Routing Fundamentals - Cisco Nexus 7000 Series Configuration Manual

Information About Layer 3 Unicast Routing
S e n d d o c u m e n t c o m m e n t s t o n e x u s 7 k - d o c f e e d b a c k @ c i s c o . c o m .

Routing Fundamentals

Routing protocols use a
measurement, such as a path bandwidth, that routing algorithms use to determine the optimal path to a
destination. To aid path determination, routing algorithms initialize and maintain routing tables, that
contain route information such as the IP destination address and the address of the next router or
hop. Destination and next-hop associations tell a router that an IP destination can be reached optimally
by sending the packet to a particular router that represents the next hop on the way to the final
destination. When a router receives an incoming packet, it checks the destination address and attempts
to associate this address with the next hop. See the
information about the route table.
Routing tables can contain other information, such as the data about the desirability of a path. Routers
compare metrics to determine optimal routes, and these metrics differ depending on the design of the
routing algorithm used. See the
Routers communicate with one another and maintain their routing tables by transmitting a variety of
messages. The routing update message is one such message that consists of all or a portion of a routing
table. By analyzing routing updates from all other routers, a router can build a detailed picture of the
network topology. A link-state advertisement, another example of a message sent between routers,
informs other routers of the link state of the sending router. You can also use link information to enable
routers to determine optimal routes to network destinations. For more information, see the
Algorithms" section on page
Packet Switching
In packet switching, a host determines that it must send a packet to another host. Having acquired a
router address by some means, the source host sends a packet addressed specifically to the router
physical (Media Access Control [MAC]-layer) address but with the IP (network layer) address of the
destination host.
The router examines the destination IP address and tries to find the IP address in the routing table. If the
router does not know how to forward the packet, it typically drops the packet. If the router knows how
to forward the packet, it changes the destination MAC address to the MAC address of the next hop router
and transmits the packet.
The next hop might be the ultimate destination host or another router that executes the same switching
decision process. As the packet moves through the internetwork, its physical address changes, but its
protocol address remains constant (see
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
1-2
metric to evaluate the best path to the destination. A metric is a standard of
"Routing Metrics" section on page
1-8.
Figure 1-1).
"Unicast RIB" section on page 1-11
1-3.
Chapter 1 Overview
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"Routing
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