Using the OSPFv3 Protocol in Your Network

Open Shortest Path First Version 3 (OSPFv3) is the OSPF routing protocol for IPv6. OSPFv3 is considered a TCP/IP Internet routing Interior Gateway Protocol (IGP). OSPFv3 distributes routing information between routers belonging to a single Autonomous System (AS). The OSPF protocol is based on link-state or SPF technology. The advantages associated with a link-state routing protocol are:

• Rapid convergence
• Reduced routing update traffic over traditional distance-vector protocols

This OSPFv3 implementation supports RFC 2740, OSPF for IPv6.

The OSPFv3 protocol is designed expressly for the TCP/IP Internet environment. OSPFv3 utilizes IP multicast when sending and receiving routing updates. Routing updates are optionally authenticated using IPsec for OSPFv3.

OSPFv3 routes IP packets based solely on the destination IP address found in the IP packet header. IP packets are not encapsulated in any further protocol headers as they transit the AS. OSPFv3 is a dynamic routing protocol in that it quickly detects topological changes in the AS, such as router interface failures, and calculates new loop-free routes after a period of convergence. This period of convergence is short and involves a minimum of routing traffic. In a link-state routing protocol, each router maintains a database describing the AS‘s topology. This database is referred to as the link-state database. Each participating router has an identical database. Each individual database entry is a particular router‘s local state made up of such information as the router‘s usable interfaces and reachable neighbors. The router distributes its local state throughout the AS by flooding.

Each network that has at least two attached routers has a designated router. The designated router generates an LSA for the network and has other special responsibilities in the running of the protocol, enabling a reduction in the number of adjacencies required on a network. This in turn reduces the amount of routing protocol traffic and the size of the link-state database.

All routers run the exact same algorithm, in parallel. From the link-state database, each router constructs a tree of shortest paths with itself as root. This shortest-path tree provides the route to each destination in the AS. Externally derived routing information appears on the tree as leaves. When several equal-cost routes to a destination exist, traffic is distributed equally among them. The cost of a route is described by a single dimensionless metric.

OSPF allows sets of networks to be grouped together. Such a grouping is called an area. The topology of an area is hidden from the rest of the AS. This information hiding enables a significant reduction in routing traffic. Also, routing within the area is determined only by the area‘s own topology, lending the area protection against bad routing data. An area is a generalization of an IP subnetted network. OSPF enables the flexible configuration of IP subnets. Each route distributed by OSPF has a destination. Two different subnets of the same IP network number may have different masks providing a different range of addresses for that subnet. This is commonly referred to as Variable Length Subnet Masking (VLSM). A packet is routed to the longest or most specific match. Host routes are considered to be subnets whose masks are “all ones” (0xffffffff).

If IPsec for OSPFv3 is enabled on the interface, OSPFv3 protocol exchanges are authenticated. This means that only trusted routers can participate in the AS‘s routing. The S- K- and 7100-Series platform supports IPsec for OSPFv3. See IPsec for OSPFv3 for a listing of supported authentication and encapsulation algorithms.

Route redistribution is supported for RIP, connected, and static routes. Route redistribution of BGP is supported on the S- and 7100-Series platforms.

The Bidirectional Forwarding Detection (BFD) protocol providing sub-second failure detection on OSPF forwarding interfaces is enabled by default on all OSPF interfaces (S-, K-Series).

An OSPF Customer Edge (CE) router can be configured as a peer to a Provider Edge (PE) router by enabling the PE-CE protocol on the PE-CE associated routers.

OSPFv3 is similar to OSPFv2 in its usage of the SPF algorithm, flooding, Designated Router (DR) election, timers, metrics, concept of a link-state database, intra/inter area and AS external routes and virtual-links. OSPFv3 differs with OSPFv2 in many respects, as outlined in OSPFv3 and OSPFv2 Differences.

OSPFv3 is not backward compatible with OSPFv2. If you need to route both IPv4 and IPv6 using OSPF, enable both OSPFv3 and OSPFv2 on the device.