Implementing IBM System Networking 10Gb Ethernet Switches

62 Implementing IBM System Networking 10Gb Ethernet Switches

When a router receives a routing update that includes changes to an entry, it updates its

routing table to reflect the new route. The metric value for the path is increased by 1, and the

sender is indicated as the next hop. RIP routers maintain only the best route (the route with

the lowest metric value) to a destination.

RIPv1

RIP version 1 use broadcast User Datagram Protocol (UDP) data packets for the regular

routing updates. The main disadvantage is that the routing updates do not carry subnet mask

information. Hence, the router cannot determine whether the route is a subnet route or a host

route. RIPv1 is of limited usage after the introduction of RIPv2. For more information about

RIPv1 and RIPv2, see RFC 1058, found at:

http://www.ietf.org/rfc/rfc1058.txt

RIPv2

RIPv2 is the most popular and preferred configuration for most networks. RIPv2 expands the

amount of useful information carried in RIP messages and provides a measure of security.

For a detailed explanation of RIPv2, see the following RFCs:

򐂰 RFC 1723, found at:

http://www.ietf.org/rfc/rfc1723.txt

򐂰 RFC 2453, found at:

http://www.ietf.org/rfc/rfc2453.txt

RIPv2 improves efficiency by using multicast UDP (address 224.0.0.9) data packets for

regular routing updates. Subnet mask information is provided in the routing updates. A

security option is added for authenticating routing updates, by using a shared password.

2.3.4 Open Shortest Path First

Open Shortest Path First (OSPF) is designed for routing traffic within a single IP domain

called an Autonomous System (AS). The AS can be divided into smaller logical units known

as

areas.

All routing devices maintain link information in their own Link State Database (LSDB). The

LSDB for all routing devices within an area is identical, but is not exchanged between

different areas. Only routing updates are exchanged between areas, reducing the impact for

maintaining routing information on a large, dynamic network.

OSPF area types

An AS can be broken into logical units known as areas. In any AS with multiple areas, one

area must be designated as area 0, known as the

backbone. The backbone acts as the

central OSPF area.

All other areas in the AS must be connected to the backbone. Areas inject summary routing

information into the backbone, which then distributes it to other areas as needed.

Chapter 2. IBM System Networking Switch 10Gb Ethernet switch features 63

OSPF defines the following types of areas (shown in Figure 2-8):

򐂰 Stub area: An area that is connected to only one other area. External route information is

not distributed into stub areas.

򐂰 Not-So-Stubby-Area (NSSA): Similar to a stub area with additional capabilities. Routes

originating from within the NSSA can be propagated to adjacent transit and backbone

areas. External routes from outside the AS can be advertised within the NSSA but are not

distributed into other areas.

򐂰 Transit Area: An area that allows area summary information to be exchanged between

routing devices. The backbone (area 0), any area that contains a virtual link to connect

two areas, and any area that is not a stub area or an NSSA are considered transit areas.

Figure 2-8 OSPF area types