Communications Server for z/OS V1R7 TCP/IP Implementation, Volume 1: Base Functions, Connectivity, and Routing

Chapter 5. Routing 151

into fewer supernet routes. Therefore, the number of network routes in the routing tables

becomes smaller for advertisements. Supernet routes are received and sent in RIP V2

messages.

RIP V2 packets are backward compatible with existing RIP V1 implementations. A RIP V1

system will process RIP V2 packets but without the RIP V2 extensions and broadcast them

as RIP V1 packets to other routers. Note that routing problems may occur when variable

subnet masks are used in mixed RIP V1 and RIP V2 systems. RIP V2 is based on a distance

vector algorithm, just as RIP V1 is.

5.1.4 IPv6 dynamic routing

Dynamic routing in a IPv6 network can be implemented in a z/OS Communications Server in

two different ways:

򐂰 IPv6 dynamic routing using router discovery

򐂰 IPv6 dynamic routing using OMPROUTE

IPv6 dynamic routing using router discovery

Enabling IPv6 router discovery in the z/OS Communications Server requires no additional

z/OS Communications Server configuration. All that is needed is at least one IPv6 interface

that is defined and started, and at least one adjacent router through that interface that is

configured for IPv6 router discovery. If these things exist, then the z/OS Communications

Server begins receiving router advertisements from the adjacent routers. Depending on the

configuration in the adjacent routers, the following types of routes may be learned from the

received router advertisements:

򐂰 Default route for which the originator of the router advertisement is the next hop

򐂰 Direct routes (no next hop) to prefixes that reside on the link shared by the z/OS

Communications Server and the originator of the router advertisement

IPv6 dynamic routing using OMPROUTE

For IPv6, OMPROUTE implements the IPv6 RIP protocol described in RFC 2080 (RIPng for

IPv6) and the IPv6 OSPF protocol described in RFC 2740 (OSPF for IPv6). It provides an

alternative to the static TCP/IP gateway definitions. The z/OS host running with OMPROUTE

becomes an active OSPF or RIP router in a TCP/IP network. Either or both of these routing

protocols can be used to dynamically maintain the host IPv6 routing table. For example,

OMPROUTE can detect when a route is created, is temporarily unavailable, or if a more

efficient route exists. If both IPv6 OSPF and IPv6 RIP protocols are used simultaneously,

IPv6 OSPF routes will be preferred over IPv6 RIP routes to the same destination.

RIPng or RIP next generation

RIP Next Generation (RIPng) is a distance vector routing protocol for IPv6 that is defined in

RFC 2080. RIPng for IPv6 is an adaptation of the RIP V2 protocol to advertise IPv6 network

prefixes. RIPng for IPv6 uses UDP port 521 to periodically advertise its routes, respond to

requests for routes, and advertise route changes.

RIPng for IPv6, like other distance vector protocols, has a maximum distance of 15, in which

15 is the accumulated cost (hop count). Locations that are a distance of 16 or further are

considered unreachable. RIPng for IPv6 is a simple routing protocol with a periodic

route-advertising mechanism designed for use in small to medium-sized IPv6 networks.

RIPng for IPv6 does not scale well to a large or very large IPv6 network.