How OSPF for IPv6 Works
OSPF is a routing protocol for IP. It is a link-state protocol, as opposed to a distance-vector protocol. Think of a link as being an interface on a networking device. A link-state protocol makes its routing decisions based on the states of the links that connect source and destination machines. The state of a link is a description of that interface and its relationship to its neighboring networking devices. The interface information includes the IPv6 prefix of the interface, the network mask, the type of network it is connected to, the routers connected to that network, and so on. This information is propagated in various type of link-state advertisements (LSAs).
A router's collection of LSA data is stored in a link-state database. The contents of the database, when subjected to the Dijkstra algorithm, result in the creation of the OSPF routing table. The difference between the database and the routing table is that the database contains a complete collection of raw data; the routing table contains a list of shortest paths to known destinations via specific router interface ports.
OSPF version 3, which is described in RFC 2740, supports IPv6.
Comparison of OSPF for IPv6 and OSPF Version 2
Much of the OSPF for IPv6 feature is the same as in OSPF version 2. OSPF version 3 for IPv6, which is described in RFC 2740, expands on OSPF version 2 to provide support for IPv6 routing prefixes and the larger size of IPv6 addresses.
In OSPF for IPv6, a routing process does not need to be explicitly created. Enabling OSPF for IPv6 on an interface will cause a routing process, and its associated configuration, to be created.
In OSPF for IPv6, each interface must be enabled using commands in interface configuration mode. This feature is different from OSPF version 2, in which interfaces are indirectly enabled using the router configuration mode.
When using an nonbroadcast multiaccess (NBMA) interface in OSPF for IPv6, users must manually configure the router with the list of neighbors. Neighboring routers are identified by their router ID.
In IPv6, users can configure many address prefixes on an interface. In OSPF for IPv6, all address prefixes on an interface are included by default. Users cannot select some address prefixes to be imported into OSPF for IPv6; either all address prefixes on an interface are imported, or no address prefixes on an interface are imported.
Unlike OSPF version 2, multiple instances of OSPF for IPv6 can be run on a link.
LSA Types for IPv6
The following list describes LSA types, each of which has a different purpose:
- Router LSAs (Type 1)—Describes the link state and costs of a router's links to the area. These LSAs are flooded within an area only. The LSA indicates if the router is an Area Border Router (ABR) or Autonomous System Boundary Router (ASBR), and if it is one end of a virtual link. Type 1 LSAs are also used to advertise stub networks. In OSPF for IPv6, these LSAs have no address information and are network-protocol-independent. In OSPF for IPv6, router interface information may be spread across multiple router LSAs. Receivers must concatenate all router LSAs originated by a given router when running the SPF calculation.
- Network LSAs (Type 2)—Describes the link-state and cost information for all routers attached to the network. This LSA is an aggregation of all the link-state and cost information in the network. Only a designated router tracks this information and can generate a network LSA. In OSPF for IPv6, network LSAs have no address information and are network-protocol-independent.
- Interarea-prefix LSAs for ABRs (Type 3)—Advertises internal networks to routers in other areas (interarea routes). Type 3 LSAs may represent a single network or a set of networks summarized into one advertisement. Only ABRs generate summary LSAs. In OSPF for IPv6, addresses for these LSAs are expressed as prefix, prefix length instead of address, mask. The default route is expressed as a prefix with length 0.
- Interarea-router LSAs for ASBRs (Type 4)—Advertise the location of an ASBR. Routers that are trying to reach an external network use these advertisements to determine the best path to the next hop. ASBRs generate Type 4 LSAs.
- Autonomous system external LSAs (Type 5)—Redistributes routes from another AS, usually from a different routing protocol into OSPF. In OSPF for IPv6, addresses for these LSAs are expressed as prefix, prefix length instead of address, mask. The default route is expressed as a prefix with length 0.
- Link LSAs (Type 8)—Have local-link flooding scope and are never flooded beyond the link with which they are associated. Link LSAs provide the link-local address of the router to all other routers attached to the link, inform other routers attached to the link of a list of IPv6 prefixes to associate with the link, and allow the router to assert a collection of Options bits to associate with the network LSA that will be originated for the link.
- Intra-Area-Prefix LSAs (Type 9)—A router can originate multiple intra-area-prefix LSAs for each router or transit network, each with a unique link-state ID. The link-state ID for each intra-area-prefix LSA describes its association to either the router LSA or the network LSA and contains prefixes for stub and transit networks.
An address prefix occurs in almost all newly defined LSAs. The prefix is represented by three fields: PrefixLength, PrefixOptions, and Address Prefix. In OSPF for IPv6, addresses for these LSAs are expressed as prefix, prefix length instead of address, mask. The default route is expressed as a prefix with length 0. Type 3 and Type 9 LSAs carry all IPv6 prefix information that, in IPv4, is included in router LSAs and network LSAs. The Options field in certain LSAs (router LSAs, network LSAs, interarea-router LSAs, and link LSAs) has been expanded to 24 bits to provide support for OSPF in IPv6.
In OSPF for IPv6, the sole function of link-state ID in interarea-prefix LSAs, interarea-router LSAs, and autonomous-system external LSAs is to identify individual pieces of the link-state database. All addresses or router IDs that are expressed by the link-state ID in OSPF version 2 are carried in the body of the LSA in OSPF for IPv6.
The link-state ID in network LSAs and link LSAs is always the interface ID of the originating router on the link being described. For this reason, network LSAs and link LSAs are now the only LSAs whose size cannot be limited. A network LSA must list all routers connected to the link, and a link LSA must list all of the address prefixes of a router on the link.
NBMA in OSPF for IPv6
On NBMA networks, the designated router (DR) or backup DR (BDR) performs the LSA flooding. On point-to-point networks, flooding simply goes out an interface directly to a neighbor.
Routers that share a common segment (Layer 2 link between two interfaces) become neighbors on that segment. OSPF uses the Hello protocol, periodically sending hello packets out each interface. Routers become neighbors when they see themselves listed in the neighbor's hello packet. After two routers become neighbors, they may proceed to exchange and synchronize their databases, which creates an adjacency. Not all neighboring routers have an adjacency.
On point-to-point and point-to-multipoint networks, the software floods routing updates to immediate neighbors. There is no DR or BDR; all routing information is flooded to each networking device.
On broadcast or NBMA segments only, OSPF minimizes the amount of information being exchanged on a segment by choosing one router to be a DR and one router to be a BDR. Thus, the routers on the segment have a central point of contact for information exchange. Instead of each router exchanging routing updates with every other router on the segment, each router exchanges information with the DR and BDR. The DR and BDR relay the information to the other routers.
The software looks at the priority of the routers on the segment to determine which routers will be the DR and BDR. The router with the highest priority is elected the DR. If there is a tie, then the router with the higher router ID takes precedence. After the DR is elected, the BDR is elected the same way. A router with a router priority set to zero is ineligible to become the DR or BDR.
When using NBMA in OSPF for IPv6, you cannot automatically detect neighbors. On an NBMA interface, you must configure your neighbors manually using interface configuration mode.
Force SPF in OSPF for IPv6
When the process keyword is used with the clear ipv6 ospf command, the OSPF database is cleared and repopulated, and then the shortest path first (SPF) algorithm is performed. When the force-spf keyword is used with the clear ipv6 ospf command, the OSPF database is not cleared before the SPF algorithm is performed.
Load Balancing in OSPF for IPv6
When a router learns multiple routes to a specific network via multiple routing processes (or routing protocols), it installs the route with the lowest administrative distance in the routing table. Sometimes the router must select a route from among many learned via the same routing process with the same administrative distance. In this case, the router chooses the path with the lowest cost (or metric) to the destination. Each routing process calculates its cost differently and the costs may need to be manipulated in order to achieve load balancing.
OSPF performs load balancing automatically in the following way. If OSPF finds that it can reach a destination through more than one interface and each path has the same cost, it installs each path in the routing table. The only restriction on the number of paths to the same destination is controlled by the maximum-paths command. The default maximum paths is 16, and the range is from 1 to 64.
Importing Addresses into OSPF for IPv6
When importing the set of addresses specified on an interface on which OSPF for IPv6 is running into OSPF for IPv6, users cannot select specific addresses to be imported. Either all addresses are imported, or no addresses are imported.
OSPF for IPv6 Customization
You can customize OSPF for IPv6 for your network, but you likely will not need to do so. The defaults for OSPF in IPv6 are set to meet the requirements of most customers and features. If you must change the defaults, refer to the IPv4 configuration guide and the IPv6 command reference to find the appropriate syntax.
 | Caution Be careful when changing the defaults. Changing defaults will affect your OSPF for IPv6 network, possibly adversely. |
How to Implement OSPF for IPv6
This section contains the following procedures:
- Enabling OSPF for IPv6 on an Interface (required)
- Configuring NBMA Interfaces (optional)
- Forcing an SPF Calculation (optional)
- Verifying OSPF for IPv6 Configuration and Operation (optional)
Enabling OSPF for IPv6 on an Interface
This task explains how to enable OSPF for IPv6 routing and configure OSPF for IPv6 on each interface. By default, OSPF for IPv6 routing is disabled and OSPF for IPv6 is not configured on an interface.
SUMMARY STEPS
4. ipv6 ospf process-id area area-id [instance instance-id]
DETAILED STEPS
Configuring NBMA Interfaces
You can customize OSPF for IPv6 in your network to use NBMA interfaces. OSPF for IPv6 cannot automatically detect neighbors over NBMA interfaces. On an NBMA interface, you must configure your neighbors manually using interface configuration mode.
Prerequisites
Before you configure NBMA interfaces, you must perform the following tasks:
Restrictions
You cannot automatically detect neighbors when using NBMA interfaces. You must manually configure your router to detect neighbors when using an NBMA interface.
SUMMARY STEPS
4. frame-relay map ipv6 ipv6-address dlci [broadcast] [cisco] [ietf] [payload-compression {packet-by-packet frf9 stac [hardware-options] data-stream stac [hardware-options]}]
5. ipv6 ospf neighbor ipv6-address [priority number] [poll-interval seconds] [cost number] [database-filter all out]
DETAILED STEPS
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Forcing an SPF Calculation
This task explains how to start the SPF algorithm without first clearing the OSPF database.
SUMMARY STEPS
2. clear ipv6 ospf [process-id] {process force-spf redistribution counters [neighbor [neighbor-interface]]}
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Verifying OSPF for IPv6 Configuration and Operation
This task explains how to display information to verify the configuration and operation of OSPF for IPv6.
SUMMARY STEPS
2. show ipv6 ospf [process-id] [area-id] interface [interface-type interface-number]
3. show ipv6 ospf [process-id] [area-id]
DETAILED STEPS
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What to Do Next
For output examples of the commands used to verify OSPF for IPv6 configuration and operation, refer to the IPv6 for Cisco IOS Command Reference.
Configuration Examples for Implementing OSPF for IPv6
This section provides the following configuration examples:
Enabling OSPF for IPv6 on an Interface Configuration Example
The following example configures an OSPF routing process 109 to run on the interface and puts it in area 1:
Configuring NBMA Interfaces Configuration Example
The following example configures an OSPF neighboring router with the IPv6 address of FE80::A8BB:CCFF:FE00:C01.
Forcing SPF Configuration Example
The following example triggers SPF to redo the SPF and repopulate the routing tables:
