OSPFv2

OSPFv2 is protocol 89.

Terms

LS — Link State

LSA — Link State Advertisement

LSDB — Link-State Database

RID — Router ID

Unique 32-bit number to identify the router in a graph
Doesn’t have to be an on-the-box IP, but is usually a loopback
Should be easy to identify from a Router-ID subnet

Process ID

Where the databases lives
Not transmitted
Allows multiple OSPFv2 processes

DR — Designated Router

Network vertex for a broadcast or NBMA network
Used to reduce the number of FULL adjacencies

Advertising Router

Router that created the LSA
Value in this field is the RID

The Update Rule

Routers can only modify LSAs they originate
Its RID is inside the “Advertising Router” field

LS Sequence

The first sequence number in any LSA is 0x80000001
Higher sequence numbers are newer LSAs

LS Checksum

Used to ensure the LSA was transmitted without corruption
Everything is checked except LS Age

LS Age

LSAs time out in an hour
Refreshed every 30 minutes
LSA Age increments traveling through routers

IFF — If and only if

Packet types

Type	Name	Short Name	Purpose
1	Hello	Hello	OSPFv2 puts the neighbor ID into it’s hello messages
2	Database Description	DBD/DDP	A LSA that contains LSA headers, “I have these LSAs”
3	LS Request	LSR	Requesting a specific LSA
4	LS Update	LSU	Send a specific LSA
5	LS Acknowledgment	LSAck	Acknowledging a specific LSA

Hello packets

These things must match for an adjacency to form

Hello time
Dead time
Subnet
Subnet mask
Interface MTU
Area
Area flags (NSSA, Stub)
Is DR/BDR enabled
Authentication

These must not match

Router ID

Check with debug ip ospf event

Identical databases

Each router can perform its own SPT via Dijkstra’s algorithm.

LSAs are flooded throughout an area, all routers in the same area should have the same LSAs and same database.

R1# show ip ospf database database-summary  | s Area 0
Area 0 database summary
  LSA Type      Count    Delete   Maxage
  Router        5        0        0       
  Network       5        0        0       
  Summary Net   8        0        0       
  Summary ASBR  2        0        0       
  Type-7 Ext    0        0        0       
    Prefixes redistributed in Type-7  0
  Opaque Link   0        0        0       
  Opaque Area   0        0        0       
  Subtotal      20       0        0

R2# show ip ospf database database-summary | s Area 0
Area 0 database summary
  LSA Type      Count    Delete   Maxage
  Router        5        0        0       
  Network       5        0        0       
  Summary Net   8        0        0       
  Summary ASBR  2        0        0       
  Type-7 Ext    0        0        0       
    Prefixes redistributed in Type-7  0
  Opaque Link   0        0        0       
  Opaque Area   0        0        0       
  Subtotal      20       0        0

Can also check with checksums

show ip ospf | i Checksum

Adjacency state machine

State	Description
Down	OSPFv2 is running, no hello packets received yet.
Attempt	NBMA mode, the router has sent OSPFv2 packets.
Init	The router sees hello packets.
2-Way	The router sees it’s own router-id in the hello packet.
ExStart	Routers vote on who exchanges LSDB first.
Loading	Router DB has been exchanged, router is requesting specific LSAs.
Full	LSDBs for this area are identical on both sides.

Routing hierarchy

OSPFv2 has four levels of routing hierarchy.

Preference	Route	Purpose
1	O	Intra-area (same area)
2	O IA	Inter-area (same OSPFv2 domain)
3	E1	External type 1 (seed metric + IGP metric)
4	E2	External type 2 (just seed metric)

The bit E is what makes E1 and E2 routes. The bit being set is an E2 route, which is considered less preferred.

Default route

OSPFv2 has two ways of originating a default route.

default-information originate if a default route is present.

default-information originate always do it anyway.

Cost

Should be set manually on each node.

The defaults make 100Mbps and above all the same cost.

auto-cost reference-bandwidth 40,000

Area summary

These will show up as an O IA route in OSPFv2, and a route-to-null on the ABR.

Requires a route present in the RIB

v4

router ospf 1
 router-id 2.2.2.2
 area 1 range 10.0.0.0 255.255.224.0

v6

router ospfv3 1
 !
 address-family ipv6 unicast
  area 1 range 2001:DB8::/56

Route-Filtering

You can use the same command to tell the router to … exclude these routes from the backbone, via the not-advertise keyword.

Using range

The area command is now a route-filter.

Range v4

router ospf 1
 router-id 2.2.2.2
 area 1 range 10.0.0.0 255.255.224.0 not-advertise

Range v6

router ospfv3 1
 !
 address-family ipv6 unicast
  area 1 range 2001:DB8::/56 not-advertise
 exit-address-family

Using Filter-Lists

These are a bit harder to use, in and out are inbound and outbound to the area.

For this topology

             Area 0                               Area 1            
                                                               
                                 |           10.0.10.0/24           
                                 |         2001:db8:0:10/64         
                                 |                            +----+
                              +----+       +------------------+ R3 |
+----+                        |    +-------+                  +----+
| R1 +------------------------+ R2 |                        
+----+                        |    +------+     
             10.0.0.0/24      +----+      |                   +----+
           2001:db8:0:0/64       |        +-------------------+ R4 |
                                 |           10.0.20.0/24     +----+
                                 |         2001:db8:0:20/64

ip prefix-list PREFIX_LIST_LOOPBACK_v4 seq 10 deny 1.1.1.1/32
ip prefix-list PREFIX_LIST_LOOPBACK_v4 seq 20 deny 2.2.2.2/32
ip prefix-list PREFIX_LIST_LOOPBACK_v4 seq 30 deny 3.3.3.3/32
!
router ospf 1
 area 0 filter-list prefix PREFIX_LIST_LOOPBACK_v4 in
 area 1 filter-list prefix PREFIX_LIST_LOOPBACK_v4 in

!
ipv6 prefix-list PREFIX_LIST_v6 seq 10 deny FD::1/128
ipv6 prefix-list PREFIX_LIST_v6 seq 20 deny FD::3/128
ipv6 prefix-list PREFIX_LIST_v6 seq 30 deny FD::4/128
!
router ospfv3 1
 !
 address-family ipv6 unicast
  area 0 filter-list prefix PREFIX_LIST_v6 in
  area 1 filter-list prefix PREFIX_LIST_v6 in

OSPF graceful restart

Restart OSPF without dropping adjacencies
Keeps the OSPF table in memory while OSPF rebuilds.

Area types

I made a chart.

References

RFC 2328 - OSPF Version 2

J. T. Moy, OSPF: Anatomy of an Internet Routing Protocol. Reading, MA: Addison-Wesley, 1998.

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