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ITEC 275
Computer Networks – Switching, Routing, and
WANs
Week 7
Robert D’Andrea
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.
Example1: Present location is /abc/xyz, I am want
to remove /abc/xyz/read/hello.txt file.
Using relative path:
rm read/hello.txt
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 (/proc/bus/usb), moves from
its most specific information (the DNS name for a
computer called "host-a") 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 when
hardware and software fail.
Route Summarization
Classful Boundary Summarization
Routing Tables
Dual Stack and Tunneling IPv4/IPv6
Bridge Protocol Data Unit (BPDU)
What are BPDUs?
BPDUs are data messages that are exchanged
across the switches within an extended LAN that uses a
spanning tree protocol topology. BPDU packets contain
information on ports, addresses, priorities and costs and
ensure that the data ends up where it was intended to go.
BPDU messages are exchanged across bridges to detect
loops in a network topology. The loops are then removed
by shutting down selected bridge interfaces and placing
redundant switch ports in a backup, or blocked, state.
Bridge Protocol Data Unit (BPDU)
Stateless Auto-configuration
Stateless Auto-configuration
Stateless Auto Configuration is an important
feature offered by the IPv6 protocol. It allows the various
devices attached to an IPv6 network to connect to the
Internet using the Stateless Auto Configuration without
requiring any intermediate IP support in the form of a
Dynamic Host Configuration Protocol (DHCP) server. A
DHCP server holds a pool of IP addresses that are
dynamically assigned for a specified amount of time to
the requesting node in a Local Area Network (LAN).
Stateless Auto-configuration
Stateless Auto Configuration is a boom for the Network
Administrators since it has automated the IP address configuration
of individual network devices. Earlier, configuration of the IP
addresses was a manual process requiring support of a DHCP
server. However, IPv6 allows the network devices to automatically
acquire IP addresses and also has provision for
renumbering/reallocation of the IP addresses en masse. With a
rapid increase in the number of network devices connected to the
Internet, this feature was long overdue. It simplifies the process of
IP address allocation by doing away with the need of DHCP
servers and also allows a more streamlined assignment of network
addresses there by facilitating unique identification of network
devices over the Internet.
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
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
STP 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
Connectivity to different segments
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 product feature. It supports
the concept of a switch edge port.
• UplinkFast and Backbone Fast. UpLinkFast is a
Cisco product 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 feature. 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.
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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
• A metric is 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 before hand, 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
Routing Algorithms
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 ter 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
Routing Algorithms
Cisco’s On- Demand Routing
The Cisco Discovery Protocol (CDP) is a Cisco
proprietary protocol that, among other things, is used to
discover other Cisco devices on either broadcast or nonbroadcast 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 (ODR) 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
ODR is a feature that provides IP routing for stub sites
with minimum overhead. The overhead of a general, dynamic
routing protocol is avoided without incurring the configuration
and management overhead of static routing.
A stub router can be thought of as a spoke router in a
hub-and-spoke network topology where the only router to
which the spoke is adjacent is the hub router. In such a network
topology, the IP routing information required to represent this
topology is fairly simple. These stub routers commonly have a
WAN connection to the hub router, and a small number of
LAN network segments (stub networks) are directly connected
to the stub router.
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/feature 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
ODR (On Demand Routing) allows routing information from
hub/spoke topology to be exchanged with hub and entered into hub
routing table without running any standard routing protocol.
ODR is designed to be used in a partially meshed environment
(e.g. Frame Relay networks) where a hub router maintains one link each
to multiple stub routers (spokes routers). Therefore, for any spoke to
communicate with another spoke, such traffic must pass through the
hub.
If each stub network is simply made up of the stub router and
multiple hosts (much like remote location of a corporate organization).
There is no need to run any routing protocols on the stub since all host
are connected and the routes can be exchanged with hub router using
ODR. Hub routing will install the routes in routing/forwarding tables as
connected routes with next-hop address of each stub respectively.
Routing Algorithms
ODR uses CDP protocol that runs by default on
all Cisco devices. CDP is used by Cisco devices to
learn and retrieve basic information about their
connected neighbors. Therefore, disabling CDP on a
router will also disable propagation of ODR traffic.
The primary benefit of using ODR is that
it is not CPU intensive and it consumes very
little bandwidth.
Routing Algorithms
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
Interior Routing Protocols
RIPv1 – classful
RIPv2 – classless
OSPF – Build an entire topology
ISIS –
IGRP – older CISCO protocol
EIGRP – Enhanced Interior CISCO protocol
Exterior Routing Protocol
BGP
Default Routing Example
Routing Protocols Provide
1.
2.
3.
4.
5.
Discovery of new networks
Automatic route updating
Best path determination
Failover – load balancing
Eliminates human error
Default Routing Example
Distance Vector Routing Protocols
Neighboring routers communicate with
each other to keep their routing tables updated. A
view of the entire network is through all routers
connected together.
Link State Routing Protocols
Share link information (up or down) build a
routing table based on the topology that has been
built.
Default Routing
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
Link-State
• Routing Information Protocol
• Open Shortest Path First (OSPF)
(RIP) Version 1 and 2
• Intermediate System-to• Interior Gateway Routing
Intermediate System (IS-IS)
Protocol (IGRP)
• Enhanced IGRP
• Border Gateway Protocol (BGP)
Routing Protocols
Routers talk to routers on the network, sharing information with
each other.
Routed protocol: IP
Routing protocol: A protocol used by a router.
Distance Vector routing protocols include
RIP, RIPv1, RIPv2, IGRP, and EIGRP
Routers communicate with neighboring routers.
Distance = Metric
Routing Protocols
Link State routing protocols include
OSPF and ISIS
Routers communicate with all other routers. They exchange link
state information to build a topology of the entire network.
What direction of interface are you going out of?
Link State refers to the interface connections or “links” to other
routers and networks.
Routing Protocols
What is convergence?
Convergence is when all routers in the network have the
same picture of the network.
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, super netting)
– Supports variable-length subnet masking (VLSM)
• Includes simple authentication to foil crackers
from 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 100Mbps 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.
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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, an
EGPversion 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?