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    11Sep, 2024
    IS-IS Operation and Design – Routing Protocol Characteristics, EIGRP, and IS-IS

    This section discusses IS-IS areas, designated routers, authentication, and the Network Entity Title (NET). IS-IS defines areas differently from OSPF; area boundaries are links and not routers. IS-IS has no BDRs. Because IS-IS is an OSI protocol, it uses a NET to identify each router.

    IS-IS NET Addressing

    Although you can configure IS-IS to route IP, the communication between routers uses OSI PDUs. The NET is the OSI address used for each router to communicate with OSI PDUs. A NET address ranges from 8 to 20 bytes in length and is hexadecimal. It consists of an authority and format identifier (AFI), area ID, system ID, and selector (SEL), as shown in Figure 3-10. The system ID must be unique within the network. An example of an IS-IS NET is 49.0001.1290.6600.1001.00, which consists of the following parts:

    Figure 3-10 IS-IS NET

    • AFI: 49
    • Area ID: 0001
    • System ID: 1290.6600.1001
    • SEL: 00

    Level 2 routers use the area ID. The system ID must be the same length for all routers in an area. For Cisco routers, it must be 6 bytes in length. Usually, a router MAC address identifies each unique router. The SEL is configured as 00. You configure the NET with the router isis command. In this example, the domain authority and format identifier (AFI) is 49, the area is 0001, the system ID is 00aa.0101.0001, and the SEL is 00:

    router isis
    net 49.0001.00aa.0101.0001.00

    IS-IS DRs

    As with OSPF, IS-IS selects DRs on multiaccess networks. It does not choose a backup DR, as OSPF does. By default, the priority value is 64. You can change the priority value to a value from 0 to 127. If you set the priority to 0, the router is not eligible to become a DR for that network. IS-IS uses the highest system ID to select the DR if there is a tie with the priorities. On point-to-point networks, the priority is 0 because no DR is elected. In IS-IS, all routers in a multiaccess network establish adjacencies with all others in the subnetwork, and IS-IS neighbors become adjacent upon the discovery of one another. Both of these characteristics are different from OSPF behavior.

    IS-IS Interface Types

    IS-IS has only two network types (or interface types): point-to-point and broadcast. Unlike OSPF, IS-IS does not have nonbroadcast and point-to-multipoint interface types.

    IS-IS Area Design

    IS-IS uses a two-level hierarchy. Routers are configured to route Level 1 (L1), Level 2 (L2), or both Level 1 and Level 2 (L1/L2). Level 1 routers are like OSPF internal routers in a Cisco totally stubby area. (OSPF is covered in Chapter 4.) An L2 router is similar to an OSPF backbone router. A router that has both Level 1 and 2 routes is similar to an OSPF area border router (ABR). IS-IS does not define a backbone area like OSPF’s area 0, but you can consider the IS-IS backbone a continuous path of adjacencies among Level 2 ISs. When you are creating a backbone, there should never be Level 1 routers between L2-only or L1/L2 routers.

    L1/L2 routers maintain separate link-state databases for the L1 routes and for the L2 routes. Also, the L1/L2 routers do not advertise L2 routes to the L1 area. L1 routers do not have information about destinations outside the area and use L1 routes to the L1/L2 routers to reach outside destinations.

    As shown in Figure 3-11, IS-IS areas are not bounded by the L1/L2 routers but by the links between L1/L2 routers and L2 backbone routers.

    Figure 3-11 IS-IS Areas and Router Types

    With IS-IS, you can create three types of topologies:

    • Flat
    • Hierarchical
    • Hybrid

    The IS-IS flat design consists of a single area. As shown in Figure 3-12, all routers are on the same level. There is no summarization with this design. This design is recommended for a small network.

    Figure 3-12 IS-IS Flat Topology Design

    An IS-IS hierarchical topology is recommended for large networks. A backbone area can be created, much as with OSPF, and can have other areas connected via L1/L2 routers, as shown in Figure 3-13. This design allows for network summarization.

      

    Figure 3-13 IS-IS Hierarchical Topology Design

    A hybrid IS-IS design is recommended for networks that might not be large enough for a backbone area. As shown in Figure 3-14, the IS-IS areas are connected using L1/L2 routers. This design allows for summarization between areas.

    Figure 3-14 IS-IS Hybrid Topology Design

    8Aug, 2024
    IS-IS Authentication – Routing Protocol Characteristics, EIGRP, and IS-IS

    IS-IS supports three types of plaintext authentication: link authentication, area authentication, and domain authentication. All these types support only plaintext password authentication. An RFC draft has added support for IS-IS MD5. The design recommendation is not to use any plaintext authentication and to use MD5 hashing for authentication. With MD5, a cryptographic hash is used instead of plaintext, and the password is never included in the PDU, thus making it more secure.

    Routers in a common subnetwork (such as Ethernet or a private line) use link authentication. The plaintext password must be common only between the routers in the link. Level 1 and Level 2 routers use separate passwords. With area authentication, all routers in the area must use authentication and must have the same password.

    Only L2 and L1/L2 routers use domain authentication. All L2 and L1/L2 routers must be configured for authentication and must use the same password.

    IS-IS for IPv6

    IS-IS in IPv6 functions the same as IS-IS in IPv4 and offers many of the same benefits. IPv6 enhancements to IS-IS allow IS-IS to advertise IPv6 prefixes in addition to IPv4 and OSI routes. Extensions to the IS-IS command-line interface (CLI) allow configuration of IPv6-specific parameters. IPv6 IS-IS extends the address families supported by IS-IS to include IPv6, in addition to OSI and IPv4.

    IS-IS supports IPv6 as a separate protocol, as described in RFC 5308. The network layer protocol ID (NLPID) 142 (0x8E) identifies it as IPv6. Its operation and design retain the same characteristics as with IS-IS support for IPv4.

    Multi-topologies for IS-IS are described in RFC 5120: M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs). M-ISIS allows for creating independent IP topologies within a single IS-IS domain. IPv4, IPv6, or IPv4/IPv6 may be configured on the interface for either Level 1, Level 2, or Level 1/2 routers. But if IPv4 and IPv6 are configured on the same interface, they must be running the same IS-IS level. It is important to know that if IPv4 and IPv6 are sharing the same topologies with the same router levels, there is no need for multi-topology or transition features.

    IS-IS Summary

    The characteristics of IS-IS are as follows:

    • It is a link-state protocol.
    • It uses OSI CNLP to communicate with routers.
    • It is a classless protocol (and supports VLSM and CIDR).
    • The default metric is set to 10 for all active interfaces.
    • IS-IS has two interface types: point-to-point and broadcast.
    • It uses a single path metric, with a single link maximum of 64 and a path maximum of 1024.
    • It sends partial route updates only when there are changes.
    • IS-IS authentication uses plaintext passwords.
    • The administrative distance is 115.
    • It is used in large networks and is sometimes attractive as compared to OSPF and EIGRP.
    • It is described in ISO/IEC 10589, reprinted by the IETF as RFC 1142.
    • IS-IS provides support for IPv4 and IPv6 as separate topologies.
    11May, 2024
    OSPFv2 – OSPF, BGP, and Route Manipulation

    OSPFv2

    RFC 2328 defines Open Shortest Path First version 2 (OSPFv2), a link-state routing protocol that uses Dijkstra’s shortest-path first (SPF) algorithm to calculate paths to destinations. OSPFv2 is used in IPv4 networks. OSPF was created for use in large networks where RIP failed. OSPF improved the speed of convergence, provided for the use of variable-length subnet masking (VLSM), and improved the path calculation.

    In OSPF, each router sends link-state advertisements (LSAs) about itself and its links to all other routers in the area. Note that it does not send routing tables but rather sends link-state information about its interfaces. Then, each router individually calculates the best routes to the destination by running the SPF algorithm. Each OSPF router in an area maintains an identical database describing the area’s topology. The routing table at each router is individually constructed using the local copy of this database to construct a shortest-path tree.

    OSPFv2 is a classless routing protocol that permits the use of VLSM. With Cisco routers, OSPF also supports equal-cost multipath load balancing and neighbor authentication. OSPF uses multicast addresses to communicate between routers. OSPF uses IP protocol 89.

    This section covers OSPF theory and design concepts. It discusses OSPF LSAs, area types, and router types. OSPF uses a two-layer hierarchy with a backbone area at the top and all other areas below. Routers send LSAs informing other routers of the status of their interfaces. The use of LSAs and the characteristics of OSPF areas are important concepts to understand for the ENSLD 300-420 exam.

    OSPFv2 Metric

    The metric that OSPFv2 uses is cost. It is an unsigned 16-bit integer in the range 1 to 65,535. The default cost for interfaces is calculated based on the bandwidth, using the formula 108 / BW, where BW is the bandwidth of the interface expressed as a full integer of bits per second (bps). If the result is smaller than 1, the cost is set to 1. A 10BASE-T (10 Mbps = 107 bps) interface has a cost of 108 / 107 = 10. OSPF performs a summation of the costs to reach a destination; the lowest cost is the preferred path. Table 4-2 shows some sample interface metrics.

    Table 4-2 OSPF Default Interface Costs

    Interface TypeOSPF Cost
    10 Gigabit Ethernet108/1010 = .01 → 1
    Gigabit Ethernet108/109 = .1 → 1
    OC-3 (155 Mbps).64516 → 1
    Fast Ethernet108/108 = 1
    DS-3 (45 Mbps)2
    Ethernet108/107 = 10
    T164
    512 kbps195
    256 kbps390

    The default reference bandwidth used to calculate OSPF costs is 108 (cost = 108 / BW). Notice that for technologies that support speeds greater than 100 Mbps, the default metric gets set to 1 without regard for the network’s different speeds.

    Because OSPF was developed prior to high-speed WAN and LAN technologies, the default metric for 100 Mbps was 1. Cisco provides a method to modify the default reference bandwidth. The cost metric can be modified on every interface. It is highly recommended that you change the default reference bandwidth to a higher number on all routers in the OSPF network if OSPF links have a speed higher than 100 Mbps.

    10Apr, 2024
    OSPFv2 Adjacencies and Hello Timers – OSPF, BGP, and Route Manipulation

    OSPF uses Hello packets for neighbor discovery. The default Hello interval is 10 seconds (30 seconds for nonbroadcast multiaccess [NBMA] networks). For point-to-point networks, the Hello interval is 10 seconds. Hellos are multicast to 224.0.0.5 (ALLSPFRouters). Hello packets include information such as the router ID, area ID, authentication, and router priority.

    After two routers exchange Hello packets and set two-way communication, they establish adjacencies. To become OSPF neighbors, the following values must match:

    • Area ID
    • Authentication password
    • Hello and Dead Intervals
    • Stub flag
    • MTU size
    OSPF Message Types

    OSPF uses five different message types to exchange information between routers:

    • Type 1: Hello packets: This message type is used to discover and maintain neighbor relationships.
    • Type 2: Database Description (DBD) packets: This message type is used to summarize database information and ensure that the databases are in sync.
    • Type 3: Link-State Request (LSR) packets: This message type is used for database downloads. It is sent to OSPF neighbors to request the most recent version of missing LSRs.
    • Type 4: Link-State Update (LSU) packets: This message type is used to update the database. LSU packets are used for the flooding of LSAs and for sending LSA responses to LSR packets.
    • Type 5: Link-State Acknowledgement (LSAck) packets: This message type is used to acknowledge the flooding of LSAs. Multiple LSAs can be acknowledged in a single LSAck packet.

    DBD and LSR packets are used for database synchronization after adjacencies are formed between OSPF routers.

    Figure 4-1 shows a point-to-point network and an NBMA network. For point-to-point networks, valid neighbors always become adjacent and communicate using multicast address 224.0.0.5. For broadcast (Ethernet) and NBMA networks (Frame Relay), all routers become adjacent to the designated router (DR) and backup designated router (BDR) but not to each other. All routers reply to the DR and BDR using the multicast address 224.0.0.6. The section “OSPF DRs,” later in this chapter, covers the DR concept.

    Figure 4-1 OSPF Networks

    On OSPF point-to-multipoint nonbroadcast networks, it is necessary to configure the set of neighbors that are directly reachable over the point-to-multipoint network. Each neighbor is identified by its IP address on the point-to-multipoint network. Nonbroadcast point-to-multipoint networks do not elect DRs, so the DR eligibility of configured neighbors is undefined. OSPF communication in point-to-point networks uses unicast or multicast addresses for neighbor communication.

    OSPF virtual links send unicast OSPF packets. Later in this chapter, the section “Virtual Links” discusses virtual links.

    20Mar, 2024
    OSPFv2 Areas – OSPF, BGP, and Route Manipulation

    As a network grows, the initial flooding and database maintenance of LSAs can burden a router’s CPU. OSPF uses areas to reduce these effects. An area is a logical grouping of routers and links that divides the network. Routers share link-state information with only the routers in their areas. This setup reduces the size of the database and the cost of computing the SPF tree at each router.

    Using a topology with multiple areas provides the following benefits:

    • The segmentation of the network reduces the number of SFP tree calculations.
    • The segmentation of the network reduces the amount of LSA flooding.
    • Multi-area design allows for summarization at the area border routers (ABRs).
    • One OSPF area hides the topology from another area.

    Each area is assigned a 32-bit integer number. Area 0 (or 0.0.0.0) is reserved for the backbone area. Every OSPF network should have a backbone area. The backbone area must exist in any internetwork using OSPF as a routing protocol over multiple areas. As you can see in Figure 4-2, communication between Area 1 and Area 2 must flow through Area 0. This communication can be internal to a single router that has interfaces directly connected to Areas 0, 1, and 2.

    Figure 4-2 OSPF Areas

    Intra-area traffic is packets passed between routers in a single area.

    OSPF Area Design Considerations

    A CCNP enterprise designer should be aware of a few considerations in the design of OSPF areas. First, in a hub-and-spoke design, you have a remote branch keep the OSPF boundary at the hub side, as shown in Figure 4-3. This allows the branch router to calculate SPF only within its own area and limits the LSA flooding. If the OSPF Area 0 boundary were extended to the branch, the branch router would have to do OSPF calculations for Area 0 and its own area, and LSAs would flood over the WAN link.

    Figure 4-3 OSPF Area Design

    The second design consideration is to avoid grouping remote branches into a single area. Having all remote branches in the same area is not scalable. Instead, place each remote branch in its own area to limit LSA flooding and SPF recalculations.

    10Feb, 2024
    OSPF Router Types – OSPF, BGP, and Route Manipulation

    OSPF classifies participating routers based on their place and function in the area architecture. Figure 4-4 shows OSPF router types.

    Figure 4-4 OSPF Router Types

    Table 4-3 explains the router types shown in Figure 4-4.

    Table 4-3 OSPF Router Types

    TypeDescription
    Internal routerAny router whose interfaces all belong to the same OSPF area. Such a router keeps only one link-state database.
    Area border router (ABR)A router that is connected to more than one area. Such a router maintains a link-state database for each area it belongs to. These routers generate summary LSAs.
    Autonomous system boundary router (ASBR)A router that injects external LSAs into the OSPF database (redistribution). These external routes are learned either through other routing protocols or static routes.
    Backbone routerA router with at least one interface attached to Area 0.

    Tip

    An OSPF router can be an ABR, an ASBR, and a backbone router at the same time. A router is an ABR if it has an interface on Area 0 and another interface in another area. A router is a backbone router if it has one or more interfaces in Area 0. A router is an ASBR if it redistributes external routes into the OSPF network.

    OSPF DRs

    On multiaccess networks (such as Ethernet), some routers get selected as DRs. The purpose of a DR is to collect LSAs for the multiaccess network and to forward the LSAs to all non-DR routers; this arrangement reduces the amount of LSA traffic generated. A router can be the DR for one multiaccess network and not the DR in another attached multiaccess network.

    The DR also floods the network LSAs to the rest of the area. OSPF also selects a BDR; it takes over the function of the DR if the DR fails. Both the DR and the BDR become adjacent to all routers in the multiaccess network. All routers that are not a DR or a BDR are sometimes called DROTHERs. These routers are only adjacent to the DR and BDR. The DR generates a Type 2 (network) LSA, which advertises all other routers on the multiaccess segment. This allows the DROTHERs to get the Type 1 LSAs. OSPF routers multicast LSAs only to adjacent routers. DROTHERs multicast packets to the DR and BDR using the multicast address 224.0.0.6 (ALLDRouters). The DR floods updates using ALLSPFRouters (224.0.0.5).

    DR and BDR selection is based on an OSPF DR interface priority. The default value is 1, and the highest priority determines the DR. In a tie, OSPF uses the numerically highest router ID. The router ID is the IP address of the configured loopback interface. The router ID is the highest configured loopback address, or if the loopback is not configured, it’s the highest physical address. Routers with a priority of 0 are not considered for DR/BDR selection. The dotted lines in Figure 4-5 show the adjacencies in the network.

    Figure 4-5 DRs

    In Figure 4-5, Router A is configured with a priority of 10, and Router B is configured with a priority of 5. Assuming that these routers are turned on simultaneously, Router A becomes the DR for the Ethernet network. Router C has a lower priority and becomes adjacent to Router A and Router B but not to Router D. Router D has a priority of 0 and therefore is not a candidate to become a DR or BDR.

    If you introduce to a network a new router that has a higher priority than that of the current DR and BDR, it does not become the selected DR unless both the DR and the BDR fail. If the DR fails, the current BDR becomes the DR.

    1Jan, 2024
    LSA Types – OSPF, BGP, and Route Manipulation

    OSPF routers generate LSAs that are flooded within an area, between areas, and throughout the entire autonomous system. OSPF defines different LSA types for participating routers, DRs, ABRs, and ASBRs. Understanding the LSA types can help you with other OSPF concepts. Table 4-4 describes the major LSA types. For the ENSLD 300-420 exam, you need to know OSPF LSAs by type code (number) and by type name. Note that there are other LSA types that are not covered in this book.

    Type 1 and Type 2 LSAs are intra-area LSAs that have an area-flooding scope. Type 3 LSAs provide summaries of destinations outside the local area but within the OSPF domain. Information carried in OSPF type 3 LSAs includes the network address, the network mask, and a metric. Type 4 LSAs provide reachability about the ASBR. Type 3 and Type 4 LSAs are interarea LSAs that have an area-flooding scope. ABRs exchange Type 3 and Type 4 LSAs. Type 5 LSAs advertise external destinations. Type 5 LSAs have a domain-flooding scope, which means they are flooded throughout all areas. Type 7 LSAs are originated by ASBRs in a not-so-stubby area (NSSA) and are similar to the Type 5 LSA and only flooded within the NSSA.

    Table 4-4 Major LSA Types

    Type CodeTypeDescription
    1Router LSAProduced by every router. Includes all the router’s links, interfaces, link states, and costs. This LSA type is flooded within a single area and does not travel into other areas.
    2Network LSAProduced by every DR on every broadcast or NBMA network. It lists all the routers in the multiaccess network. This LSA type is contained within an area.
    3Summary LSA for ABRsProduced by ABRs. It is sent into an area to advertise destinations outside the area.
    4Summary LSA for ASBRsOriginated by ABRs. Sent into an area by the ABR to advertise the IP addresses of the ASBRs. It does not advertise networks outside the OSPF network; only the ASBR does that.
    5Autonomous system external LSAOriginated by ASBRs. Advertises destinations external to the OSPF autonomous system, flooded throughout the whole OSPF autonomous system.
    7Not-so-stubby area (NSSA) external LSAOriginated by ASBRs in an NSSA. It is not flooded throughout the OSPF autonomous system but only to the NSSA. Similar to the Type 5 LSA.
    Autonomous System External Path Types

    The two types of autonomous system external paths are Type 1 (E1) and Type 2 (E2), and they are associated with Type 5 LSAs. ASBRs advertise external destinations whose cost can be just a redistribution metric (E2) or a redistribution metric plus the costs of each segment (E1) used to reach the ASBR.

    By default, external routes are Type 2, which is the metric (cost) used in the redistribution. Type 1 external routes have a metric that is the sum of the redistribution cost plus the cost of the path to reach the ASBR.

    11Nov, 2023
    OSPF Stub Area Types – OSPF, BGP, and Route Manipulation

    OSPF provides support for stub areas. The concept is to reduce the number of interarea or external LSAs that get flooded into a stub area. RFC 2328 defines OSPF stub areas. RFC 1587 defines support for NSSAs. Cisco routers use totally stubby areas, such as Area 2, as shown in Figure 4-6.

    Figure 4-6 OSPF Stub Networks

    Stub Areas

    Consider Area 1 in Figure 4-6. Its only path to the external networks is via the ABR through Area 0. All external routes are flooded to all areas in the OSPF autonomous system. You can configure an area as a stub area to prevent OSPF external LSAs (Type 5) from being flooded into that area. A single default route is injected into the stub area instead. If multiple ABRs exist in a stub area, all of them inject the default route. Traffic originating within the stub area routes to the closest ABR.

    Note that network summary LSAs (Type 3) from other areas are still flooded into the stub area’s Area 1.

    Totally Stubby Areas

    Let’s take Area 1 in Figure 4-6 one step further. The only path to get from Area 1 to Area 0 and other areas is through the ABR. A totally stubby area does not flood network summary LSAs (Type 3). It stifles Type 4 LSAs, as well. Like regular stub areas, totally stubby areas do not flood Type 5 LSAs. A totally stubby area sends just a single LSA for the default route. If multiple ABRs exist in a totally stubby area, all ABRs inject the default route. Traffic originating within the totally stubby area routes to the closest ABR.

    NSSAs

    Notice that Area 2 in Figure 4-6 has an ASBR. If this area is configured as an NSSA, it generates the external LSAs (Type 7) into the OSPF system while retaining the characteristics of a stub area to the rest of the autonomous system. There are two options for the ABR. First, the ABR for Area 2 can translate the NSSA external LSAs (Type 7) to autonomous system external LSAs (Type 5) and flood the rest of the internetwork. Second, the ABR is not configured to convert the NSSA external LSAs to Type 5 external LSAs; therefore, the NSSA external LSAs remain within the NSSA.

    There is also an NSSA totally stub area. The difference is that the default NSSA has no default route unless the ABR is explicitly configured to advertise one. The NSSA totally stub area does receive a default route.

    20Aug, 2023
    Virtual Links – OSPF, BGP, and Route Manipulation

    OSPF requires that all areas be connected to a backbone router. Sometimes, WAN link provisioning or failures can prevent an OSPF area from being directly connected to a backbone router. You can use virtual links to temporarily connect (virtually) an area to the backbone.

    In Figure 4-7, Area 4 is not directly connected to the backbone. A virtual link is configured between Router A and Router B. The flow of the virtual link is unidirectional and must be configured in each router of the link. Area 2 becomes the transit area through which the virtual link is configured. Traffic between Areas 2 and 4 does not flow directly to Router B. Instead, the traffic must flow to Router A to reach Area 0 and then pass through the virtual link.

    Figure 4-7 OSPF Virtual Link

    OSPFv2 Router Authentication

    OSPFv2 supports the authentication of routes using 64-bit plaintext, cryptographic Message Digest 5 (MD5), and Secure Hash (SHA) Standard authentication. Authentication can be performed on a per-area or per-interface basis. Plaintext authentication passwords do not need to be the same for the routers throughout the area, but they must be the same between neighbors.

    MD5 authentication provides higher security than plaintext authentication. As with plaintext authentication, passwords do not have to be the same throughout an area, but they do need to be the same between neighbors. SHA-1 authentication is recommended because it is the most secure.

    OSPF supports the National Institute of Standards and Technology (NIST) Secure Hash Standard family of algorithms for authentication.

    OSPFv2 Summary

    OSPFv2 is used in large enterprise IPv4 networks. The network topology must be hierarchical. OSPF is used in the enterprise campus building access, distribution, and core layers. OSPF is also used in the enterprise data center, WAN/MAN, and branch offices.

    The characteristics of OSPFv2 follow:

    • Is a link-state routing protocol
    • Uses IP protocol 89
    • Is a classless protocol (supports VLSM and CIDR)
    • Uses cost as the metric (based on interface bandwidth, by default)
    • Provides fast convergence, using link-state updates and SPF calculation
    • Uses reduced bandwidth thanks to sending LSA updates only when changes occur
    • Labels routes as intra-area, interarea, external Type 1, or external Type 2
    • Supports authentication
    • Uses the Dijkstra algorithm to calculate the SPF tree
    • Has a default administrative distance of 110
    • Uses multicast address 224.0.0.5 (ALLSPFRouters) and multicast address 224.0.0.6 (ALLDRouters)
    • Provides good scalability and recommended for large networks
    11Jun, 2023
    OSPFv3 – OSPF, BGP, and Route Manipulation

    RFC 5340 describes Open Shortest Path First version 3 (OSPFv3) as a routing protocol for IPv6 networks. OSPFv3 initially supported IPv6 networks only, but it has been updated to support both IPv6 and IPv4 networks. OSPF algorithms and mechanisms, such as flooding, router types, designated router election, backbone areas, stub areas and NSSAs, and SPF calculations, remain the same. Changes are made for OSPF to support IPv6 addresses, address hierarchy, and IPv6 for transport. OSPFv3 uses multicast group FF02::5 for all OSPF routers and FF02::6 for all designated routers.

    OSPFv3 Changes from OSPFv2

    The following are the major changes from OSPFv2 to OSPFv3: 

    • IPv6 only: This newer version of OSPF runs over IPv6 only.
    • Support for IPv6 addressing: New LSAs have been created to carry IPv6 addresses and prefixes.
    • Per-link processing: OSPFv2 uses per-subnet processing. With link processing, routers in the same link can belong to multiple subnets.
    • Address semantics removed: Addresses are removed from the router and network LSAs. These LSAs now provide topology information.
    • No authentication in OSPFv3: OSPFv3 uses IPsec, the IPv6 authentication scheme.
    • New LSA: There is a new LSA for local-link flooding scope.
    • New intra-area-prefix LSA: This LSA, which carries all the IPv6 prefix information, is similar to an OSPFv2 router and network LSAs.
    • Neighbors identified by router ID: Neighbors are always identified by the router ID. This does not occur in OSPFv2 point-to-point and broadcast networks.
    • Options field changes: Two Options bits, the R bit and the V6 bit, have been added to the Options field for processing router LSAs during the SPF calculation.

    Note

    In OSPFv3, the router IDs, area IDs, and LSA link-state IDs remain 32 bits. Larger IPv6 addresses cannot be used.

    OSPFv3 Areas and Router Types

    OSPFv3 retains the same structure and concepts as OSPFv2. The area topology, interfaces, neighbors, link-state database, and routing table remain the same. RFC 2740 does not define new area types or router types.

    The OSPF areas shown in Figure 4-2 and the router types shown in Figure 4-4 remain the same. These are the router types in relationship to the OSPF areas:

    • Internal router: An internal router is any router whose interfaces all belong to the same OSPF area. Such a router keeps only one link-state database.
    • ABR: An ABR is connected to more than one area, where one area is Area 0. Such a router maintains a link-state database for each area it belongs to. ABRs generate summary LSAs.
    • ASBR: This type of router injects external LSAs into the OSPF database (redistribution). These external routes are learned via either other routing protocols or static routes.
    • Backbone router: A backbone router has at least one interface attached to Area 0.