table; it is common – even necessary – in small networks, such as a home or small business where the default route simply sends all non-local traffic to the Internet service provider. The default route can be manually configured (as a static route); learned by dynamic routing protocols; or be obtained by DHCP. A router can run more than one routing protocol at a time, particularly if it serves as an autonomous system border router between parts of a network that run different routing protocols; if it does so, then redistribution may be used (usually selectively) to share information between the different protocols running on the same router. Besides deciding to which interface a packet is forwarded, which is handled primarily via the routing table, a router also has to manage congestion when packets arrive at a rate higher than the router can process. Three policies commonly used are tail drop, random early detection (RED), and weighted random early detection (WRED). Tail drop is the simplest and most easily implemented: the router simply drops new incoming packets once buffer space in the router is exhausted. RED probabilistically drops datagrams early when the queue exceeds a pre-configured portion of the buffer, until reaching a pre-determined maximum, when it drops all incoming packets, thus reverting to tail drop. WRED can be configured to drop packets more readily dependent on the type of traffic. Another function a router performs is traffic classification and deciding which packet should be processed first. This is managed through QoS, which is critical when Voice over IP is deployed, so as not to introduce excessive latency. Yet another function a router performs is called policy-based routing where special rules are constructed to override the rules derived from the routing table when a packet forwarding decision is made. Some of the functions may be performed through an application-specific integrated circuit (ASIC) to avoid overhead of scheduling CPU time to process the packets. Others may have to be performed through the CPU as these packets need special attention that cannot be handled by an ASIC. See also Mobile broadband modem Modem Residential gateway Switch virtual interface Wireless router Notes References External links Internet architecture Hardware routers Networking hardware Server appliance Computer networking