Networker's BLOG: July 2007

OSPF Topic

Overview

- open standard

Link-State Routing Protocols

Characteristics

- respond quickly to network changes

- send triggered updates when a network changes occur

- send periodic updates (link-state refresh) , every 30 minutes

- generate routing updates only when a change occurs in the network topology

- when a link change state, the device that detects the change creates a LSA concerning that link

- LSA propagates to all neighboring devices using a multicast address

- each routing devices takes a copy of the LSA and updates its LSDB and forward the LSA to all neighboring devices within the area

- LSDB is used to calculate the best paths

- Link-state routers find the best paths to a destination by applying Dijkstra’s algorithm also known as SPF against the LSDB to build the SPF tree

- The best path are then selected from the SPF tree and placed in the routing table

- Examples: OSPF and IS-IS

- Each router has full view of the network topology

- All routers must keep a copy of the LSDB, the more OSPF routers, the larger LSDB

Link-State Data Structures

a. Neighbor Table

- also known as the adjacency database

- contains list of recognized neighbors

b. Topology Table

- referred to as LSDB, identical LSDB for all routers w/in an area

- contains all routers and their attached links in the area

c. Routing Table

- also known as forwarding database

- contains all the list of best paths to destinations

Drawback of Link-state protocol

- memory resources

OSPF Area Structure

- link-state routing requires a hierarchical network structure, meaning that all area must connect directly to area 0

- this 2 level hierarchy consists of

a. transit area (backbone or area 0)

- primary function is fast and efficient movement of packets

- interconnect other area types

- end users are not found within a transit area

b. regular areas (nonbackbone areas)

- primary function is to connect end users and resources

- by default, does not allow traffic from another area

- subtypes include standard area, stub area, totally stubby area, and not-so-stubby area (NSSA)

- minimizes routing table entries

- localizes impact of a topology change within an area

- detailed LSA stops at the area boundary

- when a router or link fails, that information is flooded along adjacencies only to the routers in the local area

- routers outside the area do not receive this information

- all interarea traffic must pass through the backbone area, area 0

Area Terminology

- routers that make up area0 are known as backbone routers

- all other areas connect directly to backbone area 0

- an area border router (ABR) connects area 0 to the nonbackbone areas

Characteristic of Area Border Router

- it separates LSA flooding zone

- becomes primary point for area address summarization, only ABR and ASBR can do summarization

- functions regularly as the source for default routes

- maintains the LSDB for each area with which it is connected

OSPF Adjacency Database

- router running link-state protocol must first establish neighbor adjacencies with its neighboring routers, this is done by exchanging hello packets

router sends/receives hello packets to/from its neighboring routers, the format of the destination address is typically multicast
routers exchange hello packets subject to protocol-specific parameters, such as wether the neighbor is in the same AS and area.
routers declare the neighbor up when the exchange is complete
after adjacency using helo packets, they synchronize their LSDB’s by echanging LSA’s and confirming the receipt of LSAs from the adjacent router

Forming OSPF Adjacencies

P2P WAN Links

- both neighbors become fully adjacent

LAN Links

- neighbors form a full adjacency with DR and BDR

- DR forwards the updates from one neighbor on the LAN to all other neighbors on that LAN

- main function of a DR is to ensure that all of the routers on the same LAN have identical database

- the DR passes its database to any new routers that come up

- routers on the LAN also maintain a partial-neighbor relationship, a two-way adjacency state, with the other routes on the LAN that are not DR or BDR called DROTHERS

LSA

- LSAs report the state of the routers and the links between routers

- Link-state information must be synchronized between routers

- LSAs are reliable, there is a method of acknowledging the delivery of LSA

- LSAs are flooded throughout the area

- LSAs have a sequence number and a set lifetime so that each router recognizes that it is the most up-to-date version of the LSA

- LSAs are periodically refreshed to confirm topology information before it ages out of the link-state-database, link-state age of 30 minutes by default

- Only by reliably flooding the link-state info can every router in the area or domain ensure that it has the latest, most accurate view of the network

When router receives LSU, it does the following:

- if LSA does not exist, the router adds the entry to its LSDB, sends a links state acknowledgement (LSAck) back, floods the info to other routers, runs SPF and updates its routing table

- if the entry already exist and the received LSA has the same sequence number, the router ignores the LSA entry

- if the entry already exists but the LSA includes newer info(has a higher sequence number), the router adds it on LSDB, sends LSAck back , floods the info, runs SPF and update its routing table

- if entry already exist but the LSA includes older info, it sends an LSU to the sender with newer information

Calculating the OSPF Metric

- uses Dijkstra (Edgar Dijkstra) algorithm to calculate the best paths

- every router in an area has the identical link-state database

- each router in the area places itself into the root of the tree that is built

- the best path is calculated with respect to the lowest total cost of links to a specific destination

- best routes are put into the forwarding database (routing database)

- by default, the interface cost is calculated based on the its configured bandwidth

- ospf cost can also be manually defined for each interface, which overrides the default cost value

II. OSPF Packet Types

5 Packet Types

Type 1: Hello

- discovers neighbors and builds adjacencies between them

Type 2: DBD Database description

- checks for database synchronization between routers

Type 3: LSR Link-State Request

- request specific link state records from router to router

Type 4: LSU Link-state Update

- sends specifically requested link-state records

Type 5: LSAck Link-state Acknowledgement

- acknowledges the other packet types

OSPF Packet Header Format

- all 5 OSPF packet types are encapsulated directly into an IP payload

- the OSPF packet does not use TCP or UDP

- it has defined its own acknowledgement routine using an acknowledgment packet (OSPF packet type 5)

- in the IP Header, a protocol identifier of 89 defines all OSPF packets

- each OSPF packet types begins with the same header format, it has these ff fields:

a. Version Number: For OSPF version 2

b. Type: Differentiates 5 OSPF Packet Types

c. Packet Length: Length of OSPF packet in bytes

d. Router ID: defines which router is the source of the packet

e. Area ID: defines the area where the packet originated

f. Checksum: used for packet header error detection to ensure all OSPF packet was not corrupted during transmission

g. Authentication Type: formats for authentication, clear-text or md5

h. Authentication: used in authentication scheme

i. Data (for hello packet) : includes list of all neighbors

j. Data (for DBD packet) : contains summary of the LSDB, which includes all known router ID and their last sequence number

k. Data (for LSR pacekt): contains the type of LSU needed and the router ID that the needed LSU

l. Data (for LSU packet): contains full link-state advertisement LSA entries

m. Data (for LSAck packet): empty

Establishing OSPF Neighbor Adjacencies

- hello protocol establishes and maintains neighbor relationship

- each interface participating in OSPF uses IP multicast address 224.0.0.5 to send hello packets periodically

Hello Packet

Router ID

- 32-bit number that uniquely identifies the router

- the highest IP address on an active interface is chosen by default, unless the loopback interface or its manually configured

- breaks ties during DR and BDR selection process

Hello and Dead Interval – must be the same on neighboring routers

- hello interval specifies the frequency at which a router sends hello packets

every 10 secs default on multiaccess/P2P/broadcast network

every 30 secs on NBMA

- dead interval is the time in secs that a router waits to hear from a neighbor before declaring the neighboring router out of service

4 times the hello by default

Neighbors

- the neighbor field lists the adjacent routers

Area ID – must be the same

- to communicate, routers interfaces must belong to the same OSPF area

Router Priority

- 8-bit number that indicates the priority of a router

- used for selecting DR and BDR

DR and BDR IP Addresses

- ip address of DR and BDR for the specific network

Authentication Password – must be the same if enabled

- if its enabled the routers must exchange the same password

Stub Area Flag

- special area, designating a stub area is a technique that reduces routing updates by replacing them with a default route

Note: After DR and BDR are selected, newly added router will establish adjacencies with the DR and BDR only.

Exchanging and Synchronizing LSDBs

- once adjacency is formed, OSPF must exchange and synchronized the LSDBs between routers

Neighbor Relationship Process

- when routers running OSPF initialize, an exchange process of hello protocol is the first procedure

- if let say RouterA is enabled on a LAN and is in DOWN STATE because it has not exchanged information from other router

- it begins sending hello packet through each of its interfaces participating OSPF, the helo packet is sent out using the multicast address 224.0.0.5

- all directly connected routers running OSPF receive the hello packet from RouterA and add RouterA to their list of neighbors. This state is the INITIAL STATE (init)

- all routers that received the hello packet send a unicast reply hello packet to RouterA with the corresponding information

- the neighbor field in the hello packet includes all neighboring routers and RouterA

- when RouterA receives these hello packets, it add all the routers that had its routerID in their hello packets to its own neighbor relationship database, this state is referred to as the TWO-WAY STATE

- at this point all routers that have each other in their lists of neighbors have established bidirectional communication

- if the link type is broadcast, generally LAN like Ethernet, then a DR and BDR must be first selected.

- the DR forms bidirectional adjacencies with all other routers on the LAN link, this process must occur before the routers can begin exchanging link-state info

- periodically every 10 secs by default on broadcast networks the routers within a network exchange hello packets to ensure that communication is still working

- the hello updates include the DR, BDR and the list of routers whose hello packets have been received by the router

- after the DR and BDR have been selected, the routers are considered to be in the EXSTART STATE and they are ready to discover the link-state information about the internetwork and create their LSDBs

- the process used to discover the network routes is the exchange protocol and get the routers to FULL STATE of communication

- the first step in this process(FULL STATE) is for the DR and BDR to establish adjacencies with each of the other routers

- when adjacent routers are in a FULL STATE, they do not repeat the exchange protocol unless the FULL STATE changes

Exchange Protocol

- in the EXSTART STATE, the DR and BDR establish adjacencies with each router in the network

- during these process master-slave relationship is created between each router and its adjacent DR and BDR

- the router with the higher router ID acts as the master during the exchange process

- the master and slave routers exchange one or more DBD packets, the routers are in the EXCHANGE STATE

- a DBD includes information about the LSA entry header that appears in the LSDB of the router

- each LSA entry header includes info about the link-state type, address of the advertising router, cost of the link and the sequence number

- the router uses these sequence number to determine the “newness” of the received link-state information

- when the router receives the DBD, it acknowledges the receipt of the DBD using LSAck packet

- it compares the info it receives with the info it has

- if the DBD has a more up-to-date link-state entry, then the router sends an LSR to the other router

- the process of sending LSRs is called the LOADING STATE

- the router responds with the complete information about the requested entry in an LSU packet

- when the router receives an LSU, it sends an LSAck

- the router adds the new link-state entries to its LSDB

- when 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 router traffic

- at this point all the routers in the area should have identical LSDBs

Note: Only the DR exchanges and synchronizes link-state information with the routers to which it has established adjacencies

Maintaining Network Routes

- in a link-state environment it is very important for the LSDBs (Topology Table) of all routers to stay synchronized

- when there is a change in a link-state, the routers use a flooding process to notify the other routers in the network of the change

- LSUs provide the mechanism for flooding LSAs

Flooding Process Steps

- a router notices a change in a link state and multicast an LSU packet to all OSPF DRs and BDRs at 224.0.0.6. an LSU packet may contain several distinct LSAs

- the DR acknowledges the receipt of the change and floods the LSU to other on the network using the OSPF multicast address 224.0.0.5

- after receiving the LSU, each router responds to the the DR with an LSAck, each LSA must be acknowledge separately

- if a router is connected to other networks, it floods the LSU to those other networks by forwarding the LSU to the DR of the multicaccess network

- the DR in turn multicast the LSU to the other routers on the network

- the router updates its LSDB using the LSU that includes the changed LSA

- it the recomputes the SPF algorithm against the updated database after a short delay and updates the routing table as necessary

- summaries of individual link-state entries, not the complete link-state entries are sent every 30 minutes to ensure LSDB synchronization

- each link-state entry has a timer to determine when the LSA refresh update must be sent

- each link-state entry also has a max age of 60 minutes

- if a link-state entry has not been refreshed within 60 minutes, it is removed from the LSDB

III. Configuring OSPF Routing

Enables OSPF routing process

Router(config)# router ospf

Defines the interfaces that OSPF will runon

Router(config-router)# network area

Optional method to enable OSPF explicitly on an interface

Router(config-if)# ip ospf area

Parameters

process-id :

- number to identify the OSPF routing process

- does not need to match process IDs on other routers

wildcard-mask:

- specifying the interface address use 0.0.0.0

- matches all interface address use a combination of 0.0.0.0 255.255.255.255

area-id:

- specifiy ospf area to be associated with the address

- can be a decimal number or dotted decimal notaion similar to an IP address

Single Area

Multiple Area

Configuring a Router ID

- for an ospf routing process to start successfully, it must able to determine an OSPF router ID

- the router is known OSPF by the OSPF router ID number

- LSDBs use the OSPF router ID to differentiate one router from the nex

- by default, the router ID si the highest IP address on an active interface at the moment of OSPF process startup
- the interface does not have to be part of the OSPF process, but it has to be up

- a loopback interface can override the OSPF router ID

- if a loopback interface exist, the router ID is the highest IP address on any active loopback interface

- the OSPF router-id command can be used to override the OSPF router ID

- once the OSPF router ID is set, it does not change even if the interface that the router is using for the router ID goes down

- the router ID changes only if the router reloads of if the OSPF routing process restarts

Configuring the Loopback interface

Configuring the router-id

Verifying ospf router id

Verifying OSPF Operation

show ip protocols

- displays ip routing protocol parameters about timers, filters, metrics, networks

show ip route ospf

- displays OSPF routes known to the router

0 – code represents OSPF routes

IA – interarea

[110/782] – AD and total cost of the route to subnet 10.2.1.0/24

show ip ospf interface

- displays OSPF process ID, routerID, OSPF network type, DR/BDR timers, neighbor adjacency

- verifies that the interface is configured in the intended areas

show ip ospf neighbor

- displays all list of neighbors, OSPF router ID, OSPF priority and their neighbor adjacency state and the dead timer

IV. OSPF NETWORK TYPES

3 Types of defined by OSPF

1. Point-to-point

- a network that joins a single pair of routers

2. Broadcast

- muliaccess broadcast network, such as Ethernet

3. Nonbroadcast multiaccess (NBMA)

- a network that interconnects more than two routes but that has no broadcast capability, examples are FR, ATM and X.25

Adjacency on Point-to-Point Link

- usually a serial interface running either PPP or HDLC

- may also be a point-to-point subinterface running Frame Relay or ATM

- No DR or BDR election required

- OSPF autodetects this interface type

- OSPF packets are sent using multicast 224.0.0.5

- default OSPF hello and dead intervals on point-to-point links are 10 secs and 40 secs repectively

Adjacency for Broadcast Network Link

- generally these are LAN technologies like Ethernet and Token Ring

- DR and BDR selection are required

- all neighbor routers form full adjacencies with the DR and BDR only

- packets to the DR and BDR use 224.0.0.6

- packets from DR to all other routers use 224.0.0.5

Selecting DR and BDR

- the router with the highest priority value is the DR

- the router with the second-highest priority value is the BDR

- the default for the interface OSPF priority is 1, in case of a tie, the router ID is used

- the router with the highest router ID becomes the DR

- the router with the second-highest router ID becomes the DR

- a router with a priority set to 0 (zero) cannot become a DR or BDR

- a router that is not the DR or BDR is called a DROTHER

- if a router with the higher priority value gets added to then network, it does not preempt(substitute) the DR and BDR, the only time that a DR or BDR changes is when one of them is out of service

- if the DR is out of service, the BDR becomes the DR and a new BDR is selected, if the BDR is out of service, a new BDR is elected

- to determine whether the DR is out of service, the BDR uses the wait timer

- the BDR does not confirm that the DR is forwarding LSAs before the timer expires, then the BDR assumes that the DR is out of service

Note:

The highest IP address on an active interface is normally used as the router ID. However, you can override this selection by configuring an IP address on a loopback interface or using the router-id configuration command.

In a multicaccess environment, each network segment has its own DR and BDR. A router also can be a DR on one segment and a regular router on another segment.

Setting priority for DR election

Adjacency for an NBMA network

- to implement broadcasting or multicasting on an NBMA network, the router replicates the packets to be broadcast or multicast and sends them individually on each permanent virtual circuit (PVC), this process is CPU and bandwidth intensive

- default hello and dead intervals on NBMA interface are 30 secs and 120 secs

- DR and BDR need to have fully meshed connectivity with all other routers

- OSPF neighbors are not automatically discovered by the router

OSPF over Frame Relay Configuration

Types of FR topologies

1. Star Topology

- the most common FR network topology

- the central router provides multiple connection because it typically uses a single interface to interconnect multiple PVCs

2. Full-mesh Topology

- most costly

- all routers have virtual circuits to all other destinations, n(n-1)2 where n is the number of nodes

3. Partial-mesh Topology

- not all sites have direct access to a central site

RFC 2328 OSPF over NBMA Topology Modes of Operation

a. NBMA

- neighbors must be manually configured, DR and BDR election is required

b. Point-to-multipoint

- this mode treats the nonbroadcast network as a collection of point-to-point links

- routers automatically identify their neighboring routers but do not elect a DR and BDR

Note: The main advantage of point-to-multipoint mode is that it requires less manual configuration and the main advantage of the nonbroadcast mode is that there is less overhead traffic.

Cisco has identified the ff additional modes:

- Point-to-multipoint nonbroadcsat

- Broadcast

- Point-to-point

Selecting the OSPF Network Type for NBMA Netoworks

OSPF over Frame Relay NBMA Configuration

- emulates operation over a broadcast network

- DR and BDR are elected on the NBMA network

- Routers are usually fully meshed, if not fully meshed DR and BDR are selected

- Neighboring routers are statically defined to start the DR and BDR election

- For flooding, the LSU packet must be replicated for each PVC

- Frame Relay, ATM and X.25 networks default to OSPF nonbroadcast mode

- priority set to 0 for routersB and C because its not a full-meshed topology

- this ensures RouterA to become the DR, no BDR

OSPF over Frame Relay Point-to-Multipoint Configuration

- are designed to work with partial-mesh or star topologies

- in RFC 2328, OSPF treats all router-to-router connection as if they were point-to-point

- DR are not used and type2 LSA is not flooded

Point-to-Multipoint mode has the ff properties

- does not require a fully meshed network

- does not require a static neighbor configuration: because point-to-multipoint mode treats the network as a collection of point-to-point links

- uses one IP subnet: all routers are on one IP subnet

- duplicates LSA packets: when flooding the router must replicate the LSU

- 30 sec hello is used

- This mode is RFC 2328 compliant

Point-to-Multipoint Nonbroadcast

- Cisco extension to RFC-compliant point-to-multipoint mode

- must statically define neighbors, like nonbroadcast

- like point-to-multipoint mode, DR and BDR are not elected

Using Subinterface in OSPF over Frame Relay

- is used when only two nodes exist

V. Link-State Advertisement

4 OSPF Router Types

1. Internal routers

- routers that have all their interface in the same area and have identical LSDBs

2. Backbone router

- routers that sit in the perimeter of the backbone area and have at least one interface connected to area0

3. ABR

- routers that have interface attached to multiple areas

- maintain separate LSDBs for each area to which they connect

- exit points for the area

4. ASBR

- routers that have at least one interface attached to an external internetwork(another AS) such as non-OSPF network

Note:

A router can exist as more than one router type.

A router has a separate LSDB for each area to which it connects.

Two routers belonging to the same area maintain identical LSDBs for that area.

OSPF Virtual Links

- a virtual link is a link that allows discontiguous area0s to be connected

- or that allows a disconnected area to be connected to area 0

- area not connected to area 0

- recommended for backup or temporary connection

Configuring Virutal Links

OSPF LSA Types

1. Type 1 : Router LSA (same area)

- most common type, includes list of directly attached links

- flooding on one/same area only, does not cross ABR

- the link-state ID is the originating router ID

- describes a router if it an ABR or ASBR

LSA Type 1 Link Types

Link Type 1:

- p2p to another rouer, lsa-id is neighboring router ID

Link Type 2:

- connection to transit network, lsa-id is ip add of the DR

Link Type 3:

- connection to a stub network, lsa-id is ip network number

Link Type 4:

- virtual link, lsa-id neighboring router ID

2. Type 2 : Network LSA (same area only for DR)

- DR generate network lsa for every transit broadcast or NBMA network

- flooded in its area only, does not cross ABR

- the link-state ID is the IP interface address of the DR

- contains list of attached routers that make up the transit network, DR itself and the subnet mask used on the link

3. Type 3 : Summary LSA (different area)

- ABR generate summary lsa

- advertising any networks owned by an area to the rest of the areas in the OSPF AS

- link-stated id is set to the network number, mask is also advertised

- summary lsa are flooded throughout a singe area only but are generated by ABRs to flood into other areas

- summary LSA do not by default contained summary routes

4. Type 4 : Summary LSA (IP Address ASBR)

- generated by an ABR only when an ASBR exists within an area

- link-state id is set to the ASBR router ID

5. Type 5 : External LSA (summary from ASBR)

- describes routes to networks outside the OSPF AS

- generated by ASBR and are flooded to the entire AS

- link-stated ID is the external network number

6. Type 6 :

- used in multicast OSPF applications

7. Type 7:

- used in NSSAs

8. Type 8:

- used in internetworking OSPF and BGP

9. Types 9, 10, and 11

- for future upgrades to OPSF, like MPLS

Interpreting the Routing Table: Types of Routes

Calculating the COSTS for E1 and E2 Routes

E1: Calculates cost by adding the external cost to the internal cost of each link that the packet crosses. Used if multiple ASBRs are advertising

E2: Default. Externcal cost only . Used if only one ASBR advertising

Configuring OSPF LSDB Overload Protection

- protects against misconfigured routers that can cause excessive LSAs and drain local CPU and memory resources

Changing the Cost Metric

VI. OSPF Route Summarization

- OSPF has the ability to summarize routes at area and AS boundaries

- summarization is impt because it reduces OSPF LSA flooding, LSDB, routing table size

- with route summarization only summarize routes are advertised to the backbone area(area 0)

2 Types of Summarization

1. Interarea route summarization

- occurs on ABRs and applies to routes from within each area

- it does not apply to external routes injected into OSPF via redistribution

2. Exrenal route summarization

- is specific to external routes that are injected into OSPF via redistribution

- only ASBRs summarize external routes

Configuring OSPF Route Summarization

Note: IOS software creates a summary route to interface null 0 when manual summarization is configured.

Default Route on OSPF

- a default route is injected into OSPF as an external LSA type 5

- default route distribution is not ON by default

VII. Configuring OSPF Special Area Types

- the purpose behind stub areas is to inject default routes into an area so that external and summary link-state advertisement

- stub areas are designed to reduce the amount of flooding, LSDB size and routing table size within the area

OSPF Area Types

1. Standard area:

- this default area accepts link updates, route summaries and external routes

2. Backbone area (transit area)

- labeled as area 0, all other area connect to this and exchange route info

- this include all the properties of a standard OSPF area

3. Stub area

- this area does not accept information about routes external to the AS

- if routers need to route outside the AS, they use default route, noted as 0.0.0.0

- stub areas cannot contain ASBR

4. Totally stubby area

- this area does not accept external AS routes or summary routes from other areas internal to the AS

- if routers need to send packet to an external network it uses a default route

- cannot contain ASBRs

5. NSSA

- OSPF RFC

- this area defines a special LSA type 7

- it allows ASBRs and benefits similar to stub and totally stubby area

Note: Stub and Totally Stubby areas do not carry any external routes,LSA type 5.

Qualification for Stub and Totally Stubby

- there is a single exit point from that area

- if there are multiple exits, one or more ABR inject a default into the stub area

- all OSPF routers including the ABR and internal routers must be configured as stub routers before they can become neighbors and exchange infor

- there is no ABR inside the stub area

- the area is not the backbone area, area0

- the area is not needed as a transit area for virtual links

- area 2 is defined as stub area, no routes from the external AS are forwarded into the stub area

- R3 automatically advertises 0.0.0.0 (default route) with a default metric cost of 1 to the stub area

- each router in the stub area must be configured with the area stub command

Routes that appear in the routing table of R4 are as follows

- intra-area routes, designated with an O in the routing table

- default-route and interarea routes, designated with an IA in the routing table

- default route is denoted with an asterisk (O *IA)

Configuring Totally Stubby Areas

- Cisco proprietary, it blocks type 5 LSA as well as summary type 3 and type 4 LSA from entering the area

- Totally stubby area recognizes only intra-area routes and default route 0.0.0.0

- ABRs inject the default summary link 0.0.0.0 into the totally stubby area

- all routes advertised into area 1 (from area0 and the external AS) default to 0.0.0.0

- default route cost is set to 5 on router2 and to 10 on rotuer4

- both default routes are advertised into area 1

- only ABR use the no-summary command to keep summary LSAs from being propagated into another area

Configuring NSSA

- described by RFC 3101, non proprietary extension of the existing stub area feature that allows the injection of external routes in a limited fashion into the stub area

- creates a Type 7 LSA, generated by ASBR and NSSA ABR translates it into a type 5 LSA

- ABR sends a default route into the NSSA instead of external routes from other ASBRs

- Type 7 LSA is described in the routing table as an O N2 or O N1 (N means NSSA)

- N1 means that the metric is calculated like external type 1

- N2 means that the metric is calculated like external type 2, default is O N2

- R2 is the NSSA ABR, this router converts LSA type 7 into type 5 for advertisement into the backbone area0

- To cause R2 to generate an O *N2 default route (O *N2 0.0.0.0/0) into the NSSA, use the default-information originate option

- R2 the NSSA ABR automatically generates the O *N2 default route into the NSSA area with the no-summary option, so the default-information-originate option is not required

- All other routers in the NSSA area require area 1 nssa command only

Verifying All Stub Area Types