CCNA 2 Module 6
Routing and Routing Protocols
Routing
• Routing is moving data from one network
to another based on the Layer 3 address.
• Routers look at the destination address in
the layer 3 header and forward the data
out the appropriate interface.
• Network numbers and their associated
interfaces are kept in the router’s routing
table.
Different Types of Routes
There are basically three types of routes:
1. Static Routes
2. Dynamic Routes
3. Default Routes
Static Routes
• Static routes must be manually entered by
the network administrator.
• Static routes are configured according to
the destination (remote) network number,
subnet mask and next hop address or the
exiting interface of the local router.
• Static routes are configured from global
configuration mode and are represented
by an ‘S - ’ in the routing table.
Static Routes
• Because static routes must be configured
manually, any network topology changes
require the network administrator to add
and delete static routes to account for the
changes.
• Because of the extra administrative
requirements, static routing does not have
the scalability of dynamic routing.
Static Route Configuration
Static Route-Outgoing Interface
Gateway = exiting local interface
Static Route-Next Hop Address
Gateway = next hop address
Administrative Distance
• The administrative distance is an optional
parameter that gives a measure of the reliability
of the route.
• A lower value for the administrative distance
indicates the more reliable route.
– Default Admin Dist = 1 and Max is 255
• A static route to a dest. network with a higher
admin distance then the routing protocol for the
same network can be used as a backup route.
This is called a “Floating Static Route”
Administrative Distance
Static Default Routes
• Default routes are used to route packets with
destinations that do not match any of the other
routes in the routing table.
• Routers are typically configured with a default
route for Internet-bound traffic.
• Static default routes are configured exactly like
any other static route except the destination
network and subnet mask are 0.0.0.0
• This is called the “quad zero” route.
Router(config)# ip route 0.0.0.0 0.0.0.0 172.16.0.1
Dynamic Routing
• Dynamic routing is achieved by use of a
routing protocol
• A routing protocol allows one router to
share information with other routers
regarding the networks it knows about and
their distance from that router (metric).
• The information a router gets from another
router is used to build and maintain a
routing table.
Routing Protocols
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•
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Examples of routing protocols are:
Routing Information Protocol (RIP)
Interior Gateway Routing Protocol (IGRP)
Enhanced Interior Gateway Routing
Protocol (EIGRP)
Open Shortest Path First (OSPF)
Border Gateway Protocol (BGP)
Routing Protocols
Routing protocols can be broken into
different categories according to whether
they are:
1. External or Internal to the Autonomous System
2. Distance Vector or Link-State
3. Classful or Classless
Interior and Exterior Protocols
• A routing protocol can be used to route
traffic within an Autonomous System (AS),
in which case the routing protocol would
be “interior”.
• A routing protocol can also be used to
route traffic from one AS to another, in
which case it would be “exterior”.
• Interior and exterior refer to internal to the
AS or external to the AS.
Autonomous Systems
• An autonomous system (AS) is a collection of
networks under a common administration
sharing a common routing strategy.
• To the outside world, an AS is viewed as a single
entity.
• The American Registry of Internet Numbers
(ARIN), a service provider, or an administrator
assigns an identifying number to each AS.
• This autonomous system number is a 16 bit
number.
Interior and Exterior Protocols
• Interior Gateway Protocols:
– RIP
– IGRP and EIGRP
– OSPF
• Exterior Gateway Protocols:
– BGP
Distance Vector and Link-State
•
Most routing algorithms can be classified into
one of two categories:
1. distance vector (RIP, IGRP)
2. link-state (OSPF)
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The distance vector routing approach
determines the direction (vector) and distance
(metric) to any link in the internetwork.
The link-state approach, also called shortest
path first, recreates the topology of the entire
internetwork.
Routing Metrics
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Routers decide how close a network is to
itself by using a routing metric.
Routing metrics can be calculated
according to:
1.
2.
3.
4.
5.
Hop count
Bandwidth
Reliability
Load
Delay
Routing Metric
Composite Metric
Distance Vector Concepts
• Distance vector routing algorithms pass periodic
copies of a routing table (updates) from router to
router.
• Each router receives the entire routing table from
its directly connected neighbor routers.
• This same step-by-step process occurs in all
directions between neighbor routers until all
routers have a complete routing table of the
entire internetwork.
• Once this happens, the network has “converged.”
Distance Vector Updates
Routing Information Protocol
Routing Information Protocol (RIP) was
originally specified in RFC 1058. Its key
characteristics include the following:
1. It is a distance vector routing protocol.
2. Hop count is used as the metric for path
selection.
3. If the hop count is greater than 15, the
packet is discarded (Max hop count = 15)
4. Routing updates are broadcast every 30
seconds, by default.
5. RIP is a classful routing protocol
Interior Gateway Routing Protocol
1.
2.
3.
4.
Interior Gateway Routing Protocol (IGRP) is a
proprietary protocol developed by Cisco. Some
of the IGRP key design characteristics
emphasize the following:
It is a distance vector routing protocol.
Bandwidth, load, delay and reliability are used
to create a composite metric.
Routing updates are broadcast every 90
seconds, by default.
IGRP is a classful routing protocol
Link-State Concepts
• Link-state algorithms are also known as
Dijkstras algorithm or as SPF (shortest path first)
algorithms.
• Link-state routing algorithms maintain a complex
database of topology information.
• A link-state routing algorithm maintains full
knowledge of distant routers and how they
interconnect.
• Link-state routers exchange Link State
Advertisements (LSAs) to update network
reachability.
OSPF Route Calculation
Link-State Routing
Link-state routing uses:
1. Link-state advertisements (LSAs) – A link-state
advertisement (LSA) is a small packet of routing
information that is sent between routers.
2. Topological database – A topological database is a
collection of information gathered from LSAs.
3. SPF algorithm – The shortest path first (SPF)
algorithm is a calculation performed on the database
resulting in the SPF tree.
4. Routing tables – A list of the known paths and
interfaces for only the best routes to a destination.
Open Shortest Path First
1.
2.
3.
4.
5.
6.
Open Shortest Path First (OSPF) is a nonproprietary
link-state routing protocol. The key characteristics of
OSPF are as follows:
It is a link-state routing protocol.
Open standard routing protocol described in RFC 2328.
Uses the SPF algorithm to calculate the lowest cost to a
destination.
Routing updates are flooded as topology changes occur
to a multicast address.
Update packets are passed across the network in eventtriggered updates, so convergence is fast.
OSPF is a classless routing protocol.
Enhanced IGRP - Hybrid
1.
2.
3.
4.
5.
6.
EIGRP is a Cisco proprietary enhanced distance vector
routing protocol. The key characteristics of EIGRP are
as follows:
It is an enhanced distance vector routing protocol.
Uses load balancing.
Uses a combination of distance vector (metric) and linkstate features (updates).
Uses Diffused Update Algorithm (DUAL) to calculate the
shortest path.
Routing updates are triggered by topology changes.
EIGRP is a classless routing protocol.
Classful and Classless
• Classful routing protocols such as RIP and IGRP do
not support classless addresses or Variable Length
Subnet Masks (VLSM) unless all subnet masks are
the same!
• Classful routing protocols can only route for specific
subnets, if all subnet masks are the same!
• Classless routing protocols (RIPv2, EIGRP, OSPF)
support classless address and VLSM by sending
subnet mask information as well as network
information as part of the routing update.
• Classless protocols support routing between
individual subnets of different length subnet masks.