OSPF

(Open Shortest Path First)

Link-State Protocol / Classless / Triggered Updates/ AD of 110

Characteristics

Link-State Protocol Characteristics

  1. Responds quickly to network changes
  2. Send triggered updates when a network change occurs
  3. Send periodic updates, known as link-state refresh, at long time intervals, such as every 30 minutes.
  4. Uses a 2 layer are hierarchy
    1. Transit Area – primary function is the fast and efficient movement of IP packets.  OSPF area 0 (backbone) is a transit area.
    1. Regular Area – primary function is to connect users and resources.  By default, they do not allow traffic from another area to use its links to reach other areas

OSPF Area Characteristics

  1. Minimizes routing table entries
  2. Localizes the impact of a topology change within an area.
  3. Detailed LSA flooding stops at the area boundary
  4. Requires a hierarchical network design

ABR Characteristics

  1. Separates LSA flooding zones
  2. Becomes the primary point for area address summarization
  3. Functions regularly as the source of default routes
  4. Maintains the LSDB for each area with which it is involved.

OSPF Packet Types

 

  1. Hello – Discovers neighbors and builds adjacencies between them.
  2. Database Description (DBD) – Checks for database synchronization between routers.
  3. Link-state request (LSR) –Requests specific link-state records from another router.
  4. Link-state update (LSU) – Sends specifically requested link-state records.
  5. Link-state acknowledgement (ACK) – Acknowledges other packet types.

Link Header

IP Header

Protocol ID No. 89

OSPF Packet Type

Link Trailer

OSPF Packet Header

Version

Type

Packet Length

Router ID

Area ID

Checksum

Authentication Type

Authentication

Data

  • Version Number – For OSPF version 2
  • Packet Type – Differentiates the five OSPF packet types
  • Packet Length – The length of the OSPF packet in bytes.
  • Router ID – Defines which router is the packet’s source.
  • Area ID – Defines the area where the packet originated
  • Checksum – Used for packet header error detection to ensure that the OSPF packet was not corrupted during transmission.
  • Authentication Type – An option in OSPF that describes either no authentication, cleartext passwords, or encrypted Message Digest 5 (MD5) formats for router authentication
  • Data
    • For the Hello Packet – Contains a list of known neighbors
    • For the DBD Packet – Contains a summary of the LSDB, which includes all known router IDs and their last sequence number among other fields
    • For the LSR Packet – Contains the type of LSU needed and the router ID of the needed LSU
    • For the LSU Packet – Contains the full LSA entry.  Multiple LSA entries can fit one OSPF Packet update.
    • For the LSAck Packet – This is empty.

OSPF Hello Packet

Version

1

Packet  length

Router ID

Area ID

Checksum

Authentication Type

Authentication

Authentication

Network Mark

Hello Interval

Options

Router Priority

Router Dead Interval

Designated Router

Backup Designated Router

Neighbor

 

·                           Network mask - The network mask associated with this interface

·                           Options - The optional capabilities supported by the router.

·                           Hello Interval - The number of seconds between this router's Hello packets.

·                           Router Priority - This router's Router Priority.  Used in (Backup) Designated Router election.  If set to 0, the router will be ineligible to become (Backup) Designated Router.

·                           Router Dead Interval - The number of seconds before declaring a silent router down.

·                           Designated Router - The identity of the Designated Router for this network, in the view of the sending router.  The Designated Router is identified here by its IP interface address on the network.  Set to 0.0.0.0 if there is no Designated Router.

·                           Backup Designated Router - The identity of the Backup Designated Router for this network, in the view of the sending router.  The Backup Designated Router is identified here by its IP interface address on the network. Set to 0.0.0.0 if there is no Backup Designated Router.

·                           Neighbor - The Router IDs of each router from whom valid Hello packets have been seen recently on the network.  Recently means in the last Router Dead Interval seconds.

 OSPF Neighbor Adjacency States

 DOWN

The router has not exchanged any information with any other routers.  It begins sending a hello packet through each of its interfaces participating in OSPF, even though it does not understand the identity of the DR or any other routers.  The hello packets are sent out using the multicast address of 24.0.0.5.

INIT

All directly connected routers running OSPF receive the hello packet from the router and add the router to their list of neighbors.  All routers that received the hello packet send a unicast reply packet to the sending router with their corresponding information.  The neighbor field in the hello packet includes all other neighboring routers, including the initial sending router.

2-WAY

When the initial sending router received the hello packets, it adds all the routers that had its Router ID in their hello packets to its own neighborship database.  At this point, all routers that have each other in their list of neighbors have established bidirectional communication.

EXSTART

If the link type is a broadcast network (i.e. Ethernet, etc.), a DR and BDR must be selected.  The DR forms bidirectional adjacencies between all other routers on the LAN link.  This process must occur before the routers can begin exchanging link-state information.  After the DR and BDR have been selected, the routers are considered to be in the exstart state.  The master and slave relationship is created between each router and its adjacent DR and BDR

EXCHANGE

After the master and slave routers have been determined they will begin to exchange one or more DBD packets.  The routers are now in the exchange state.  The DBD includes information about the LSA entry header that appears in the router’s LSDB.  Each LSA entry header includes information about the link-state type, the address of the advertising router, the link’s cost and the sequence number.

LOADING

When the router receives the DBD it acknowledges the receipt of the DBD using a LSA packet and compares the information it received with the other information it has in its own LSDB.  If the DBD has a more up to date link-state entry, the router sends an LSR to the other router.  The other route responds with the complete information about the requested entry in an LSU packet.

FULL

As soon as all LSRs have been satisfied for a given router, the adjacent routers are considered synchronized and in a full state.  The routers must be in a full state before they can route traffic.

 OSPF Adjacency Behaviors

Point-to-Point Networks

  • Joins a single pair of routers.
  • Router dynamically detects its neighboring routers by multicasting hello packets to all SPF routers using address 224.0.0.5
  • Neighboring routers become adjacent whenever they can communicate directly
  • No DR or BDR election is performed.
  • The default hello interval is 10 seconds
  • The default dead interval is 40 seconds

Broadcast Networks

  • DR and BDR election is performed
  • Adjacency is the relationship between a router and its BR and BDR.
  • The BDR does not perform any DR functions when the DR is operating.
  • The BDR performs the DR tasks only if the DR fails.
  • The BDR receives the information, but the DR performs the LSA forwarding and LSDB synchronization tasks.

Nonbroadcast Multiaccess Networks

  • DR and BDR election is performed in full mesh topologies only.
  • Examples of NBMA networks are Frame Relay, ATM and X.25.
OSPF Mode NBMA Preferred Topology Subnet Address Hello Timer Adjacency RFC or Cisco Example

 

NBMA Fully Meshed Same 30 sec Manual Configuration

DR/BDR elected

RFC Frame Relay configured on a serial interface
Broadcast Fully Meshed Same 10 sec Automatic

DR/BDR elected

Cisco LAN interface such as Ethernet
Point-to-Multipoint Partial or Star Mesh Same 30 sec Automatic

No DR/BDR

RFC OSPF over FR mode that eliminates the need for a DR
Point-to-multipoint Nonbroadcast Partial or Star Mesh Same 30 sec Manual Configuration

No DR/BDR

Cisco OSPF over FR mode that eliminates the need for a DR
Point-to-point Partial or Star using a subinterface Different for each subinterface 10 sec Automatic

No DR/BDR

Cisco T1 Serial Interface

OSPF in NBMA Frame Relay

Router (config-if)#ip ospf network {broadcast | non-broadcast | point-to-point |           point-to-multipoint [non-broadcast]}

broadcast

(Cisco Mode)

  • Makes the WAN interface appear to be a LAN
  • One IP Subnet
  • Uses a multicast OSPF hello packet to automatically discover the neighbors.
  • DR and BDR elected
  • Requires a full-mesh topology
non-broadcast (NBMA)

(RFC-Compliant)

  • One IP Subnet
  • Neighbors must be manually configured
  • DR and BDR elected
  • DR and BDR need to have full connectivity with all other routers.
  • Typically used in partial-mesh topology
point-to-point

(Cisco)

  • One IP Subnet
  • No DR and BDR election
  • Used only when two routers need to form an adjacency on a pair of interfaces
  • Interfaces can be either WAN or LAN
point-to-multipoint

(RFC-Compliant)

  • One IP Subnet
  • Uses a multicast OSPF hello packet to automatically discover the neighbors
  • DR and BDR not required.  The router sends additional LSAs with more information about the neighboring routers
  • Typically used in partial-mesh topology
point-to-multipoint non-broadcast (a subset of point-to-multipoint)

(Cisco)

  • If multicast and broadcast are not enabled on the VCs, the RFC-compliant point-to-multipoint mode cannot be used, because the router cannot dynamically discover its neighboring routers using hello multicast packets.
  • Neighbors must be manually configured in the case
  • DR and BDR election not required.

Benefits of Using Multiple OSPF Areas

Reduced frequency of SPF calculations – Because detailed route information exists within each area, it is not necessary to flood all link-state changes to all other areas.  Therefore, all routers that are affected by the change need to recalculate SPF.

Smaller routing tables – Within multiple areas, detailed route entries for specific networks with the area remain in the area.  Instead of advertising the explicit routes outside the area, routers can be configured to summarize the routes into one or more summary addresses.  Advertising these summaries reduces the number of LSAs propagated between areas but keeps the network reachable.

Reduced link-state update (LSU) overhead – Rather than sending an LSU about each network within an area, a router can advertise a single summarized route or a small number of routes between areas, thereby reducing the overhead associated with LSUs when they cross areas.

OSPF Router Types

Backbone router – A backbone router (BR) is a router with an interface to the backbone area. An ABR is a BR, though the converse need not be true.

Internal router – A router is called an internal router (IR) if it has only OSPF adjacencies with routers in the same area.

Area border router – An area border router (ABR) is a router that connects one or more OSPF areas to the main backbone network. It is considered a member of all areas it is connected to. An ABR keeps multiple copies of the link-state database in memory, one for each area.

Autonomous system boundary router – An autonomous system boundary router (ASBR) is a router that is connected to more than one AS and that exchanges routing information with routers in other AS’s. ASBR’s typically also run a non-IGP routing protocol, such as BGP. An ASBR is used to distribute routes received from other ASs throughout its own AS.

Designated and Backup Designated Routers

Designated router – A designated router (DR) is the router elected by the network by elections. The DR is elected based on the following default criteria:

  • If the priority setting on a OSPF router is set to 0, that means it can NEVER become a DR or BDR.
  • When a DR fails and the BDR takes over, there is another election to see who becomes the replacement BDR.
  • The router sending the Hello packets with the highest priority.
  • If two or more routers tie with the highest priority setting, the router sending the Hello with the highest RID (Router ID) wins.
  • (NOTE) A RID is the highest logical (loopback) IP address configured on a router, if no logical/loopback IP address is set then the Router uses the highest IP address configured on its interfaces. (e.g. 192.168.0.1 would be higher than 10.1.1.2)
  • Usually the router with the second highest priority number becomes the BDR (Backup Designated Router)
  • The range of priority values range from 1 – 255, with a higher value increasing its chances of becoming DR or BDR.
  • IF a HIGHER priority OSPF router comes online AFTER the election has taken place, it will not become DR or BDR until (at least) the DR and BDR fail.
  • DR’s exist for the purpose of reducing network traffic by providing a source for routing updates; the DR maintains a complete topology table of the network and sends the updates to the other routers via multicast. This way all the routers do not have to constantly update each other, and can rather get all their updates from a single source. The use of multicasting further reduces the network load. DRs and BDRs are always setup/elected on Broadcast networks (Ethernet). DR’s can also be elected on NBMA (Non-Broadcast Multi-Access) networks such as Frame Relay. DRs or BDRs do not configure on point-to-point links (such as a point-to-point WAN connection) because the bandwidth between two hosts cannot be further optimized.

Backup designated router – A backup designated router (BDR) is a router that becomes the designated router if the current designated router has a problem or fails. The BDR is the OSPF router with second highest priority at the time of the last election

OSPF LSA Types

Type 1Router LSA – the router lists the links to other routers or networks in the same area, together with the metric. Type 1 LSAs are flooded across their own area only.

Type 2Network LSA – the designated router on a broadcast segment (e.g. Ethernet) lists which routers are joined together by the segment. Type 2 LSAs are flooded across their own area only.

Type 3Summary LSA – an Area Border Router (ABR) takes information it has learned on one of its attached areas and summarizes it before sending it out on other areas it is connected to. This helps scalability by removing detailed topology information for other areas, because their routing information is summarized into just an address prefix and metric.

Type 4ASBR-Summary LSA – this is needed because Type 5 External LSAs are flooded to all areas and the detailed next-hop information may not be available in those other areas. This is solved by an Area Border Router flooding the information for the router where the type 5 originated.

Type 5External LSA – these LSAs contain information imported into OSPF from other routing processes. They are flooded to all areas (except stub areas). For “External Type 1” LSAs routing decisions are made by adding the OSPF metric to get to the ASBR and the external metric from there on, while for “External Type 2” LSAs only the external metric is used.

Type 6Group Membership LSA – this was defined for Multicast extensions to OSPF (MOSPF), a multicast routing protocol which is not in general use.

Type 7 – Routers in a Not-so-stubby-area (NSSA) do not receive external LSAs from Area Border Routers, but are allowed to send external routing information for redistribution. They use type 7 LSAs to tell the ABRs about these external routes, which the Area Border Router then translates to type 5 external LSAs and floods as normal to the rest of the OSPF network.

Type 8 – a link-local only LSA for the IPv6 version of OSPF, which is known as OSPFv3. A type 8 LSA is used to give information about link-local addresses and a list of IPv6 addresses on the link.

Type 9 – a link-local “opaque” LSA (defined by RFC2370) in OSPFv2 and the Inter-Area-Prefix LSA in OSPFv3.

Type 10 – an area-local “opaque” LSA as defined by RFC2370. Opaque LSAs contain information which should be flooded by other routers even if the router is not able to understand the extended information itself. Typically type 10 LSAs are used for traffic engineering extensions to OSPF, flooding extra information about links beyond just their metric, such as link bandwidth and color.

Type 11 – an “opaque” LSA defined by RFC2370, which is flooded everywhere except stub areas. This is the opaque equivalent of the type 5 external LSA.

OSPF LSDB Contents

Link ID – Identifies each LSA

ADV Router – Advertising router – the LSA’s source center.

Age – The maximum age counter in seconds.  The maximum age is 1 hour, or 3600 seconds

Seq# – The LSA’s sequence number. It begins at 0x80000001 and increases with each update of the LSA.

Checksum – Checksum of the individual LSA to ensure reliable receipt of that LSA.

Link Count – The total number of directly attached links used only on router LSAs.  The link count includes all point-to-point, transit, and stubby links.  Except for point-to-point serial links, which count as two, all other serial links count as one, and each Ethernet link counts as one.

OSPF Route Types

Route Designator

Description
O OSPF intra-area

(Router LSA)

Networks from within the router’s area.

Advertises by way of router LSAs.

O IA OSPF inter-area

(Summary LSA)

Networks from outside the router’s area but within the OSPF autonomous system.
O E1 Type 1 external routes

External Cost + Internal Cost

Networks outside the router’s autonomous system.

Advertised by way of external LSAs.

O E2 (Default) Type 2 external routes

External Cost ONLY

OSPF Summarization Types

Interarea route summarization – Occurs on the ABRs and applies to routers from within the area.  It does not apply to external routes injected into OSPF via redistribution.

External route summarization – Specific to external routed that are injected into OSPF via route redistribution.  Only ASBRs generally summarize external routes.

OSPF Area Types

Standard area – This default area accepts link updates, route summaries and external routes.

Backbone area – The backbone area (also known as area 0) forms the core of an OSPF network. All other areas are connected to it, and inter-area routing happens via a router connected to the backbone area. It is the logical and physical structure for the ‘autonomous system’ (AS) and is attached to multiple areas. The backbone area is responsible for distributing routing information between nonbackbone areas. The backbone must be contiguous, but it does not need to be physically contiguous; backbone connectivity can be established and maintained through the configuration of virtual links.

All OSPF areas must connect to the backbone area.

Stub area – A stub area is an area which does not receive external routes. External routes are defined as routes which were distributed in OSPF from another routing protocol. Therefore, stub areas typically need to rely on a default route to send traffic to routes outside the present domain. This implies that AS-external routes (Type 5 LSAs) are not fed into Stub Areas.

Totally stubby area – A totally stubby area (TSA) is similar to a stub area, however this area does not allow summary routes in addition to the external routes, that is, inter-area (IA) routes are not summarized into totally stubby areas. The only way for traffic to get routed outside of the area is a default route which is the only Type-3 LSA advertised into the area. When there is only one route out of the area, fewer routing decisions have to be made by the route processor, which lowers system resource utilization.

Not-so-stubby area – A not-so-stubby area (NSSA) is a type of stub area that can import autonomous system (AS) external routes and send them to the backbone, but cannot receive AS external routes from the backbone or other areas. Cisco also implements a proprietary version of a NSSA called a NSSA totally stubby area. It takes on the attributes of a TSA, meaning that type 3 and type 4 summary routes are not flooded into this type of area.

OSPF Virtual Links

Virtual links are created to link areas not directly connected to area 0 to area 0.

The following are two major reasons for using virtual links:

  • Backup
  • Temporary Connection

Facts

  1. Flooding of LSAs ensures that all routing devices update their databases before updating routing tables to reflect the new topology.
  1. LSAs are stored in the topology table (an LSDB).
  1. The best paths in the LSDB are forwarded to the routing table.
  1. OSPF areas require a hierarchical structure, meaning that all areas must connect directly to area 0.
  1. Cisco recommends no more than 50 to 100 routers per area.
  1. Area border routers (ABR) connect area 0 to the backbone areas.
  1. The DR and BDR maintain a partial-neighbor relationship, a 2-way adjacency, with the other non-DR and non-BDR routers (DROTHERs) on the LAN.
  1. LSAs are reliable; there is a method for acknowledgement and delivery
  1. LSAs are flooded throughout an area
  1. LSAs have a sequence number and a set lifetime, so each router recognizes that is has the most up-to-date version of the LSA.
  1. LSAs are periodically refreshed to confirm topology information before they age out of the LSDB.
  1. For OSPF, the default behavior is that the interface cost is based on its configured bandwidth.
  1. The default timer value for OSPF is 30 minutes (expressed in seconds in the link-state age field).
  1. Summaries of the individual link-state entries, not the complete link-state entries, are sent every 30 minutes to ensure LSDB synchronization.  This interval is called the LSRefreshTime.
  1. An LSA never remains in the database longer than the maximum age of 1 hour without a refresh.
  1. After a DR and BDR are selected, any router added to the network establishes adjacencies with the DR and BDR only.
  1. The hello packet is sent out using the multicast address 224.0.0.5.
  1. A router will use the multicast address of 224.0.0.6 to reach the BDR.
  1. The DR will send transmissions to the other area routers using multicast address 224.0.0.5.
  1. The sequence number field in a link-state record is 32 bits ling.  It is used to detect old or redundant LSA records.  The larger the number, the more recent the LSA.
  1. The highest IP address of any physical interface is chosen as the router ID.
  1. A loopback address is always preferred over an interface address, because the loopback address never goes down.  If there is more than one loopback address, the highest loopback address becomes the router ID.
  1. If the loopback address is published using the network command, the other routers can ping this address for testing purposes, and a private IP address can be used to save a registered public IP addresses.
  1. The router with the highest priority value is the DR
  1. If a router with a higher priority value gets added to the network, it does not preempt the DR and BDR.  The only time a DR and BDR changes is if one of them is out of service.
  1. To figure out how many VCs need to be implemented in a full-mesh topology, use the formula n(n-1)/2, where n is the number of nodes in the network.
  1. The main advantage of point-to-multipoint mode is that it requires less manual configuration.
  1. The main advantage of NBMA mode is that there is less overhead traffic compared to point-to-multipoint mode.
  1. In point-to-multipoint networks the hello and dead timers on neighboring interfaces must match for the neighbors to form successful adjacencies.
  1. The default mode on a point-to-point Frame Relay subinterface is point-to-point mode.
  2. The default mode on a Frame Relay point-to-multipoint subinterface is nonbroadcast mode.
  1. The default mode on a Frame Relay interface is also nonbroadcast mode.
  1. ASBRs can import non-OSPF network information to the OSPF network and vice versa; this process is called route redistribution.
  1. All types of LSAs have 20-byte LSA headers.
  1. The link-state ID of the Type 1 LSA is the router’s originating ID.
  1. The link-state ID of the Type 2 LSA is the DR’s IP interface address.
  1. The link-state ID of the Type 3 LSA is the destination network number.
  1. The link-state ID of the Type 4 LSA is the router ID of the described ASBR.
  1. The link-state ID of the Type 5 LSA is the external network number.
  1. When an ABR or ASBR receives summary or external LSAs, it adds them to its
  1. LSDB and floods them to their local area
  1. By default, OSPF calculates the OSPF metric for an interface according to the interface’s inverse bandwidth.
  1. By default, summary LSAs are not summarized.
  1. Only ASBRs can generally summarize external routes.
  1. Manual summarization for OSPF is off by default.
  1. An OSPF router does not, by default, generate a default route into the ospf domain.
  1. A stub area is typically created using a hub-and-spoke topology, with the spoke being the stub area.
  1. The hello packet in stub areas contains a stub area flag that must match on neighboring routers.
  1. A totally stubby area blocks type 5 LSAs and summary type 3 and type 4 LSAs (Interarea routes) from entering the area.
  1. Using totally stubby areas is typically a better solution than using stub areas as long as the ABR is a Cisco router.
  2. The ABR advertises a default route with a cost of 1.
  1. Redistribution into an NSSA area creates a special type of LSA known as type 7 which can exist only in an NSSA area.  An NSSA ASBR generated this LSAs and an NSSA ABR translates iit into a type 5 LSA, which gets propagated into the OSPF domain.
  1. LSAs learned through the virtual link have the DoNotAge (DNA) option set.  The DNA technique is required to prevent excessive flooding over the virtual link.

Acronyms

DBD

Database Description

LSR

Link-State Request

LSU

Link-State Update

LSAck

Link-State-Acknowledgement

DR

Designated Router

BDR

Backup Designated Router

ABR

Area Border Router

ASBR

Autonomous System Boundary Router

RID

Router ID

DROTHER

A router that is not the DR or BDR

NBMA

Non-Broadcast Multiaccess

P2P

Point-to-Point

P2MP

Point-to-Multipoint

SPF

Shortest Path First

LSDB

Link State Database

NSSA

Not-So-Stubby-Area

Show Commands

sh ip ospf Displays the OSPF router ID (RID), OSPF timers, the number of times the SPF algorithm has been executed, and LSA information.
sh ip ospf interface Verifies that the interfaces are configures in the intended areas.  In addition, this command displays the timer intervals (including the hello interval), OSPF cost, and neighbor adjacencies
sh ip ospf neighbor detail Displays a detailed list of neighbors, including their RID, OSPF priorities, neighbor adjacency state (init, full, etc.) and the dead timer.
sh ip route ospf Displays the OSPF routes known to the router.  One of the best ways to determine connectivity between the local router and the rest of the network.
sh ip ospf database Displays the contents of the OSPF topological database maintained by the router.  Will also show the RID and process ID
sh ip ospf border-routers Displays the internal OSPF routing table entries to area border routers (ABRs) and autonomous system boundary routers (ASBRs).
sh ip ospf virtual-links Displays the OSPF virtual links.
sh ip ospf database nssa-external Displays the specific details of each LSA type 7 update in the database.

Debug Commands

debug ip ospf Starts the console display of translation entries being created.
debug ip ospf adj Starts the console display of OSPF adjacency-related events on the router
debug ip ospf events Starts the console display of OSPF related events, such as adjacencies, flooding information, designated router selection, and SPF calculation of the router.
debug ip ospf packet Starts the console display of OSPF packets received.
debug ip ospf lsa-generation Starts the console display of OSPF link-state advertisement generation-related events on the router.
debug ip ospf spf Starts the console display of the shortest path first (SPF) calculation-related events on the router

Configuration Commands

router ospf process-id Enables the OSPF process on the router
network address wildcard-mask area area-id Identifies the network that belongs to the OSPF process
router-id ip –address Manually configures the router ID
clear ip ospf process Restarts the OSPF process on the router.
ip ospf priority  priority number Used to designate which router interfaces on a multiaccess link are the DR and BDR.
neighbor ip-address [priority number] [poll-interval sec] [cost number] Statically defines adjacent relationships in NBMA networks using nonbroadcast mode.
interface serial number.subinterface-number {multipoint | point-to-point} Creates and defines a subinterface.
bandwidth value Assigns an interface a bandwidth value to determine OSPF cost.
auto-cost bandwidth-reference ref-bw This command is used for interfaces faster than 100 Mbps.
ip ospf cost value(1-65,535) This overrides the default OSPF cost. The lower the number the better the link.
area area-id range address-mask Instructs the ABR to summarize routes for a specific area before injecting them into a different area via the backbone as type 3 summary LSAs.
summary-address {{address mask} | {prefix-mask}} [not-advertise] [tag tag] Instructs the ASBR or the ABR to summarize external routed before injecting them into the OSPF domain as type 5 external LSAs
default-information originate [always] [metric metric-value] [metric-type type-value] [route-map mapname] Generates a default external route into an OSPF routing domain.
area area-id stub Configures an area as stub.
area area-id stub no-summary Configures an area as totally stubby.
area area-id nssa Configures an area as Not So Stubby
area area-id default-cost cost Changes the default cost of an area.
default-information-originate Causes the NSSA ABR to generate an O*N2 default route into the NSSA area.
area area-id virtual-link router-id [authentication [message-digest | null ]] [hello-interval seconds] [retransmit-interval seconds] [transmit-delay seconds] [dead-interval seconds] [[authentication-key key] | [message-digest-key key-id md5 key] Creates a virtual link to area 0
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