Cisco ASA 5505 Configuration Manual page 354

Information About Routing
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 contain route information. Route information varies depending on the
routing algorithm used.
Routing algorithms fill routing tables with a variety of information. Destination/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 contain 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's 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.
Asymmetric routing is not supported on the adaptive security appliance.
Note
Supported Route Types
There are several types of route types that a router can use. The adaptive security appliance uses the
following route types:
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•
•
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Static Versus Dynamic
Static routing algorithms are hardly algorithms at all, but are table mappings established by the network
administrator before the beginning of routing. These mappings do not change unless the network
administrator alters them. Algorithms that use static routes are simple to design and work well in
environments where network traffic is relatively predictable and where network design is relatively
simple.
Because static routing systems cannot react to network changes, they generally are considered unsuitable
for today's large, constantly changing networks. Most of the dominant routing algorithms today are
dynamic routing algorithms, which adjust to changing network circumstances by analyzing incoming
routing update messages. If the message indicates that a network change has occurred, the routing
software recalculates routes and sends out new routing update messages. These messages permeate the
network, stimulating routers to rerun their algorithms and change their routing tables accordingly.
Cisco ASA 5500 Series Configuration Guide using ASDM
18-2
Static Versus Dynamic, page 18-2
Single-Path Versus Multipath, page 18-3
Flat Versus Hierarchical, page 18-3
Link-State Versus Distance Vector, page 18-3
Chapter 18 Information About Routing
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