Gigabit Ethernet Switch User's Manual

WGS3 Layer 3 Switch User’s Manual
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single network. These nodes must therefore use a smaller subnet mask than that used by the switch or
other relevant network devices.
Note that extensive use of Proxy ARP can adversely affect the performance of the switch because it may
lead to increased ARP traffic and increased search time for larger ARP address tables.
6.2.6 Routing Protocols
The switch supports both static and dynamic routing.
• Static routing requires routing information to be stored in the switch, either manually or when a
connection is set up by an application outside the switch.
• Dynamic routing uses a routing protocol to exchange routing information, calculate routing tables, and
respond to changes in the status or loading of the network.
Dynamic routing involves the determination and updating of all the routing information required for packet
forwarding.
• Handling routing protocols
• Updating the routing table
• Updating the Layer 3 switching database
The switch supports RIP and RIP-2 dynamic routing protocols.
6.2.6.1 RIP and RIP-2 Dynamic Routing Protocols
The RIP protocol is the most widely used routing protocol. The RIP protocol uses a distance
vector-based approach to routing. Routes are determined on the basis of minimizing the distance vector,
or hop count, which serves as a rough estimate of transmission cost. Each router broadcasts its
advertisement every 30 seconds, together with any updates to its routing table. This allows all routers on
the network to learn consistent tables of next hop links which lead to relevant subnets. Just as Layer 2
switches use the Spanning Tree Algorithm to prevent loops, routers also use methods for preventing
loops that would cause endless retransmission of data traffic. RIP utilizes the following three methods to
prevent loops from occurring:
• Split horizon— never propagate routes back to an interface port from which they have been acquired.
• Poison reverse— propagate routes back to an interface port from which they have been acquired, but
set the distance vector metrics to infinity. (This provides faster convergence.)
• Triggered updates— whenever a route gets changed, broadcast an update message after waiting for a
short random delay, but without waiting for the periodic cycle.
RIP-2 is a compatible upgrade to RIP. RIP-2 adds useful capabilities for plain text authentication, multiple
independent RIP domains, variable length subnet masks, and multicast transmissions for route
advertising (RFC 1388).
There are several serious problems with RIP that you should consider before deciding which routing
protocol to use for your network. First of all, RIP (version 1) has no knowledge of subnets, both RIP
versions can take a long time to converge on a new route after the failure of a link or router during which
time routing loops may occur, and its small hop count limitation of 15 restricts its use to smaller networks.