state on a specific interface:

show ip ospf interface br0

Example output:

br0 is up
  Internet Address 172.16.1.254/24, Area 0.0.0.0
  Router ID 172.16.1.254, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 172.16.1.254, Interface Address 172.16.1.254
  Backup Designated Router (ID) 172.16.1.24, Interface Address 172.16.1.24
  Timer intervals configured, Hello 10s, Dead 40s, Retransmit 5
  Neighbor Count is 1, Adjacent neighbor count is 1

View the link-state database:

show ip ospf database

View OSPF routes installed in the routing table alongside all other route sources:

show ip route ospf