Cisco ASA 5505 Configuration Manual page 353

Information About
This chapter describes underlying concepts of how routing behaves within the adaptive security
appliance, and the routing protocols that are supported.
The chapter includes the following sections:
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Information About Routing
Routing is the act of moving information across an internetwork from a source to a destination. Along
the way, at least one intermediate node typically is encountered. Routing involves two basic activities:
determining optimal routing paths and transporting information groups (typically called packets)
through an internetwork. In the context of the routing process, the latter of these is referred to as packet
switching. Although packet switching is relatively straightforward, path determination can be very
complex.
Switching
Switching algorithms is relatively simple; it is the same for most routing protocols. In most cases, a host
determines that it must send a packet to another host. Having acquired a router's address by some means,
the source host sends a packet addressed specifically to a router's physical (Media Access Control
[MAC]-layer) address, this time with the protocol (network layer) address of the destination host.
As it examines the packet's destination protocol address, the router determines that it either knows or
does not know how to forward the packet to the next hop. 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, however, it changes
the destination physical address to that of the next hop and transmits the packet.
The next hop may be the ultimate destination host. If not, the next hop is usually another router, which
executes the same switching decision process. As the packet moves through the internetwork, its
physical address changes, but its protocol address remains constant.
OL-20339-01
Information About Routing, page 18-1
How Routing Behaves Within the Adaptive Security Appliance, page 18-4
Supported Internet Protocols for Routing, page 18-5
Information About the Routing Table, page 18-5
Information About IPv6 Support, page 18-8
Disabling Proxy ARPs, page 18-11
C H A P T E R
Routing
Cisco ASA 5500 Series Configuration Guide using ASDM
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