ROUTE Course
OSPF
OSPF FEATURES
OSPF is an open standards routing protocol
This works by using the Dijkstra algorithm
OSPF provides the following features:
 Minimizes routing update traffic
 Allows scalability (e.g. RIP is limited to 15 hops)
 Has unlimited hop count
 Supports VLSM/CIDR
 Allows multi-vendor deployment (open standard)
OSPF Tables
There are three type of tables
 Neighbor
 Topology
 Routing
Areas Reasons and Features
 A larger topology database requires more memory on
each router.
 Processing the larger topology database with the SPF
algorithm requires more processing power
 A single interface status change, anywhere in the
internetwork forces every router to run SPF again.
 A router can be a member of more than one area (ABR)
 All routers in the same area have same topology database
 When multiple areas exist, there must always be an area 0
(the backbone) to which other areas connect
OSPF Area
 Autonomous System Border Router (ASBR)
ROUTER ID
 The Router ID (RID) is an IP address used to identify the
router
 Cisco chooses the Router ID by using the highest IP
address of all configured loopback interfaces
 If no loopback interfaces are configured with addresses,
OSPF will choose the highest IP address of all active
physical interfaces.
 You can manually assign the router ID.
 The RID interface MUST always be up, therefore loopbacks
are preferred
OSPF PACKETS
Data Field of the OSPF packet contents depend on the OSPF
packet type:
• Hello packet: Contains a list of known neighbors.
• DBD packet: Contains a summary of the LSDB, which includes
all known router IDs and their last sequence number, among
several other fields.
• LSR packet: Contains the type of LSU needed and the router ID
of the router that has the needed LSU.
• LSU packet: Contains the full LSA entries. Multiple LSA entries
can fit in one OSPF update packet.
• LSAck packet: This data field is empty.
OSPF PACKETS
Hello: Discovers neighbors and
builds adjacencies between them
Database Description (DBD):
Checks for database
synchronization between routers
Link-state request (LSR): Requests
specific link-state records from
another router
LSU: Sends specifically requested
link-state records
LSAck: Acknowledges the other
packet types
Hello Packet Information
• Router ID: 32-bit
• Hello and dead intervals: must be the same on neighboring
routers (must match)
• Neighbors: list of adjacent routers
• Area ID: Contains the full LSA entries. Multiple LSA entries
can fit in one OSPF update packet (must match)
• Router Priority: 8-bit
• DR and BDR IP Addresses
• Authentication Password: (must match)
• Stub Area Flag: (must match)
Network Types
Point-to-point
 Broadcast mutliaccess
 Non-Broadcast Multiaccess (NBMA)
 The contents of the LSA (excluding the LSA header) have
changed

 OSPF
 DR
over L2 and L3 MPLS VPN
and BDR have been selected, any router added to the
broadcast network establishes full adjacencies with the DR
and BDR only
Neighbor Adjacency States
 Router
A send hello on LAN .
 All connected routers add A
on list of neighbors (init
state)
 All routers received Hello
send unicast reply to A with
corresp. Info and list on their
neighbors inc. A
 A adds the received
neighbors IDs on table (2way state)
Discovering Network Routes in BC domain
After DR and BDR selected:
 Master-slave relationship
higher RID is the Masters
(Exstart state)
 Master & slave exchange
DBD packets (Exchange
state)
DBD is LSA entries header (linkstate type, Addr. Of Advertising
router, link cost, Seq. No.)
Discovering Network Routes in BC domain
When receive DBD:
 Router Ack the DBD using
LSAck
 Compare received info with
its LSDB, send LSR for
newest LSA (Loading state)
 Other router replies with
complete info about
requested entry using LSU
 Router Ack the received LSU
 Router adds new LSAs into
LSDB (Full state)
 Attempt
state for NBMA
Link-State Advertisement “LSA”
 LSAs
advertised to routers
with split-horizon rule
 Each LSA entry has aging timer
in age field of LSA (def. 30min)
 Router originated the entry
resends the LSA with higher
seq. no. in LSU to verify link
still active
 LSA discarded when LSA reach
its maxage (60 min)
 LS entry must be refreshed
every 30 min
LINK-STATE UPDATE “LSU” OPERATION
SPF Recalculation
SFP triggered when any of the following happen:
 The LSA’s Options field has changed
 The LSA’s LS age is set to maxage
 The Length field in the LSA header has changed
 The contents of the LSA (excluding the LSA header) have
changed
An SPF calculation is performed separately for each area
in the topology database.
Adjacency for NBMA Network


DR &BDR needs full L2 connectivity with routers in NBMA
Several OSPF configuration choices are available for FR network
depending on network topology:
Nonbroadcast: B/DR are elected, neighbors are manually config
Point-to-multipoint: B/DR not req., multicast hello auto neighbors disc.
Point-to-multipoint nonbroadcast: B/DR not req, manual neighbors config
Broadcast: B/DR are elected, multicast hello auto neighbors disc.
Point-to-point: no B/DR, differ. IP subnets
Point-to-point needs less configuration, nonbroadcast less traffic
overhead
R(config-if)#ip ospf network {broadcast | non-broadcast | point-tomultipoint [non-broadcast] | point-to-point}
NBMA Mode Configuration
Fully meshed topology DR & BDR are elected
 Not Fully meshed DR & BDR are manually selected
 DR & BDR should have full connectivity with all routers
 LSUs are replicated for each PVC
 Configure nonbroadcast mode by:
Manually configuring neighbors in DR and BDR
Define OSPF network type as nonbroadcast

neighbor ip-address [priority number] [poll-interval number] [cost
number] [database-filter all]
Point-to-multipoint Mode Configuration
Not full-mesh topology
 OSPF treats this mode as several point-to-point links
 No static neighbor configuration, multicast hello discover
neighbors
 Duplicated LSA packets
 Hello interval 30 sec, and dead interval 120 sec
 One IP subnet

Point-to-multipoint Nonbroadcast Mode Configuration
Used when no broadcast and multicast
 OSPF treats this mode as several point-to-point links
 No automatic neighbor discovery, so must use static neighbor
configuration
 Bandwidth for each neighbor can be defined
 Hello interval 30 sec, and dead interval 120 sec
 One IP subnet

OSPF over Frame Relay Subinterface Configuration
Point-to-point subinterface:
Each VC gets its own subinterface
No DR and BDR, automatic neighbor discovery
 Point-to-multipoint subinterface:
Nonbroadcast id the default mode

LSA Types
LSA type 1: Router LSA
 LSA type 2: Network LSA
 LSA type 3 : Summary LSA
 LSA type 4: ASBR
 LSA type 5: AS External LSA
 LSA type 6: Multicast OSPF LSA (not supported by Cisco)
 LSA type 7: LSA for NSSA
 LSA type 8: External Attributes for BGP (Not supp. by Cisco)
 LSA type 9, 10, 11: Opaque LSA (future upgrades)

LSA Type 1
link type is defined by
(1,2,3, or 4)
 Link ID: what is on the
other end of the link which
depends on link type
 Link data: IP address of the
link, or subnet mask in case
of stub network
 Type 1 LSA includes the
OSPF cost for each link, and
whether the router is an
ABR or ASBR

Link
Type
Decription
Link ID Field Contents
1
Point-point
Neighbor RID
2
Transit Network DR’s interface address
3
Stub network
IP network/subnet
4
Virtual link
Neighbor RID
LSA Type 2
Transit Network has at least 2 directly attached OSPF routers
 LSA type 2 list all attached routers, DR, and subnet mask
 DR is responsible for advertising Network LSA
 Link-state ID is the IP address of DR

LSA Type 3
Advertises networks for an area to the rest of areas
 Type 3 summary LSA is advertised into the backbone area for
every subnet defined in the originating area
 Manual summarization at ABR should be considered
 Receiving type 3 LSA does not cause router to run SPF
 To summarize inter-area, IOS a creates summary route to null 0:

area area-id range address mask [advertise | not-advertise] [cost
cost]

Link-state ID is
destination network
number (summary
network)
LSA Type 4
Generated by an ABR only when an ASBR exists within an area
 It identifies the ASBR and provide a route to it.
 ASBR send type 1 LSA with external bit(e bit) is set
 ABR generate type 4 LSA and floods it to backbone area and into
their area
 Link-state ID is ASBR ID

LSA Type 5
Describe a routes networks outside the OSPF AS
 To reduce flooding of LSAs for external networks, summarization
should be considered at ASBR
 Link-state ID is the external network number
 To manually summarize external routes:

summary-address ip-address mask [not-advertise] [tag tag]
Types of OSPF Routes
Route Designator
description
O
OSPF intra-area (router
LSA) and network LSA
Networks within the area advertised by
router and network LSAs
O IA
OSPF interarea
(summary LSA)
Networks from outside area and within
AS advertised by summary LSAs
O E1
Type 1 external routes
Networks from outside AS advertised
by way of external LSAs.)
O E2
Type 2 external routes
Networks from outside AS advertised
by way of external LSAs.
E1: adding external cost to internal cost (when multiple
ASBRs, E2 the cost in only the external cost
PROPAGATE DEFAULT ROUTE

default-information originate
when configured on a OSPF router it becomes an ASBR, the
router already has a default route
When use always keyword to the command it advertise
regardless whether the router already has default route
default-information originate [always] [metric metric-value] [metric-type
type-value] [route-map map-name]

The default metric value for type of interfaces is “1”
VIRTUAL LINKS
A virtual link allows discontiguous area 0s to be connected, or a
disconnected area to be connected to area 0, via a transit area.
 It cannot go through more than one area, nor through stub areas.
 LSA on virtual link does not age out (DoNotAge DNA)
 To configure virtual link:

area area-id virtual-link router-id [authentication [message-digest | null]] [hello
interval seconds] [retransmit-interval seconds] [transmit-delay seconds] [deadinterval seconds] [[authentication-key key] | [message-digest-key key-id md5 key]]
SPECIAL AREA TYPES

Standard Area: Default area link updates, route summaries, and
external routes
Backbone Area: area 0, all other areas connected to this area
 Stub Area: don’t accept external routes
 Totally Stubby Area: don’t accept external routes but can send a

packet to external using a default route, no ASBR. (Cisco proprietary)

Not so-stubby area (NSSA): don’t accept external information but
instead use default route, but can allow ASBR and use type 7 LSA

Totally Stubby NSSA: Allows ASBR and does not accept external
routes or summary routes
STUB OR TOTALLY STUBBY AREA
 There
is a single exit point from that area; or if there are multiple
exits, one or more ABRs inject a default route into the stub area
and suboptimal routing paths are acceptable.
 All
OSPF routers inside the stub area, incl. ABRs, are configured
as stub routers before they become neighbors and exchange
routing information. Hello packets contains stub area flag
 The
area is not used as a transit area for virtual links, no ASBR is
inside the area, and not a backbone area (area0)
 Type
 Type
4 and 5 LSAs not permitted to flood into stub area
3, 4 and 5 LSAs are not permitted to flood into totally
stubby area
STUB OR TOTALLY STUBBY AREA
 To
configure stub area:
area area-id stub
area area-id default-cost cost “to change the default route cost,
configured on ABR”
 To
configure totally stubby area:
area area-id stub [no-summary]
area area-id default-cost cost “to change the default route cost
configured on internal routers”
NOT-SO-STUBBY AREA “NSSA”
 ASBR
exits on area generate type 7 LSA into area with propagate bit (P)
to avoid propagation loop between NSSA and BB.
 NSSA
 ABR
ABR translates the type 7 LSA to Type 5 LSA
sends default route into NSSA instead of external routes
 Routers
in NSSA set N-bit to confirm the support for NSSA. This option
checked during neighbor discovery
 Type
7 LSA described in routing table by “O N1” or “O N2”
To configure the NSSA area
Area area-id stub nssa [no-redistribution]
[default-information originate] [metric
metric] [metric-type value] [no-summary]
 When use keyword [no-summary] totally stubby NSSA is configured
ROUTE FILTERING
 No
route filtering permitted inside area due to LSDB convergence
 Route
filtering can be applied for:

Type 3 LSAs on ABR

Type 5 LSAs on ASBR “Route redistribution”

Filtering OSPF routes when adding to IP routing table
TYPE 3 LSA FILTERING
 No
route filtering permitted inside area due to LSDB convergence

area number filter-list prefix name in | out

ip prefix-list {name | number} {seq number} {deny | permit}
netowrk/length [ge value] [le value]
 Example:
ip prefix-list filter-into-area-34 seq 5 deny 10.16.3.0/24
ip prefix-list filter-into-area-34 seq 10 permit 0.0.0.0/0 le 32
router ospf 1
area 34 filter-list prefix filter-into-area-34 in
FILTERING OSPF ROUTES ADDED TO ROUTING TABLE
 Don’t
affect LSAs, LSDB flooding process, and SPF calculation
 It
is enabled by using “distribute-list in” in OSPF subcommand
 It
filter the routes from being added to router’s IP routing table
 Routes
match ACL statement with permit are added to routing table
while that match deny are filtered
 “interface
interfaceNo” can be used to compare parameters to the
route’s outgoing interface
 Example:
ip prefix-list filter-1 seq 5 deny 10.16.1.0/24
ip prefix-list filter-1 seq 10 permit 0.0.0.0/0 le 32
router ospf 1
distribute-list prefix filter-1 in
OSPF Authentication
By default OSPF do not use authentication
 Two methods: simple password, MD5
 To configure simple authentication:

Router(config-if)# ip ospf authentication
Router(config-if)# ip ospf authentication-key password
Router(config-router)# area area-id authentication

To configure MD5 authentication:
Router(config-if)# ip ospf authentication [message-digest|null]
Router(config-if)# ip ospf message-digest-key key-id md5 key

You must configure:
service password-encryption
PLANNING FOR OSPF
IP
Addressing Plan
IP subnets and addressing plan considering summarization
Network
Topology
Detailed network topology include link types, backup links, stub areas,
redistribution

OSPF Areas
LSDB table size should be considered when dividing networks into
areas, ABR and ASBR routers should be identified

OSPF routing parameters (times, areas, authentication, RID, …)
should be determined