| A | B |
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| Path determination | enables a router to evaluate the available paths to a destination and to establish the preferred handling of a packet |
| How do routers use identify the destination network (LAN) of a packet | The router uses the network address to identify the destination network (LAN) of a packet within an internetwork. |
| What functions does a router use relay a packet from one data link to another | a path determination function or a switching function |
| a path determination function | The router uses the network portion of the address to make path selections to pass the packet to the next router along the path. |
| a switching function | allows a router to accept a packet on one interface and forward it through a second interface. |
| Routed protocol | any network protocol that provides enough information in its network layer address to allow a packet to be forwarded from one host to another host based on the addressing scheme. Routed protocols define the field formats within a packet |
| Routing protocols | support a routed protocol by providing mechanisms for sharing routing information. Routing protocol messages move between the routers. A routing protocol allows the routers to communicate with other routers to update and maintain tables |
| TCP/IP examples of routing protocols are | RIP (Routing Information Protocol); IGRP (Interior Gateway Routing Protocol); EIGRP (Enhanced Interior Gateway Routing Protocol); OSPF (Open Shortest Path First) |
| Multiprotocol routing | Routers are capable of supporting multiple independent routing protocols and maintaining routing tables for several routed protocols. This capability allows a router to deliver packets from several routed protocols over the same data links. |
| Static route | administered manually by a network administrator who enters it into a router's configuration. The administrator must manually update this static route entry whenever an internetwork topology change requires an update |
| Dynamic route | After a network administrator enters configuration commands to start dynamic routing, the route knowledge is automatically updated by a routing process whenever new information is received from the internetwork. Changes in dynamic knowledge are exchanged between routers as part of the update process. |
| Why use a static route | Static routing has several useful applications. Dynamic routing tends to reveal everything known about an internetwork, for security reasons, you may want to hide parts of an internetwork. Static routing enables you to specify the information you want to reveal about restricted networks |
| Stub network | When a network is accessible by only one path, a static route to the network can be sufficient. This type of network is called a stub network. Configuring static routing to a stub network avoids the overhead of dynamic routing |
| How a default route is used | a routing table entry that directs packets to the next hop when that hop is not explicitly listed in the routing table. You can set default routes as part of the static configuration. |
| Why dynamic routing is necessary | Dynamic routing offers more flexibility. Dynamic routing protocols can also direct traffic from the same session over different paths in a network for better performance. This is known as loadsharing. |
| Dynamic routing operations | maintenance of a routing table and timely distribution of knowledge, in the form of routing updates, to other routers |
| a routing protocol describes | how to send updates; what knowledge is contained in these updates; when to send this knowledge; how to locate recipients of the updates |
| metrics most commonly used by routers | bandwidth; delay; load; reliability; hop count; ticks; cost |
| bandwidth | the data capacity of a link; (normally, a 10 Mbps Ethernet link is preferable to a 64 kbps leased line) |
| delay | the length of time required to move a packet along each link from source to destination |
| load | the amount of activity on a network resource such as a router or link |
| reliability | usually refers to the error rate of each network link |
| hop count | the number of routers a packet must travel through before reaching its destination |
| ticks | the delay on a data link using IBM PC clock ticks (approximately 55 milliseconds). |
| cost | an arbitrary value, usually based on bandwidth, monetary expense, or other measurement, that is assigned by a network administrator |
| two basic routing algorithms | distance vector; or link state |
| distance vector | The distance-vector routing approach determines the direction (vector) and distance to any link in the internetwork |
| link state | The link-state (also called shortest path first) approach re-creates the exact topology of the entire internetwork (or at least the portion in which the router is situated). |
| Hybrid Routing | The balanced hybrid approach combines aspects of the link-state and distance-vector algorithms |
| convergence | Whenever the topology of a network changes because of growth, reconfiguration, or failure, the network knowledge base must also change. The knowledge needs to reflect an accurate, consistent view of the new topology. This view is called convergence. |
| How topology changes propagate through the network of routers | When the topology in a distance-vector protocol network changes, routing table updates must occur. As with the network discovery process, topology change updates proceed step-by-step from router to router |
| Routing loops | occur if a network's slow convergence on a new configuration causes inconsistent routing entries |
| The problem of counting to infinity | loops packets continuously around the network in spite of the fundamental fact that the destination network is down |
| split-horizon updates | Routing technique in which information about routes is prevented from exiting the router interface through which that information was received. Split-horizon updates are useful in preventing routing loops |
| hold-down timers | State into which a route is placed so that routers will neither advertise the route nor accept advertisements about the route for a specific length of time (the holddown period). Holddown is used to flush bad information about a route from all routers in the network. A route is typically placed in holddown when a link in that route fails |
| SPF (shortest path first) | maintain a complex database of topology information |
| Link-state routing uses: | link-state advertisements (LSAs); a topological database; the SPF algorithm, and the resulting SPF tree; a routing table of paths and ports to each network |
| Link-state routing protocol | OSPF (Open Shortest Path First) |
