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eChapter 11: Routing

Aleksandar Radovanovic

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ePath Determination

Path determination is a network layer (Layer 3) router function. Path determination enables a router to evaluate the available paths to a destination and to establish the preferred handling of a packet.

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eIP Routing Table

An IP routing table consists of destination network address and the next hop pairs. IP routing specifies that IP datagrams travel through internetworks one hop at the time. At each stop, the next destination is calculated by matching the datagram’s destination network address with an outgoing interface. If no match is found, the datagram is sent to a default router.

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eDetermining Network Address

To determine the network address, the router extracts the IP destination address from the incoming

packet and retrieves the internal network mask.

The router then performs the logical AND operation to obtain the network

number. During this operation the host portion of the destination address

is removed.

Finally, router looks up the destination network

number, matches it with an outgoing interface and

forwards the frame to the destination IP address.

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eRouted versus routing protocol

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eStatic versus dynamic routes

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eDynamic routing

Dynamic routing is the routing that adjusts automatically to network topology or traffic changes. Also called adaptive routing.

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.

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eRouting Metrics

When a routing algorithm updates a routing table, its primary objective is to determine the best information to include in the table. The algorithm generates a number, called the metric value, for each path through the network.

Metrics may be calculated based on a single characteristic of a path, or more complex metrics may be calculated by combining several characteristics.

Typically, the smaller the metric number, the better the path.

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eClasses of routing protocols

The distance vector routing approach determines the

direction (vector) and distance to any link in the internetwork.

The link state approach recreates the exact topology of the entire internetwork (or at least the portion in which the

router is situated). This approach is also called

shortest path first.

The balanced hybrid approach combines aspects of

the link state and distance vector algorithms

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eConvergence

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eDistance-vector routing

Distance-vector-based routing algorithms pass periodic copies of a routing table from router to router

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eThe problem of routing loops

Routing loops can occur if a network's slow convergence of a new configuration causes inconsistent routing entries.

Problem: Counting to infinity Solution: the routing protocol

permits the routing loop to continue until the metric exceeds its maximum allowed value (e.g. 15 hops)

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eSplit Horizon

Information about routes is prevented from exiting the router interface through which that information was received

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eHolddown

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.

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eLink-state routing basics

A link-state routing algorithm maintains full knowledge of distant routers and how they interconnect. 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

Engineers have implemented this link-state concept in OSPF (Open Shortest Path First) routing.

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eHybrid routing protocols

Balanced-hybrid routing protocols use distance vectors with more accurate metrics to determine the best paths to destination networks. However, they differ from most distance-vector protocols by using topology changes to trigger routing database updates.

Examples of hybrid protocols are OSI's IS-IS (Intermediate System-to-Intermediate System), and Cisco's EIGRP (Enhanced Interior Gateway Routing Protocol).