Implementing IBM System Networking 10Gb Ethernet Switches

Chapter 2. IBM System Networking Switch 10Gb Ethernet switch features 61

2.3.1 Static routes

You can use static routes to forward an IP packet based on a manually configured entry. The

entry specifies a network and an IP address of a gateway, or next-hop, to that network.

2.3.2 Equal-Cost Multi-Path static routes

Equal-Cost Multi-Path (ECMP) is a forwarding mechanism that routes packets along multiple

paths of equal cost. ECMP provides equally distributed link load sharing across the paths.

The hashing algorithm used is based on the source IP address. ECMP routes allow the

switch to choose between several next hops toward a destination. The switch performs

periodic health checks (ping) on each ECMP gateway. If a gateway fails, it is removed from

the routing table, and an SNMP trap is sent.

2.3.3 Routing Information Protocol

In a routed environment, routers communicate with one another to track available routes.

Routers can learn about available routes dynamically by using the Routing Information

Protocol (RIP). IBM Networking OS supports RIP version 1 (RIPv1) and RIP version 2

(RIPv2) for exchanging TCP/IPv4 route information with other routers.

Distance vector protocol

RIP is known as a distance vector protocol. The vector is the network number and next hop,

and the distance is the metric associated with the network number. RIP identifies network

reachability based on a metric, and the metric is defined as a hop count. One hop is

considered to be the distance from one router to the next, which typically is 1. When a router

receives a routing update that contains a new or changed destination network entry, the

router adds 1 to the metric value indicated in the update and enters the network in the routing

table. The IPv4 address of the sender is used as the next hop.

Stability

RIP includes a number of other stability features that are common to many routing protocols.

For example, RIP implements the split horizon and hold-down mechanisms to prevent

incorrect routing information from being propagated.

RIP prevents routing loops from continuing indefinitely by implementing a limit on the number

of hops allowed in a path from the source to a destination. The maximum number of hops in a

path is 15. The network destination network is considered unreachable if increasing the

metric value by 1 causes the metric to be 16 (that is, infinity). This setting limits the maximum

diameter of a RIP network to less than 16 hops.

RIP is often used in stub networks and in small autonomous systems that do not have many

redundant paths.

Routing updates

RIP sends routing-update messages at regular intervals and when the network topology

changes. Each router “advertises” routing information by sending a routing information

update every 30 seconds. If a router does not receive an update from another router for 180

seconds, those routes provided by that router are declared invalid. The routes are removed

from the routing table, but they remain in the RIP routes table. After another 120 seconds

without receiving an update for those routes, the routes are removed from respective

regular updates.