Week Ten Agenda
• Attendance
• Announcements
Data Center Design update
Final exam 150 points
No class July 4 (Monday)
Make-up class on Friday July 8.
• Review Week Nine Information
• Current Week Information
• Upcoming Assignments
Week Ten Topics
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8.
Routing Protocols
Routing Table
Static versus dynamic routing
Interior Versus Exterior Routing Protocols
What is convergence?
Autonomous Systems
Definitions
Loop Free Path
Routing Protocols
• One of the primary jobs of a router is to
determine the best path to a given destination
• A router learns paths, or routes, from the static
configuration entered by an administrator or
dynamically from other routers, through
routing protocols
Routing Table Principles
Three principles regarding routing tables:
1. Every router makes its decisions alone, based on
the information it has in its routing table.
2. Different routing table may contain different
information
3. A routing table can tell how to get to a destination
but not how to get back (Asymmetric Routing)
Routing Table Objective
• To forward a packet towards its destination
network, which is the destination IP address of
the packet.
• To do this, a router needs to search the routing
table for this stored information.
Routing Tables
• Routers keep a routing table in RAM
• A routing table is a list of the best known
available routes
• Routers use this table to make decisions about
how to forward a packet
• On a Cisco router, the show IP route command
is used to view the TCP/IP routing table
• A routing table maps network prefixes to an
outbound interface.
Routing Table Specifics
Routing Table
• When RTA receives a packet destined for
192.168.4.46, it looks for the prefix
192.168.4.0/24 in the routing table
• RTA then forwards the packet out an interface,
such as Ethernet0, as directed in the routing
table
Routing Loops
• A routing loop is a network problem in which packets
continue to be routed in an endless circle
• It is caused by a router or line failure, and the
notification of the downed link has not yet reached all
the other routers
• It can also occur over time due to normal growth or
when networks are merged together
• Routing protocols utilize various techniques to lessen
the chance of a routing loop
Routing Table Specifics
• Directly connected networks-this occurs when a device is
connected to another router interface
• Remotely connected networks-this is a network that is not
directly connected to a particular router network/next hop
associations-about the networks include source of information,
network address & subnet mask, and Ip address of next-hop
router
• The show ip route command is used to view a routing table on
a Cisco router
Routing Table Specifics
Routing Protocols
• Directly Connected Routes-To visit a neighbor,
you only have to go down the street on which
you already live. This path is similar to a
directly-connected route because the
"destination" is available directly through your
"connected interface," the street.
Static Routing
• Static Connected Routes-A train uses the same
railroad tracks every time for a specified route.
This path is similar to a static route because
the path to the destination is always the same.
Application For Static Routing
• When network only consists of a few routers
Using a dynamic routing protocol in such a case does
not present any substantial benefit.
• Network is connected to Internet only through one
ISP
There is no need to use a dynamic routing protocol
across this link because the ISP represents the only
exit point to the Internet
Application For Static Routing
• Static routing is not suitable for large, complex
networks that include redundant links, multiple
protocols, and meshed topologies
Routers in complex networks must adapt to topology
changes quickly and select the best route from
multiple candidates
• When an interface goes down, all static routes
mapped to that interface are removed from the IP
routing table
Application For Static Routing
• Hub and spoke topology is used on a large network
A hub-and-spoke topology consists of a central
location (the hub or switch) and multiple branch
locations (spokes), with each spoke having only one
connection to the hub or switch.
Using dynamic routing would be unnecessary
because each branch has only one path to a given
destination-through the central location.
• Static routing is useful in networks that have a single
path to any destination network.
Static Routes in Routing Table
• Includes: network address and subnet mask
and IP address of next hop router or exit
interface
• Denoted with the code S in the routing table
• Routing tables must contain directly connected
networks in order to connect remote networks,
before static or dynamic routing can be used
Static Routing
Static Routing
• The corporate network router has only one
path to the network 172.24.4.0 connected to
RTY
• A static route is entered on RTZ
Static Routing
Routing Protocols
• Dynamic Routes-When driving a car, you can
"dynamically" choose a different path based on
traffic, weather, or other conditions. This path
is similar to a dynamic route because you can
choose a new path at many different points on
your way to the destination.
Dynamic Routing Protocol
• Are used to add remote networks to a routing
table
• Are used to discover networks
• Are used to update and maintain routing tables
Dynamic Routing Protocol
• Automatic network discovery
• Network discovery is the ability of a routing protocol to share
information about the networks that it knows about with other
routers that are also using the same routing protocol.
• Dynamic routing protocols are used to share routing
information with other routers and to maintain an up-to-date
routing table.
• Dynamic routing protocols not only make a best path
determination to various networks, they will also determine a
new best path if the initial path becomes unusable (or if the
topology changes)
Dynamic Routing Protocol
Configuring Dynamic Routing Protocols
Dynamic routing of TCP/IP can be implemented using one or
more protocols which are often grouped according to where
they are used.
• Routing protocols designed to work inside an autonomous
system are categorized as interior gateway protocols (IGPs).
• Protocols that work between autonomous systems are
classified as exterior gateway protocols (EGPs).
• Protocols can be further categorized as either distance vector
or link-state routing protocols, depending on their method of
operation.
Path Costs
Path costs are an arbitrary value, based on hop count,
bandwidth, or other calculation, that is typically
assigned by a network administrator and used by the
routing protocol to compare different routes through
an internetwork. Routing protocols use cost values to
select the best path to a certain destination: lowest
cost identifies the best path.
Interior Versus Exterior Routing Protocols
An interior gateway protocol (IGP) is a routing
protocol that is used within an autonomous system
(AS). Two types of IGP.
Distance-vector routing protocols each router does
not possess information about the full network
topology. It advertises its distances to other routers
and receives similar advertisements from other
routers. Using these routing advertisements each
router populates its routing table. In the next
advertisement cycle, a router advertises updated
information from its routing table. This process
continues until the routing tables of each router
converge to stable values.
Interior Versus Exterior Routing Protocols
Distance-vector routing protocols make routing
decisions based on hop-by-hop. A distance vector
router’s understanding of the network is based on its
neighbors definition of the topology, which could be
referred to as routing by RUMOR.
Route flapping is caused by pathological conditions
(hardware errors, software errors, configuration
errors, intermittent errors in communications links,
unreliable connections, etc.) within the network
which cause certain reach ability information to be
repeatedly advertised and withdrawn.
Interior Versus Exterior Routing Protocols
In networks, with distance vector routing
protocols flapping routes can trigger routing
updates with every state change.
Cisco trigger updates are sent when these state
changes occur. Traditionally, distance vector
protocols do not send triggered updates.
Interior Versus Exterior Routing Protocols
Link-state routing protocols, each node possesses
information about the complete network topology.
Each node then independently calculates the best next
hop from it for every possible destination in the
network using local information of the topology. The
collection of best next hops forms the routing table
for the node.
This contrasts with distance-vector routing protocols,
which work by having each node share its routing
table with its neighbors. In a link-state protocol, the
only information passed between the nodes is
information used to construct the connectivity maps.
Routing Protocols
• Interior routing protocols are designed for use
in a network that is controlled by a single
organization
• RIPv1 RIPv2, EIGRP, OSPF and IS-IS are all
Interior Gateway Protocols
Link State Analogy
• Each router has a map of the network
• Each router looks at itself as the center of the
topology
• Compare this to a “you are here” map at the
mall
• The map is the same, but the perspective
depends on where you are at the time You
Link State Routing Protocol
• The link-state algorithm is also known as
Dijkstra's algorithm or as the shortest path first
(SPF) algorithm
• The link-state routing algorithm maintains a
complex database of topology information
• The link-state routing algorithm maintains full
knowledge of distant routers and how they
interconnect. They have a complete picture of
the network
Link State Analogy
Distant Vector Versus Link State
Distant Vectors Routing Protocols
Link State Routing Protocols
RIP (v1 and v2)
OSPF
EIGRP (hybrid)
IS - IS
Exterior Gateway Routing Protocol
An exterior routing protocol is designed for use
between different networks that are under the control
of different organizations
• An exterior routing routes traffic between
autonomous systems
• These are typically used between ISPs or between a
company and an ISP
• BGPv4is the Exterior Gateway Protocol used by all
ISPs on the Internet
EGI and EGP Routing Protocol
What is Convergence
• Routers share information with each other, but
must individually recalculate their own routing
tables
• For individual routing tables to be accurate, all
routers must have a common view of the
network topology
• When all routers in a network agree on the
topology they are considered to have
converged
Why is Quick Convergence Important?
• When routers are in the process of
convergence, the network is susceptible to
routing problems because some routers learn
that a link is down while others incorrectly
believe that the link is still up
• It is virtually impossible for all routers in a
network to simultaneously detect a topology
change.
Convergence Issues
Factors affecting the convergence time include the
following:
• Routing protocol used
• Distance of the router, or the number of hops from the
point of change
• Number of routers in the network that use dynamic
routing protocols
• Bandwidth and traffic load on communications links
• Load on the router
• Traffic patterns in relation to the topology change
What are Autonomous Systems?
• An Autonomous System (AS) is a group of
routers that share similar routing policies and
operate within a single administrative domain.
• An AS can be a collection of routers running a
single IGP, or it can be a collection of routers
running different protocols all belonging to
one organization.
• In either case, the outside world views the
entire Autonomous System as a single entity.
Autonomous System
AS Numbers
• Each AS has an identifying number that is assigned by an
Internet registry or a service provider.
• This number is between 1 and 65,535.
• AS numbers within the range of 64,512 through 65,535are
reserved for private use.
• This is similar to RFC 1918 IP addresses.
• Because of the finite number of available AS numbers, an
organization must present justification of its need before it will
be assigned an AS number.
• An organization will usually be a part of the AS of their ISP
Autonomous System
Autonomous System
• Each AS has its own set of rules and policies.
• The AS number uniquely distinguish it from
other ASs around the world.
Definitions
Metric is a numeric value used by routing
protocols to help determine the best path to a
destination.
RIP uses the metric hop count number . The
lower the numeric value, the closer the
destination.
OSPF uses the metric bandwidth.
EIGRP uses bandwidth
Definitions
• Flat routing protocol is when all routing information
is spread through the entire network.
• Hierarchical routing protocol are typically classless
link-state protocols. Classless link-state protocols
route updates and subnet masks in their routing
updates. Draw diagram
• Administrative distance is the measure used by Cisco
routers to select the best path when there are two or
more different routes to the same destination from
two different routing protocols. Administrative
distance defines the reliability of a routing protocol.
Each routing protocol is prioritized in order of most
to least reliable (believable) using an administrative
distance value. A lower numerical value is preferred.
Administrative Distance
EIGRP Characteristics
EIGRP is an advanced distance vector protocol that
employs the best features of link-state routing.
OSPF Characteristics
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OSPF is the standardized protocol for routing IPv4.
Since it’s initial development, OSPF has been revised
to be implemented with the latest router protocols.
Developed for large networks (50 routers or more)
Must be a backbone area
Routers that operate on boundaries between the
backbone and non-backbone are called, Area Border
Routers (ABR)
OSPF is a link state protocol
OSPF Characteristics
When the OSPF topology table is fully populated, the
SPF algorithm calculates the shortest path to the
destination. Triggered updates and metric calculation
based on the cost of a specific link ensure quick
selection of the shortest path to the destination.
OSPF Characteristics
OSPF is link-state routing protocol
RIP and EIGRP are distance-vector (routing by rumor) routing
protocols, susceptible to routing loops, split-horizon, and other
issues.
OSPF has fast convergence
RIP hold-down timers can cause slow convergence.
OSPF supports VLSM and CIDR
RIPv1 does not
OSPF Characteristics
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Cisco’s OSPF metric is based on bandwidth
RIP is based on hop count
OSPF only sends out changes when they occur.
RIP sends entire routing table every 30 seconds,
IGRP every 90 seconds
• OSPF also uses the concept of areas to implement
hierarchical routing
• A large internetwork can be broken up into multiple
areas for management and route summarization
OSPFCharacteristics
• Two open-standard routing protocols to choose from:
RIP, simple but very limited, or
OSPF, robust but more sophisticated to
implement.
EIGRP is Cisco proprietary
OSPFCharacteristics
Autonomous System Boundary Router (ASBR) is an
area border router placed between an OSPF
autonomous system and a non-OSPF network that
operates both OSPF and an additional routing
protocol, such as RIP. ASBRs must be located in a
non-stub OSPF area.
OSPF Characteristics
OSPF Characteristics
When all routers are configured into a single area, the convention
is to use area 0(zero)
If OSPF has more than one area, it must have an area 0
Multi-area OSPF becomes more complicated to configure and
understand
OSPF Routing Domain
• Single Area OSPF uses only one area, usually Area 0
OSPF Characteristics
1. Flooding of link-state information
The first thing that happens is that each router,
on the network announces its own piece of linkstate information to all other routers on the
network. This includes who their neighboring
routers are and the cost of the link between them.
Example: “Hi, I’m Router A, and I can reach
Router B via a T1 link and I can reach Router C
via an Ethernet link.”
Each router sends these announcements to all of
the routers in the network.
OSPF Characteristics
OSPF Characteristics
2. Building a Topological Database
Each router collects all of this link-state
information from other routers and puts it
into a topological database.
3. Shortest-Path First (SPF), Dijkstra’s
Algorithm
Using this information, the routers can
recreate a topology graph of the network.
Believe it or not, this is actually a very
simple algorithm and I highly suggest you
look at it some time, or even better, take a
class on algorithms.
OSPF Characteristics
4. Shortest Path First Tree
This algorithm creates an SPF tree, with
the router making itself the root of the
tree and the other routers and links to
those routers, the various branches.
5. Routing Table
Using this information, the router creates a
routing table.
Large OSPF Networks
Large link-state table
Each router maintains a LSDB for all links in the area
The LSDB requires the use of memory
Frequent SPF calculations
A topology change in an area causes each router to
re-run SPF to rebuild the SPF tree and the routing
table.
A flapping link will affect an entire area.
SPF re-calculations are done only for changes within
that area.
Issues with large OSPFNetworks
Large routing table
Typically, the larger the area the larger the routing
table.
A larger routing table requires more memory and
takes more time to perform the route look-ups.
Solution: Divide the network into multiple areas
Link-State Advertisement (LSA) is contained inside the
link-state packets (LSPs), these advertisements are
usually multicast packets, containing information
about neighbors and path costs, that are employed be
link-state protocols. Receiving routers use LSAs to
maintain their link-state databases and, ultimately,
routing tables.
OSPF Uses “Areas”
Hierarchical routing enables you to separate large internetworks
(autonomous systems) into smaller internetworks that are called areas.
With this technique, routing still occurs between the areas (called interarea routing), but many of the smaller internal routing operations, such
as recalculating the database –re-running the SPF algorithm, are
restricted within an area
OSPF Uses “Areas”
Changes in one area are
generally not propagated
(spread) to another
Route summarization is
extensively used in multiarea OSPF
OSPF Router Types
OSPF Router Types
Internal: Routers with all their interfaces within
the same area
Backbone: Routers with at least one interface
connected to area 0
ASBR:(Autonomous System Boundary Router):
Routers that have at least one interface
connected to an external internetwork
(another autonomous system)
ABR: (Area Border Router): Routers with
interfaces attached to multiple areas.
IS - IS Characteristics
• IS-IS is an Open System Interconnection (OSI)
routing protocol originally specified by
International Organization for Standardization
(ISO)
• IS-IS is a dynamic, link-state, intra-domain,
interior gateway protocol (IGP)
• IS-IS was designed to operate in an OSI
Connectionless Network Service (CLNS)
environment
• It was not originally designed to work with the
IP protocol
IS - IS Characteristics
• Extensions were added so that IS-IS can route
IP packets
• IS-IS operates at Layer 3 (Network) of the OSI
model
• IS-IS selects routes based upon a cost metric
assigned to links in the IS-IS network
• A two-level hierarchy is used to support large
routing domains
• A large domain can be administratively
divided into areas
OSPF and IS – IS Similarities
• Classless
• Link-state databases an Dijkstra’s algorithm
• Hello packets to form and maintain
adjacencies
• Use areas to form hierarchical topologies
• Support address summarization between areas
• Link-state representation, aging, and metrics
• Update, decision, and flooding processes
• Convergence capabilities
• Deployed on ISP backbones
IS – IS and the OSI Protocol Suite
• The OSI suite of protocols were never widely
implemented at the Layers 3-7 because the
TCP/IP Protocols at these layers became the
de-facto standard.
• Layers 1 and 2 Protocols are widely used:
IEEE 802.3, FDDI, IEEE 802.5, etc.
Link State Routing Analogy
The Link State Database is similar to a map in a
shopping mall. All maps in the mall is the
same, just as the LSDB is the same in all
routers within an area. The only real physical
difference in the maps is “you are at” what
dot. By looking at your position in relation to
where you want to go, you can determine the
best way to get to your destination. Link state
routers calculate the best path to every network
within the area, from their own perspective.
OSI Terminology
• End system (ES) is any non-routing network
node (host)
• Intermediate system (IS) is a router
• An area is a logical entity formed by a set of
contiguous routers, hosts, and the data links
that connect them
• Domain is a collection of connected areas
under a common administrative
authority(think AS)
• The areas are connected to form a backbone
IS – IS is Designed to be Hierarchical
An OSI network is a hierarchy of these entities:
• Domain -any portion of an OSI network under
a common administration
• Area –a part of a domain, broken up for easier
management
• Backbone –areas connect to other areas
through the backbone
IS – IS is Hierarchical
There are four levels of routing:
• Level 0, routing between an ES (end system)
and IS (intermediate system).
• Level 1, routing between ISs in the same area
• Level 2, routing between different areas in the
same domain
• Level 3, routing between separate domains
IS – IS is Hierarchical
Why use IS – IS instead of OSPF?
• IS-IS is more scalable than OSPF because it
uses smaller LSPs (Link-State Protocol) for
advertisements
• Up to 1000 routers can reside in an IS-IS area
versus several hundred for OSPF
• IS-IS is more efficient with its updates and
requires less CPU power
• IS-IS has more timers that can be fine-tuned to
speed up convergence
EIGRP Characteristics
• Cisco proprietary, released in 1994
• EIGRP is an advanced distance-vector routing
protocol that relies on features commonly associated
with link-state protocols. (sometimes called a hybrid
routing protocol)
• Supports VLSM and CIDR
• Uses multicasts for communication –not broadcasts
• Establishes adjacencies with its neighbor routers by
using a Hello protocol
• Keeps all routes in a topology table
• Has speed and efficiency of routing updates like a
link-state protocol
EIGRP Metric Calculation
By default, EIGRP uses only these:
• Bandwidth (carrying capacity)
• Delay (end-to-end travel time)
If these are the default:
• Bandwidth (default)
• Delay (default)
When are these used?
• load
• Reliability
These values are used when the administrator
manually enters them
EIGRP Terminology
• EIGRP uses DUAL, the Diffusing Update Algorithm
to calculate routes –not Bellman-Ford algorithm.
• The lowest cost path to a destination is called the
feasible distance (FD)
• The cost of the route as advertised by the neighboring
router, is called reported distance (RD)
• The best (primary) route to a destination is called the
successor route (successor)
• The next best route, (backup), if there is one, is called
the feasible successor (FS)
EIGRP Tables
The following three tables are maintained by
EIGRP:
• Neighbor table
• Topology table
• Routing table
BGP
BGP is a path vector routing protocol.
Defined in RFC 1772
BGP is a distance vector routing protocol, in that it relies on
downstream neighbors to pass along routes from their routing table.
BGP uses a list of AS numbers through which a packet must pass to
reach a destination.
BGP Basics
•Exchange routing information between autonomous systems
•Guarantee the selection of a loop free path.
BGP4 is the first version of BGP that supports CIDR and route
aggregation.
Common IGPs such as RIP, OSPF, and EIGRP use technical metrics.
•BGP does not use technical metrics.
•BGP makes routing decisions based on network policies, or rules
(later)
•BGP does not show the details of topologies within each AS.
•BGP sees only a tree of autonomous systems.
BGP Basics
• BGP updates are carried using TCP on port 179.
In contrast, RIP updates use UDP port 520
OSPF, IGRP, EIGRP does not use a Layer 4
protocol
• Because BGP requires TCP, IP connectivity must
exist between BGP peers.
• TCP connections must also be negotiated between
them before updates can be exchanged.
• Therefore, BGP inherits those reliable, connectionoriented properties from TCP.
Loop Free Path
To guarantee loop free path selection, BGP constructs a graph of autonomous
systems based on the information exchanged between BGP neighbors.
BGP views the whole internetwork as a graph, or tree, of autonomous systems.
The connection between any two systems forms a path.
The collection of path information is expressed as a sequence of AS numbers called
the AS Path.
This sequence forms a route to reach a specific destination
BGP Operation
When two routers establish a TCP-enabled BGP connection
between each other, they are called neighbors or peers.
Each router running BGP is called a BGP speaker.
Upcoming Deadlines
• Mimic Simulator Lab Assignment 4-1-2, Basic
Routing and LAN Switching Configuration
due June 27, 2011
• Assignment 1-4-3 Data Center Design Project
Phase 3: Data Center Network Design due July
11, 2011.
• Assignement 10-1 Concept Questions 7 is due
July 4, 2011.