introduction to lans - justanswer(1).pdf · -each router builds its own link state packet (lsp)...
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TDC 363Introduction to LANs
Greg BrewsterDePaul University
OSPF
TDC 363 Greg Brewster, DePaul University 1
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Link State Routing Algorithms Open Shortest Path First (OSPF)
Message Types Operations Configuration and Verification
OSPF
Greg Brewster, DePaul University 2TDC 363
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Link-State Routing Link state routing protocols
-Also known as shortest path first algorithms
-These protocols built around Dijkstras SPF
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Link-State RoutingLink-State Routing Process How routers using Link State Routing Protocols reach
convergence-Each routers learns about its own directly connected networks
-Link state routers exchange Hello packet to meet other directly connected link state routers.
-Each router builds its own Link State Packet (LSP) which includes information about links and neighbors such as neighbor ID, link type, & bandwidth.
-After the LSP is created the router floods it to all neighbors who then store the information and then forward it until all routers have the same information.
-Once all the routers have received all the LSPs, the routers then construct a topological map of the entire network which is used to determine the best routes to all destinations.
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Link-State RoutingSending Hello Packets to
Neighbors Connected interfaces that are using
the same link state routing protocols will exchange hello packets.
Once routers learn it has neighbors they form an adjacency
-2 adjacent neighbors will exchange hello packets
-These packets will serve as a keep alive function
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Link-State RoutingBuilding the Link State Packet
Each router builds its own Link State Packet (LSP)
Contents of LSP:
-State of each directly connected link
-Includes information about neighbors such as neighbor ID, link type, & bandwidth.
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Link-State Routing LSPs are sent out under the following conditions
-Initial router start up or routing process-When there is a change in topology
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Link-State RoutingConstructing a link state data base
Routers use a database to construct a topology map of the network
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Link-State Routing
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Link-State Routing Determining the shortest path
The shortest path to a destination determined by adding the costs & finding the lowest total path cost
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Link-State Routing Once the SPF algorithm has determined the shortest
path routes, these routes are placed in the routing table.
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Link-State Routing Protocols
Routingprotocol
BuildsTopological
map
Router can independently determine the
shortest path to every network.
ConvergenceA periodic/
event driven routing updates
Useof
LSP
Distance vector
No No Slow Generally No No
Link State Yes Yes Fast Generally Yes Yes
Advantages of a Link-State Routing Protocol
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Link-State Routing ProtocolsRequirements for using a link state routing protocol Memory requirements
Typically link state routing protocols use more memory
Processing RequirementsMore CPU processing is required of link state routing protocols
Bandwidth RequirementsInitial startup of link state routing protocols can consume lots of bandwidth
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1. A Link State Internal Gateway Protocol used within most Autonomous Systems on the Internet
2. Advantages over RIP (distance-vector protocol) a) Supports many distance metrics, including physical distance, transmission
delay, queueing delay, etc. b) Adapts to changes in topology quickly. c) Supports multiple types of service such as least delay, highest throughput,
etc. d) Does load balancing over multiple paths to the same destination. e) Supports hierarchical systems:
i) Autonomous Systems ii) Areas iii) Networks iv) Subnets
f) Improved security.
Open Shortest Path First (OSPF)
Greg Brewster, DePaul University 14TDC 363
1. A Link State Internal Gateway Protocol used within most Autonomous Systems on the Internet
2. Advantages over RIP (distance-vector protocol)
a) Supports many distance metrics, including physical distance, transmission delay, queueing delay, etc.
b) Adapts to changes in topology quickly.
c) Supports multiple types of service such as least delay, highest throughput, etc.
d) Does load balancing over multiple paths to the same destination.
e) Supports hierarchical systems:
i) Autonomous Systems
ii) Areas
iii) Networks
iv) Subnets
f) Improved security.
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OSPF Link State Database Each OSPF router maintains a Link State
Database containing, for each link in the network: The Link Type Router IDs of routers connected to this link IP subnet address for this link IP subnet mask for this link Link cost Link status (up or down)
Greg Brewster, DePaul University 15TDC 363
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Basic OSPF Configuration
Router IDThis is a single IP address used to identify a router3 methods for determining the router IDConfigure with OSPF router-id command
-Takes precedence over other methodsIf router-id command not used then router chooses highest IP address of any loopback interfaces Commonly set up Loopback0 IP to become Router ID
If no loopback interfaces are configured then the highest IP address on any active interface is used Using active interface IP address is not best practice because Router ID may change if interface goes down.
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Basic OSPF ConfigurationOSPF Router ID Commands used to verify current router ID
Show ip protocolsShow ip ospfShow ip ospf interface
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OSPF Hierarchical Networks Autonomous Systems
Owned by single organization Areas
Collection of subnets and routers within an AS. Each area has an Area Number.
Routers inside an area flood information to all other routers in area
Area border routers summarize area information to pass to another area
All areas connect to backbone area (0). For TDC 363, we only study single-area OSPF all
subnets and routers will be in Area 0.Greg Brewster, DePaul University 18TDC 363
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Figure 13-17
OSPF Hierarchical Networks
Backbone router: connect only to networks in the backbone area (Area 0)
Area border router: attaches to multiple areas.
Internal router: all interfaces connected to networks in the same area.
AS boundary router (Also known as Border Router): connect to other ASes
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Introduction to OSPFOSPF Packet Types
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OSPF HELLO Packet
When router first powers up or comes online, it will first discover neighbors Multicast HELLO messages on all its
interfaces Listen for HELLO messages in response
from neighbor routers Store neighbor router information in
Adjacency Database.
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OSPF Router Tables / Databases
OSPF maintains three databases which are used to create three tables.
Database Table Description
Adjacency Database
Neighbor Table
List of all neighbors routers to which a router has established bidirectional communication.
This table is unique for each router. Can be viewed using the show ip ospf neighbor command.
Link-state Database
Topology Table
List of information about all other routers in the network. The database shows the network topology. All routers within an area have identical link-state databases. Can be viewed using the show ip ospf database command.
Forwarding Database
Routing Table
List of routes generated when an algorithm is run on the link-state database.
Each routers routing table is unique and contains information on how and where to send packets to other routers.
Can be viewed using the show ip route command.
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Introduction to OSPF Hello Packets
Maintain adjacencies and set Designated Router
OSPF Hello IntervalsUsually multicast (224.0.0.5)Sent every 10 seconds in generalSent every 30 seconds for NBMA
OSPF Dead IntervalsThis is the time that must transpirebefore the neighbor is considered down
Default time is 4 timesthe hello interval
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OSPF DATABASE DESCRIPTION Message
When other routers receive the HELLO message: If this is the first time they have heard from
this router, they will send DATABASE DESCRIPTION message
This message gives a summary outline of all information in the Link State Database of the router
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OSPF LINK STATE REQUEST Message
When a router wants more information about a particular set of links, it will send a LINK STATE REQUEST message to its neighbor.
Neighbor will respond with LINK STATE UPDATE message containing requested information
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OSPF LINK STATE UPDATE Message
When any link state changes (link goes up or down, cost changes, etc.) a router will flood a LINK STATE UPDATE message to all other routers in the Area. Link State Update contains one or more
Link State Advertisements (LSAs) from the router.
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LSU Flooding When any router receives a Link State
Update message, it checks whether this is a new update message If it is a New LSU message, then this router
will flood the message (transmit a copy out all interfaces except the one it arrived on)
If it is an Old LSU message (seen before), then this router will drop the message
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New or Old? A router determines whether an arriving LSU
message is new or old using two header fields Sequence Number
Each router remembers the Sequence Number of the last LSU seen from every other router
Each router increments Sequence Number when sending out a new LSU
Age Routers update Age field with estimated transit
time. Messages dropped when Age gets too large
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OSPF Types of Links Point-to-Point Link
Directly connects two routers with no other host or router inbetween.
Transient Link A Link with multiple routers on it (example: a
LAN) Stub Link
A link with only one router on it Virtual Link
A multi-hop connection between two routers (a tunnel) used for backup if direct link goes down.
Greg Brewster, DePaul University 29TDC 363
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Point-to-Point Links Each of the two routers measures the
metric from itself to its neighbor. Result: there are 2 metrics for each point-
to-point link one for data sent in each direction. May be same value or different.
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Transient Links and DRs Many routers connect to the same link
(multiaccess link), such as in a LAN. Problem: Its too much work to measure metrics
between all possible pairs of routers Solution:
Choose one router to be designated router (DR) Other routers only form adjacencies with DR. Cost metric is assigned from each router to designated
router. No cost from designated router to others. Designated Router is elected based on Priority value. Backup Designated Router is also elected and takes over
if DR fails.
Greg Brewster, DePaul University 31TDC 363
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OSPF in Multiaccess Networks Solution to LSA flooding issue
is the use of Designated router (DR)Backup designated router (BDR)
DR & BDR selectionRouters are elected to send & receive LSA
Sending & Receiving LSADRothers send LSAs via multicast 224.0.0.6 to DR & BDRDR forward LSA via multicast address 224.0.0.5 to all other routers
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OSPF in Multiaccess Networks
Criteria for getting elected DR/BDR1. DR: Router with the highest OSPF
interface priority.2. BDR: Router with the second highest
OSPF interface priority. 3. If OSPF interface priorities are equal, the
highest router ID is used to break the tie.
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OSPF in Multiaccess Networks
Timing of DR/BDR ElectionOccurs as soon as 1st router has its interface enabled on multiaccess networkWhen a DR is elected it remains as the DR until one of the following occurs
-The DR fails.-The OSPF process on the DR fails.-The multiaccess interface on the DR fails.
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OSPF in Multiaccess NetworksOSPF Interface Priority Manipulating the DR/BDR election process continued
Use the ip ospf priority interface command.Example:Router(config-if)#ip ospf priority {0 - 255}Priority number range 0 to 255
0 means the router cannot become the DR or BDR1 is the default priority value
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Stub Links A stub link is just a special case of a
transient network where the number of routers on the network is 1
Stub links are advertised in LSAs sent by connected router.
Greg Brewster, DePaul University 36TDC 363
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OSPF Protocol Overview When router first powers up, it will
Determine who its neighbor routers are. Download a complete Link State Database
from a neighbor router When something changes in the network
(link goes up or down, cost metric changes, etc.) Discovering router will flood Link State
Advertisements (LSAs) to all other routers in the Area to update their databases
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Link State Advertisement
An LSA packet announces the state of all links connected to a router
Four types of LSAs: Router Link LSA Network Link LSA Summary Link LSA (multi-area OSPF) External Link LSA (multi-area OSPF)
Greg Brewster, DePaul University 38TDC 363
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Router Link LSA Advertises the state of the links connected
to a true router Can be any of the 4 link types
Greg Brewster, DePaul University 39TDC 363
Figure 13-25
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Network Link LSA Advertises the state of the links connected
to a designated router
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Summary Link LSA
Two Types (not required for TDC 363): Summary Link to Network LSA
Used to provide a summary of all link information for one particular Area to all routers in another Area in the same Autonomous System
Summary Link to AS Boundary Router LSA Advertises a path to an AS Boundary router so that
Area routers can learn how to get outside of their own Autonomous System
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OSPF Metric OSPF uses path cost as metric for determining best route.
Path Cost = sum of link costs on path to destination. -Link Cost is based on bandwidth of link interfaceInterface bandwidth parameter can be changed by admin. It is not affected by clock rate command.Link Cost is calculated using the formula
Link Cost = 108 / (Interface Bandwidth)Default Link Costs
Eth interface: 10FastEth interface: 1GigEth interface: 1Serial Interface: 64
(regardless of clock rate)
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OSPF Metric COST of an OSPF route
Is the accumulated value from one router to the next
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Basic OSPF ConfigurationModifying the Cost of a link Both sides of a serial link should be configured with the
same bandwidthCommands used to modify bandwidth valueBandwidth command
Example: Router(config-if)#bandwidth bandwidth-kbpsip ospf cost command allows you to directly specify interface cost
-Example:R1(config)#interface serial 0/0/0R1(config-if)#ip ospf cost 1562
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OSPF Configuration router ospf (global config)
Creates OSPF route database, starts OSPF routing process number that manages OSPF messages, maintains database, and copies routes from database into routing table.
network netmask area Causes OSPF to advertise Subnet ID with wildcard
(inverse subnet mask) in OSPF area number and to send/receive OSPF Hellos and LSUs on all interfaces that directly connect to this subnet.
The subnet specified in the network command should be directly connected to this router.
Two routers connected to same link will only become OSPF neighbors if both of them execute network statements for the common link subnet in their OSPF configurations.
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Configure Single-area OSPFv2The network Command
This activates OSPF (sends out HELLOs) on interfaces connected to subnets 172.16.1.0/24, 172.16.3.0/30 and 192.168.10.4/30. It also includes these 3 subnets in OSPF database and OSPF Update messages.
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Specific Network Statements Type a separate specific OSPF network statement for
each directly-connected subnet on the router. Do not type a network statement for any remote subnet that is
not directly connected. Do not combine multiple subnets into a single network
statement. Example: 172.16.5.0/24 &172.16.6.0/24 are directly attached:
Correct: network 172.16.5.0 0.0.0.255 area 0 network 172.16.6.0 0.0.0.255 area 0
Wrong: (this will work in some cases, not others - you lose points in any case) network 172.16.0.0 0.0.255.255 area 0
It will always be wrong to type: network 0.0.0.0 255.255.255.255 area 0 Sometimes this works, sometimes it messes things up. But it is always sloppy and
not acceptable.
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Basic OSPF Configurationenableconfigure terminalinterface loopback0 ! For OSPF IDip address 10.1.1.1 255.255.255.255no shutdowninterface ethernet0/0ip address 130.88.55.1 255.255.255.0no shutdowninterface serial0/0ip address 130.88.56.1 255.255.255.0no shutdownrouter ospf 1network 130.88.55.0 0.0.0.255 area 0network 130.88.56.0 0.0.0.255 area 0exitexit
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OSPF Configuration - Optional router ospf (global config)
passive-interface Disables sending route updates out the specified interface
router-id A.B.C.D Sets OSPF router-id to A.B.C.D
ospf log-adjacency-changes Shows a message on router console any time an OSPF
neighbor goes up or down.
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By default, OSPF messages are forwarded out all OSPF-enabled interfaces. However, these messages really only need to be sent out interfaces connecting to other OSPF-enabled routers.
Sending out unneeded messages on a LAN affects the network in three ways: Inefficient Use of Bandwidth Inefficient Use of Resources Increased Security Risk
OSPF messages are not sent out a Passive Interface, but its IP subnet is included in OSPF database and in Update messages sent out other interfaces.
The Passive Interface feature helps limit the scope of routing update advertisements.
Configure Single-Area OSPFv2Passive Interface
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Configure Single-area OSPFv2Configuring Passive Interfaces
Use the passive-interface router configuration mode command to prevent the transmission of routing messages through a router interface, but still allow that network to be advertised to other routers.
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OSPF Interface Configuration(optional)
ip ospf cost Sets the cost for the link connected to this interface to
ip ospf retransmit-interval Sets the time between Link State Advertisement retransmissions
to seconds.
ip ospf priority Sets the OSPF priority of this interface to . Highest
priority interface on transient network will be designated router.
ip ospf hello-interval Sets the time between HELLO packets to seconds.
ip ospf dead-interval If no HELLO packets seen for seconds, then neighbor
router is declared down.Greg Brewster, DePaul University 52TDC 363
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OSPF Interface Configuration
enableconfigure terminalinterface ethernet0/0ip address 130.88.55.1 255.255.255.0ip ospf cost 10ip ospf priority 5no shutdowninterface serial0/0ip address 130.88.56.1 255.255.255.0no shutdownrouter ospf 1network 130.88.55.0 0.0.0.255 area 0network 130.88.56.0 0.0.0.255 area 0exitexit
Greg Brewster, DePaul University 53TDC 363
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OSPF Verification/Debug show ip protocols show ip ospf interface show ip ospf database show ip ospf router debug ip ospf ? adj - OSPF adjacency events events - OSPF events flood - OSPF flooding lsa-generation - OSPF lsa generation packet - OSPF packets retransmission - OSPF retransmission events tree - OSPF database tree
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Greg Brewster, DePaul University 54TDC 363
TDC 363Introduction to LANsLink-State RoutingLink-State RoutingLink-State RoutingLink-State RoutingLink-State RoutingLink-State RoutingLink-State RoutingLink-State RoutingLink-State RoutingLink-State Routing ProtocolsLink-State Routing ProtocolsOpen Shortest Path First (OSPF)OSPF Link State DatabaseBasic OSPF ConfigurationBasic OSPF ConfigurationOSPF Hierarchical NetworksSlide Number 19Introduction to OSPFOSPF HELLO PacketOSPF Router Tables / DatabasesIntroduction to OSPFOSPF DATABASE DESCRIPTION MessageOSPF LINK STATE REQUEST MessageOSPF LINK STATE UPDATE MessageLSU FloodingNew or Old?OSPF Types of LinksPoint-to-Point LinksTransient Links and DRsOSPF in Multiaccess NetworksOSPF in Multiaccess NetworksOSPF in Multiaccess NetworksOSPF in Multiaccess NetworksStub LinksOSPF Protocol OverviewLink State AdvertisementRouter Link LSANetwork Link LSASummary Link LSAOSPF MetricOSPF MetricBasic OSPF ConfigurationOSPF Configuration Configure Single-area OSPFv2The network CommandSpecific Network StatementsBasic OSPF ConfigurationOSPF Configuration - OptionalConfigure Single-Area OSPFv2Passive Interface Configure Single-area OSPFv2Configuring Passive InterfacesOSPF Interface Configuration(optional)OSPF Interface ConfigurationOSPF Verification/Debug