21-2
Cisco ASA 5500 Series Configuration Guide using the CLI
Chapter21 Routing Overview
Information About Routing
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 address by some means,
the source host sends a packet addressed specifically to a router physical (Media Access Control
[MAC]-layer) address, this time with the protocol (network layer) address of the destination host.
As it examines the packet 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.
Path Determination
Routing protocols use metrics to evaluate what path will be the best for a packet to travel. A metric is a
standard of measurement, such as path bandwidth, that is used by routing algorithms to determine the
optimal path to a destination. To aid the process of path determination, routing algorithms initialize and
maintain routing tables, which include route information. Route information varies depending on the
routing algorithm used.
Routing algorithms fill routing tables with a variety of information. Destination or next hop associations
tell a router that a particular destination can be reached optimally by sending the packet to a particular
router representing 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 a next hop.
Routing tables also can include other information, such as 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.
Routers communicate with one another and maintain their routing tables through the transmission of a
variety of messages. The routing update message is one such message that generally 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 network topology. A link-state advertisement, another example of a message sent
between routers, informs other routers of the state of the sender links. Link information also can be used
to build a complete picture of network topology to enable routers to determine optimal routes to network
destinations.
Note Asymmetric routing is only supported for Active/Active failover in multiple context mode. For more
information, see the “Configuring Active/Active Failover” section on page63-8.
Supported Route Types
There are several route types that a router can use. The ASA uses the following route types:
•Static Versus Dynamic, page21-3
•Single-Path Versus Multipath, page21-3
•Flat Versus Hierarchical, page21-3