Cisco Systems 3750E manual Configuring Ospf for IPv6, 39-20

Chapter 39 Configuring IPv6 Unicast Routing

Configuring IPv6

Configuring OSPF for IPv6

Open Shortest Path First (OSPF) is a link-state protocol for IP, which means that routing decisions are based on the states of the links that connect the source and destination devices. The state of a link is a description of the interface and its relationship to its neighboring networking devices. Interface information, which is propagated in link-state advertisements (LSAs), 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. LSA data is stored in a link-state database, which is used to create the OSPF routing table. While the database contains a collection of raw data, the routing table contains a list of shortest paths to known destinations that use specific Layer 3 ports. OSPF Version 2 (RFC 2740) supports IPv6.

OSPF for IPv6 is largely the same as OSPF Version 2 (for IPv4), expanded to provide support for IPv6 routing prefixes and the larger size of IPv6 addresses. However, there are these differences:

•Enabling OSPF for IPv6 on an interface automatically creates a routing process and its associated configuration; you do not need to explicitly create a routing process as in IPv4.

•In OSPF for IPv6, you must enable OSPF on each interface by using commands in interface configuration mode. In OSPF Version 2, interfaces are indirectly enabled by using router configuration mode.

•In IPv6, you can configure many address prefixes on an interface. All address prefixes configured on an interface are included by default; you cannot select a subset of address prefixes to import.

•Unlike OSPF Version 2, multiple instances of IPv6 can run on a link.

•OSPF Version 2 uses the 32-bit IPv4 address configured on the interface to chose an IPv4 address to use as the router ID. When you enable OSPF for IPv6 on an interface, if an IPv4 address is configured on the interface that IP address is used for the IPv6 router ID. If no IPv4 address is configured on the interface, you must use the router-idrouter configuration command to configure a router ID before the OSFP process is started.

OSPF automatically chooses a loopback interface over other interfaces and chooses the highest IP address among all loopback interfaces. If no loopback interfaces are present, OSPF selects the highest IP address in the router. You cannot configure OSPF to use any particular interface.

You can customize OSPF for IPv6 for your network, but you will most likely not need to. The defaults for OSPF in IPv6 are set to meet the requirements of most customers and features.

Note Be careful when changing the defaults for IPv6 commands. Changing the defaults might adversely affect OSPF for the IPv6 network.

Note Before you enable IPv6 OSPF on an interface, you must enable routing by using the ip routing global configuration command, enable the forwarding of IPv6 packets by using the ipv6 unicast-routingglobal configuration command, and enable IPv6 on Layer 3 interfaces on which you are enabling IPv6 OSPF.

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
39-20	OL-9775-02