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ITEC 275
Computer Networks – Switching,
Routing, and WANs
Week 7
Robert D’Andrea
Some slides provide by Priscilla
Oppenheimer and used with permission
Agenda
• Learning Activities
– Domain Name Server (DNS)
– Summarization
– Root Owner DNS
– Routing tables
– Spanning Tree Protocol
– Rapid Spanning Tree Protocol
– Static versus Dynamic Routing
– Routing Protocols and Characteristics
DNS Domain Names
Interpreting a DNS domain names
DNS has a method of noting and interpreting the
fully qualified path to a DNS domain name similar to
the way full paths to files or directories are noted or
displayed at a command prompt.
For example, a directory tree path helps point to
the exact location of a file stored on your computer. For
Windows computers, the back slash (\) indicates each
new directory that leads to the exact location of a file.
For DNS, the equivalent is a period (.) indicating each
new domain level used in a name.
Interpreting File Names
UNIX uses the concept of relative and absolute
file names. If a file name is preceded by a
forward slash (e.g. /bin), the name is absolute. If
the name is without a leading slash, it is
considered relative to your current working
directory.
Interpreting a DNS domain names
For DNS, an example of a domain name with multiple
levels is the following, a fully qualified domain name
(FQDN):
host-a.example.microsoft.com.
Unlike the file name example, a DNS FQDN, when read
from left to right, moves from its most specific
information (the DNS name for a computer called "hosta") to its highest or most general piece of information
(the trailing period (.) that indicates the root of the DNS
name tree). This example shows the four separate DNS
domain levels that lead away from the specific host
location of "host-a":
Interpreting a DNS domain names
1. The "example" domain, which corresponds to a subdomain
where the computer name "host-a" is registered for use.
2. The "microsoft" domain, which corresponds to the parent
domain that roots the "example" subdomain.
3. The "com" domain, which corresponds to the top-level domain
designated for use by business or commercial organizations that
roots the "microsoft" domain.
4. The trailing period (.), which is a standard separator character
used to qualify the full DNS domain name to the root level of the
DNS namespace tree.
Root Servers
When a computer on the Internet needs to
resolve a domain name, it uses resolver software
to do the lookup. A resolver breaks the name up
into its labels from right to left. The first
component is queried using a root server to
obtain the responsible authoritative server.
Queries for each name are performed until a
name server returns the answer of the original
query.
Interpreting a DNS domain names
As of 2013, there are 13 root name servers, with
names in the form letter.root-server.net. This
does not mean that there are only 13 physical
servers; each site uses redundant computer
equipment to provide reliable service in when
hardware and software fail on occasion.
View:
www.root-servers.org
Route Summarization
Classful Boundary Summarization
Routing Tables
Dual Stack and Tunneling IPv4/IPv6
Stateless Auto-configuration
Switching and Routing Choices
• Switching
–
–
–
–
Layer 2 transparent bridging (switching)
Multilayer switching
Spanning Tree Protocol enhancements
VLAN technologies
• Routing
–
–
–
–
Static or dynamic
Distance-vector and link-state protocols
Interior and exterior
Etc.
•
•
•
•
•
Selection Criteria for Switching and
Routing Protocols
Network traffic characteristics
Bandwidth, memory, and CPU usage
The number of peers supported
The capability to adapt to changes quickly
Support for authentication
Making Decisions
• Goals must be established
• Many options should be explored
• The consequences of the decision should be
investigated
• Contingency plans should be made
• A decision table can be used. Decision tables
are composed of rows and columns. Each row
corresponds to a single rule, with the columns
defining the conditions and actions of the
rules.
Example Decision Table
Transparent Bridging (Switching)
Tasks
• Ethernet switches and bridges use
transparent bridging.
• A transparent bridge connects one or more
LAN segments so that end systems on
different segments can communicate with
each other transparently. An end system
sends a frame to a destination without
knowing whether the destination is local or
on the other side of the bridge.
Transparent Bridging (Switching)
Tasks
• Forward frames transparently
• Learn which port to use for each MAC
address
• Flood frames when the destination unicast
address hasn’t been learned yet
• Filter frames from going out ports that
don’t include the destination address
• Flood broadcasts and multicasts
Definitions
• STP is a bridge protocol that uses the STA
(Spanning Tree Algorithm) to find
redundant links dynamically and create a
spanning-tree topology database. Bridges
exchange BPDU (Bridge Protocol Data Unit)
messages with other bridges to detect
loops.
• BPDU STP hello packet that is sent out at
configurable intervals to exchange
information among bridges in the network.
Transparent Bridging
Switching Table on a Bridge or
Switch
MAC Address
Port
08-00-07-06-41-B9
1
00-00-0C-60-7C-01
2
00-80-24-07-8C-02
3
Cisco Spanning Tree Protocol
Enhancements
• PortFast is a Cisco feature. It supports the
concept of a switch edge port.
• UplinkFast and Backbone Fast. UpLinkFast is a
Cisco feature that is configured on access layer
switches. Improves the convergence time of
STP.
• Unidirectional link detection is a hardware
failure detection between switches.
• Loop Guard is a Cisco product. Supports the
prevention of loops caused by blocking port
erroneously moving to the forwarding state.
Redundant Uplinks
Core
Layer
X
Distribution
Layer
Switch B
Access
Layer
Switch C
Primary
Uplink
Switch A
X
Secondary
Uplink
X = blocked by STP
• If a link fails, how long will STP take to recover?
• Use UplinkFast to speed convergence
Protocols for Transporting VLAN
Information
• Inter-Switch Link (ISL)
– Tagging protocol
– Cisco proprietary
• IEEE 802.1Q
– Tagging protocol
– IEEE standard
• VLAN Trunk Protocol (VTP)
– VLAN management protocol is a switch-to-switch
and switch-to-router configuration.
Protocols for Transporting VLAN
Information
• VLAN Trunk Protocol (VTP)
– The VLAN management protocol exchanges VLAN
configuration changes as they are made to the
network. VTP manages additions, deletions, and
renaming of VLANs on a campus network without
requiring manual intervention at each switch.
Selecting Routing Protocols
• They all have the same general goal:
– To share network reachability information among
routers
• They differ in many ways:
–
–
–
–
–
–
Interior versus exterior
Metrics supported hop count or bandwidth.
Dynamic versus static and default
Distance-vector versus link-sate
Classful versus classless
Scalability
Interior Versus Exterior Routing
Protocols
• Interior routing protocols are used within
an autonomous system
• Exterior routing protocols are used
between autonomous systems
Autonomous system (two definitions that are often used):
“A set of routers that presents a common routing policy to the
internetwork”
“A network or set of networks that are under the administrative control
of a single entity”
Routing Protocol Metrics
• Metric: the determining factor used by a routing
algorithm to decide which route to a network is
better than another
• Examples of metrics:
–
–
–
–
–
Bandwidth - capacity
Delay - time
Load - amount of network traffic
Reliability - error rate
Hop count - number of routers that a packet must
travel through before reaching the destination
network
– Cost - arbitrary value defined by the protocol or
administrator
Routing Algorithms
• Static routing
– Calculated beforehand, offline
• Default routing
– “If I don’t recognize the destination, just send the packet to
Router X”
• Cisco’s On-Demand Routing
– Routing for stub networks
– Uses Cisco Discovery Protocol (CDP)
• Dynamic routing protocol
– Distance-vector algorithms
– Link-state algorithms
Routing Algorithms
Stub network has only one default path to
non-local hosts and no outside network
knowledge. Non-local stub network traffic
uses a single logical path when traveling in
and out of the network.
A good example would be an individual or
group that uses only one router to link to an
internet service provider (ISP). The individual
or group are considered stub networks by the
ISP.
Routing Algorithms
The default route is the IP address of the next
hop when no other routes are known.
To configure the default route to be
192.168.1.1:
config t ip route 0.0.0.0 0.0.0.0 192.168.1.1
An interface can be used as an alternative to
and IP address. To use serial0/0 for
destinations not in the routing table, use:
ip route 0.0.0.0 0.0.0.0 serial 0/0
Routing Algorithms
A default route of a computer that is participating
in computer networking is the packet forwarding rule
(route) taking effect when no other route can be
determined for a given Internet Protocol (IP)
destination address. All packets for destinations not
established in the routing table are sent via the default
route. This route generally points to another router,
which treats the packet the same way: If a route
matches, the packet is forwarded accordingly,
otherwise the packet is forwarded to the default route
of that router. The process repeats until a packet is
delivered to the destination. Each router traversal
counts as one hop in the distance calculation for the
transmission path.
Routing Algorithms
Cisco’s On- Demand Routing
The Cisco Discovery Protocol (CDP) is a Cisco
proprietary protocol that, amongst other things, is used
to discover other Cisco devices on either broadcast or
non-broadcast media. CDP provides administrators
with information that includes the IP address, software
version, as well as the capabilities of the neighbor
device. On-Demand Routing is an enhancement to
Cisco Discovery Protocol that advertises the connected
IP prefix or prefixes of a stub router via CDP. ODR
also supports VLSM (Variable Length Subnet Mask),
which means that it can be used in just about any
network.
Routing Algorithms
Cisco’s On- Demand Routing
It is important to know that ODR is not a routing
protocol. Instead, it is simply an enhancement to
CDP that is used to dynamically propagate
routing information at Layer 2. The primary
reasons ODR is often incorrectly referred to as a
routing protocol is because it allows routers to
dynamically exchange routing information. The
second reason is because ODR is enabled using
the router odr global configuration command.
Routing Algorithms
Cisco’s On- Demand Routing
The primary benefits of using ODR is that it is
not CPU intensive and it consumes very little
bandwidth.
Routing Algorithms
Cisco’s On- Demand Routing
Static Routing Example
172.16.20.1
Router A
s0
172.16.20.2
172.16.40.1
172.16.40.2
Router B
s0
s0
s1
Router C
e0
e0
e0
172.16.10.1
172.16.30.1
172.16.50.1
Host A
172.16.10.2
Host B
Host C
172.16.30.2
172.16.50.2
RouterA(config)#ip route 172.16.50.0 255.255.255.0 172.16.20.2
Send packets for subnet 50 to 172.16.20.2 (Router B)
Default Routing Example
172.16.20.1
Router A
e0
s0
172.16.20.2
172.16.40.1
172.16.40.2
Router B
s0
s1
e0
172.16.30.1
Router C
s0
e0
172.16.50.1
172.16.10.1
Host A
172.16.10.2
Host B
Host C
172.16.30.2
172.16.50.2
RouterA(config)#ip route 0.0.0.0 0.0.0.0 172.16.20.2
If it’s not local, send it to 172.16.20.2 (Router B)
Distance-Vector Routing
• Router maintains a routing table that lists
known networks, direction (vector) to each
network, and the distance to each network
• Router periodically (every 30 seconds, for
example) transmits the routing table via a
broadcast packet that reaches all other
routers on the local segments
• Routers update their routing table, if
necessary, based on received broadcasts
Distance-Vector Routing Tables
Router A
Router B
172.16.0.0
192.168.2.0
Router A’s Routing Table
Network
172.16.0.0
192.168.2.0
Router B’s Routing Table
Distance
Send To
Network
0
1
Port 1
Router B
192.168.2.0
172.16.0.0
Distance
Send To
0
1
Port 1
Router A
Link-State Routing
• Routers send updates only when there’s a
change
• Router that detects change creates a link-state
advertisement (LSA) and sends it to neighbors
• Neighbors propagate the change to their
neighbors
• Routers update their topological database if
necessary
Distance-Vector Vs. Link-State
• Distance-vector algorithms keep a list of
networks, with next hop and distance (metric)
information
• Link-state algorithms keep a database of
routers and links between them
– Link-state algorithms think of the internetwork as
a graph instead of a list
– When changes occur, link-state algorithms apply
Dijkstra’s shortest-path algorithm to find the
shortest path between any two nodes
Link-State Routing Protocol
Choosing Between Distance-Vector
and Link-State
Choose Distance-Vector
• Simple, flat topology
• Hub-and-spoke topology
• Junior network administrators
• Convergence time not a big
concern
Choose Link-State
• Hierarchical topology
• More senior network
administrators
• Fast convergence is critical
Choosing between Distance Vector
and Link State Protocols
Distance-Vector
• Routing Information Protocol
(RIP) Version 1 and 2
• Interior Gateway Routing
Protocol (IGRP)
• Enhanced IGRP
• Border Gateway Protocol
(BGP)
Link-State
• Open Shortest Path First
(OSPF)
• Intermediate System-toIntermediate System (IS-IS)
Routing Protocol
View:
http://www.youtube.com/watch?v=CBDdM-imW7o
Dynamic Routing Protocols
View:
http://www.youtube.com/watch?v=gXOKjnZR4Tg
Routing Information Protocol (RIP)
• First standard routing protocol developed for TCP/IP
environments
– RIP Version 1 is documented in RFC 1058 (1988)
– RIP Version 2 is documented in RFC 2453 (1998)
• Easy to configure and troubleshoot
• Broadcasts its routing table every 30 seconds; 25 routes per
packet
• Uses a single routing metric (hop count) to measure the
distance to a destination network; max hop count is 15
RIP V2 Features
• Includes the subnet mask with route updates
– Supports prefix routing (classless routing, supernetting)
– Supports variable-length subnet masking (VLSM)
• Includes simple authentication to foil crackers
sending routing updates
IGRP Solved Problems with RIP
• 15-hop limitation in RIP
– IGRP supports 255 hops
• Reliance on just one metric (hop count)
– IGRP uses bandwidth, delay, reliability, load
– (By default just uses bandwidth and delay)
• RIP's 30-second update timer
– IGRP uses 90 seconds
EIGRP
• Adjusts to changes in internetwork very
quickly
• Incremental updates contain only changes,
not full routing table
• Updates are delivered reliably
• Router keeps track of neighbors’ routing
tables and uses them as feasible successor
• Same metric as IGRP, but more granularity
(32 bits instead of 24 bits)
Open Shortest Path First (OSPF)
•
•
•
•
•
Open standard, defined in RFC 2328
Adjusts to changes quickly
Supports very large internetworks
Does not use a lot of bandwidth
Authenticates protocol exchanges to
meet security goals
• Is an IP routing protocol
that is completely link
state.
OSPF Metric
• A single dimensionless value called cost. A
network administrator assigns an OSPF cost
to each router interface on the path to a
network. The lower the cost, the more likely
the interface is to be used to forward data
traffic.
• On a Cisco router, the cost of an interface
defaults to 100,000,000 divided by the
bandwidth for the interface. For example, a
100-Mbps Ethernet interface has a cost of 1.
OSPF Areas Connected via Area Border
Routers (ABRs)
Area 0 (Backbone)
ABR
Area 1
ABR
Area 2
ABR
Area 3
IS-IS
•
•
•
•
Intermediate System-to-Intermediate System.
Link-state routing protocol.
Designed by the ISO for the OSI protocols.
Integrated IS-IS handles IP also.
IS-IS
IS-IS is a routing protocol designed to
move information efficiently within a computer
network, a group of physically connected
computers or similar devices. It accomplishes
this by determining the best route for datagrams
through a packet-switched network. The protocol
was defined in ISO/IEC 10589:2002 as an
international standard within the Open Systems
Interconnections (OSI) reference design
Border Gateway Protocol (BGP)
• Allows routers in different autonomous
systems to exchange routing information
– Exterior routing protocol
– Used on the Internet among large ISPs and major
companies
• Supports route aggregation
• Main metric is the length of the list of
autonomous system numbers, but BGP also
supports routing based on policies
Border Gateway Protocol (BGP)
Internet Protocol (EGP)
During the early days of the Internet, EGP
version 3 (EGP3) was used to
interconnect Autonomous Systems.
Currently, BGP version 4 is the accepted
standard for Internet routing and has essentially
replaced the more limited EGP3.
Internet Control Message Protocol
(ICMP)
• ICMP works at the Network layer and is used
by IP for many different services. ICMP is a
management protocol and messaging service
for IP. Its messages are carried as IP
datagrams.
Routing Protocols Summary
Distant vector:
RIP and IGRP
Link-state:
OSPF
Hybrid:
EIGRP
Summary
• The selection of switching and routing protocols
should be based on an analysis of
– Goals
– Scalability and performance characteristics of the
protocols
• Transparent bridging is used on modern switches
– But other choices involve enhancements to STP and
protocols for transporting VLAN information
• There are many types of routing protocols and
many choices within each type
Review Questions
• What are some options for enhancing the
Spanning Tree Protocol?
• What factors will help you decide whether
distance-vector or link-state routing is best for
your design customer?
• What factors will help you select a specific
routing protocol?
• Why do static and default routing still play a role
in many modern network designs?
This Week’s Outcomes
•
•
•
•
Spanning Tree Protocol
Rapid Spanning Tree Protocol
Static versus Dynamic Routing
Routing Protocols and Characteristics
Due this week
• 6-1 – Concept questions 5
Q&A
• Questions, comments, concerns?