University of the Western Cape
Chapter 11: Routing
Aleksandar Radovanovic
Path Determination
University of the Western Cape

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.
IP Routing Table
University of the Western Cape

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.
Determining Network Address
University of the Western Cape
The router then performs the logical AND
operation to obtain the network number.
Finally, router looks up the
To determine the network
During this operation the host portion of the
destination network number,
address, the router extracts
destination address is removed.
matches it with an outgoing
the IP destination address
interface and forwards the
from the incoming packet
frame to the destination IP
and retrieves the internal
address.
network mask.
University of the Western Cape
Routed versus routing protocol
University of the Western Cape
Static versus dynamic routes
Dynamic routing

University of the Western Cape

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.
Routing Metrics
University of the Western Cape



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.
Classes of routing protocols
University of the Western Cape
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
University of the Western Cape
Convergence
Distance-vector routing
University of the Western Cape

Distance-vector-based routing algorithms pass periodic copies of a routing
table from router to router
The problem of routing loops


University of the Western Cape

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)
Split Horizon
University of the Western Cape

Information about routes is
prevented from exiting the
router interface through
which that information was
received
Holddown

University of the Western Cape

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.
Link-state routing basics

University of the Western Cape





A link-state routing
algorithm maintains
full knowledge of
distant routers and
how they
interconnect. Linkstate 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 linkstate concept in OSPF
(Open Shortest Path
First) routing.
Hybrid routing protocols

University of the Western Cape

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-toIntermediate System), and Cisco's EIGRP (Enhanced Interior Gateway Routing
Protocol).