LAN Switching and WAN Networks
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LAN Switching and WAN Networks
Topic 6 - OSPF
What we have done so far!
19/04/23Richard Hancock2
Looked at the basic switching concepts and configuration from the Cisco IOS CLI
Looked at VLANs and seen how to configure basic VLANs on switches
Looked at VLAN trunking and seen how to setup trunks between switches and routers
Looked at switch port security features Looked at the Virtual Trunking Protocol
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Objectives
19/04/23Richard Hancock3
Be able to describe OSPF characteristics Be able to define what an area is Be able to describe what the OSPF metric is a
nd how it is used Be able to describe how a router using OSPF is
uniquely identified Be able to describe how a router using OSPF f
orm adjacencies with out routers also using OSPF
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OSPF characteristics
19/04/23Richard Hancock4
Developed in reference to the limitation of RIP in large enterprise networks
Based on open standards Runs on most routers Uses the SPF algorithm to provide a loop-free
topology Fast convergence with triggered and incremental
updates via LSA’s (it’s a link state protocol) Classless protocol allowing for VLSM and route
summarisation However, requires more memory, extra CPU
processing power, careful design, complex to troubleshoot and configure (multi-area designs)
OSPF areas
19/04/23Richard Hancock5
OSPF uses Autonomous Systems and areas Areas basically control when and how much
routing information is shared across a network Area 0 is the backbone Areas 1-65,535 are “areas off the backbone”
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OSPF areas
19/04/23Richard Hancock6
Route summarisation would mean that Area 1, 2 and 3 do not need to know all the subnets in each others area, only their own areas
Metric
19/04/23Richard Hancock7
OSPF uses cost Cost is the inverse of the bandwidth of a link
The faster the speed of the link, the lower the cost Preferred path is the one with the lowest cost
OSPF supports load-balancing Six equal-cost paths to a single destination
On synchronous serial links the bandwidth defaults to 1544Kbps despite the clock rate settings Bandwidth needs configuring specifically so that
load-balancing works properly This is important is there are multiple synchronous
serial paths to a destination and the paths have different clock rates
Router identities
19/04/23Richard Hancock8
Each router in an OSPF network requires a unique ID
The ID is included in OSPF messages The ID is based on:
The highest IP address on any loopback interfaces, OR
The highest IP address on it’s active interfaces If there are no active interfaces or loopback
interfaces then the OSPF process will not start Loopback interfaces are recommended
because they are always up
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Finding neighbours
19/04/23Richard Hancock9
OSPF uses LSA’s to learn about neighbours OSPF generates hello LSA’s every 10 seconds If a router does not receive a hello LSA within 40 seconds
from a neighbour it declares that neighbour dead OSPF routers build an adjacency that makes them
neighbours To do this the following must match on both routers:
The area number and its type The hello and dead interval timers The OSPF password, if configured The area stub tag
OSPF routers go through three states to form an adjacency: Down state – no exchanges Init state – destination router receives a hello and adds it to it’s
neighbour list Two-way state – new and destination routers exchange hello
packets
Designated and backup routers
19/04/23Richard Hancock10
OSPF also uses a client/server design when establishing adjacencies
Each segment will have a designated router (DR) and a backup designated router (BDR)
New routers form adjacencies with the DR and BDR A router talks to the DR using 224.0.0.6 DR and BDR talk to routers using 224.0.0.5
The router with the highest router ID (IP address) becomes the DR BDR is based upon the next highest router ID (IP
address) If the DR fails the BDR takes over and another
router becomes the BDR
Summary
19/04/23Richard Hancock11
OSPF is an open routing protocol and very popular
It works on most routers It uses areas to control how much routing
information is passed around a network It uses cost (an inverse of bandwidth) as it’s
metric It uniquely identifies a router using either the
highest IP address on a loopback interface or the highest IP address on an active interface
Using a loopback address for identity purposes is recommended
Tutorial
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Click on the icon below to run the tutorial and work through it until completion
Tutorial
So, what do you know now?
19/04/23Richard Hancock13
How does a router in an OSPF network become a designated router?
What happens if a designated router fails How often are OSPF LSA hello messages sent? What happens if a neighbour doesn't receive a
hello LSA packet from it’s neighbour after 40 seconds?
What must be configured to prevent errors with the load balancing feature of OSPF?
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How all this relates to the assignment
19/04/23Richard Hancock14
You will need to know how to configure OSPF for the case study and the skills test
You need to be able to perform these configuration on physical Cisco kit
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Questions...
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...are there any?
19/04/23Richard Hancock16
End!
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