1. University of the Western Cape Chapter 11: Routing Aleksandar Radovanovic

2. University of the Western Cape Path 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.

3. University of the Western Cape IP 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.

4. University of the Western Cape Determining 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.

5. University of the Western Cape Routed versus routing protocol

6. University of the Western Cape Static versus dynamic routes

7. University of the Western Cape Dynamic 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.

8. University of the Western Cape Routing 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.

9. University of the Western Cape Classes 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

10. University of the Western Cape Convergence

11. University of the Western Cape Distance-vector routing Distance-vector-based routing algorithms pass periodic copies of a routing table from router to router

12. University of the Western Cape The 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)

13. University of the Western Cape Split Horizon Information about routes is prevented from exiting the router interface through which that information was received

14. University of the Western Cape Holddown 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.

15. University of the Western Cape Link-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.

16. University of the Western Cape Hybrid 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).