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3.2.b Configure simple OSPFv2/v3 environments, including multiple normal areas, summarization, and filtering (neighbor adjacency, point-to-point, and broadcast network types, and passive-interface)

4 min read ENCOR 350-401 v1.2

Aligned to Cisco's 350-401 ENCOR v1.2 exam topics.

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OSPF is predictable when you keep the pieces straight:

  1. Routers become neighbors on a link.
  2. Neighbors exchange link-state information.
  3. Each router builds a link-state database.
  4. Each router runs SPF and installs the best routes.

Troubleshooting OSPF means finding which step failed.

  1. Same subnet + area on the link?
  2. Hellos seen? (passive? ACL 89? wrong timers?)
  3. Neighbor state 2-WAY or FULL?
  4. LSDB has router/network LSAs from peer?
  5. SPF runs · OSPF route in routing table

If adjacency stalls in EXSTART or EXCHANGE, suspect MTU mismatch. If it never leaves INIT, suspect hello timers, ACLs, or passive interfaces.

OSPFv2 basic configuration

OSPFv2 carries IPv4 routes.

interface Loopback0
 ip address 1.1.1.1 255.255.255.255
!
interface GigabitEthernet0/0
 description To R2
 ip address 10.12.0.1 255.255.255.252
 ip ospf network point-to-point
!
router ospf 10
 router-id 1.1.1.1
 passive-interface default
 no passive-interface GigabitEthernet0/0
 network 10.12.0.0 0.0.0.3 area 0
 network 1.1.1.1 0.0.0.0 area 0

Verify:

show ip ospf neighbor
show ip ospf interface brief
show ip route ospf
show ip ospf database

OSPF network types

On Ethernet, OSPF defaults to broadcast network type. Broadcast networks elect a DR and BDR. On a point-to-point routed link, you can simplify behavior:

interface GigabitEthernet0/0
 ip ospf network point-to-point

Point-to-point network type avoids DR/BDR election on a link where only two routers exist.

Broadcast Ethernet segment (many routers):
  R1 ←
  R2 both FULL, no DR/BDR election

Check network type:

show ip ospf interface gi0/0

Neighbor adjacency checklist

When neighbors do not form, check:

  • Same subnet.
  • Same area.
  • Matching hello/dead timers.
  • Matching network type expectations.
  • Matching authentication, if configured.
  • MTU compatibility.
  • Interface not passive.
  • ACLs are not blocking protocol 89.
  • Unique router IDs.

Useful commands:

show ip ospf interface gi0/0
show ip ospf neighbor
show ip protocols
show run interface gi0/0
show run | section router ospf

Read neighbor state in context. FULL is expected between point-to-point neighbors and between a router and the DR/BDR on broadcast networks. 2-WAY is normal between DROTHER routers on the same broadcast segment; it is not automatically a failure. EXSTART or EXCHANGE that never completes usually points to an MTU, network-type, duplicate router ID, or packet-filtering problem. INIT usually means one-way hello reception.

That single paragraph prevents a lot of bad troubleshooting. Do not chase a fake problem just because a broadcast Ethernet neighbor is 2-WAY/DROTHER.

Multiple normal areas

A simple multi-area design:

Area 1
R1
Area 0
R2
Area 2

R1 and R2 are ABRs if they connect area 0 to another area.

On R1:

router ospf 10
 router-id 1.1.1.1
 network 10.10.1.0 0.0.0.255 area 1
 network 10.0.12.0 0.0.0.3 area 0

On R2:

router ospf 10
 router-id 2.2.2.2
 network 10.0.12.0 0.0.0.3 area 0
 network 10.20.2.0 0.0.0.255 area 2

Verify inter-area routes:

show ip route ospf

Look for O IA routes.

Summarization

OSPF summarization is done on area boundaries for inter-area routes.

On an ABR summarizing area 1 networks into area 0:

router ospf 10
 area 1 range 10.10.0.0 255.255.0.0

Verify:

show ip route ospf
show ip ospf database summary

A good summary reduces routing table size and hides internal instability. A bad summary can blackhole traffic when an ABR advertises reachability for a broad range but cannot actually forward to every covered prefix.

Be precise: the ABR should have a more-specific route for the destination inside the summarized range. If it advertises 10.10.0.0/16 but the only existing internal networks are 10.10.1.0/24 and 10.10.2.0/24, traffic for 10.10.99.0/24 can be drawn toward the ABR and then dropped. The summary is not wrong because it is broad; it is wrong when the covered prefixes do not match real reachability or the discard behavior is misunderstood.

Filtering

One common inter-area filter uses a prefix list and area filter list.

ip prefix-list BLOCK-LAB seq 5 deny 10.10.50.0/24
ip prefix-list BLOCK-LAB seq 10 permit 0.0.0.0/0 le 32
!
router ospf 10
 area 1 filter-list prefix BLOCK-LAB out

This filters routes from area 1 as they are advertised out of the area by the ABR.

Verify:

show ip prefix-list BLOCK-LAB
show ip route 10.10.50.0
show ip ospf database summary

Filtering is easy to overuse. Know where it applies and what direction means.

Passive interfaces

A passive interface advertises the connected network but does not form neighbor adjacencies.

Good default:

router ospf 10
 passive-interface default
 no passive-interface GigabitEthernet0/0

This keeps OSPF hellos off user-facing networks.

OSPFv3 basic configuration

OSPFv3 carries IPv6 routes, and modern IOS XE can also use OSPFv3 address families. The simple exam-friendly version enables IPv6 routing, assigns IPv6 addresses, and enables OSPFv3 per interface.

ipv6 unicast-routing
!
interface Loopback0
 ipv6 address 2001:db8:1::1/128
 ospfv3 10 ipv6 area 0
!
interface GigabitEthernet0/0
 description To R2
 ipv6 address 2001:db8:12::1/64
 ospfv3 10 ipv6 area 0
 ospfv3 network point-to-point
!
router ospfv3 10
 router-id 1.1.1.1

Verify:

show ospfv3 neighbor
show ospfv3 interface brief
show ipv6 route ospf
show ospfv3 database

OSPFv3 still needs a router ID. If no IPv4 address exists, configure the router ID manually.

Lab: Build multi-area OSPF

Topology

LAN-A · area 1 -> R1 -> R2 · area 0 -> R3 · area 0 -> LAN-C · area 2

Tasks

  1. Configure area 0 between R1-R2 and R2-R3.
  2. Configure area 1 on R1 LAN.
  3. Configure area 2 on R3 LAN.
  4. Make user-facing LAN interfaces passive.
  5. Summarize area 1 on R1.
  6. Verify that R3 sees a summary instead of every area 1 subnet.

Example on R1:

router ospf 10
 router-id 1.1.1.1
 passive-interface default
 no passive-interface gi0/0
 network 10.0.12.0 0.0.0.3 area 0
 network 10.10.0.0 0.0.255.255 area 1
 area 1 range 10.10.0.0 255.255.0.0

Exam traps

  • Passive interfaces advertise networks but do not form neighbors.
  • OSPF area, timers, MTU, authentication, and network type can prevent adjacency.
  • On broadcast networks, DR/BDR election affects adjacency state.
  • O means intra-area; O IA means inter-area.
  • Summarization happens at ABRs for inter-area routes.
  • A summary can attract traffic for covered prefixes the ABR cannot forward to.
  • OSPFv3 needs ipv6 unicast-routing and a router ID.

Quick checklist

  1. Did neighbors form?
  2. Are neighbors in the expected state?
  3. Are routes missing from the LSDB or only from the routing table?
  4. Is the interface passive?
  5. Is the route intra-area, inter-area, or external?
  6. Is summarization or filtering changing the route?
  7. For OSPFv3, is IPv6 routing enabled and router ID set?