Ex 10 OSPF
Transcript of Ex 10 OSPF
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Distance Vector vs. Link State Routing
With distance vector routing, each node has
information only about the next hop: Node A: to reach F go to B
Node B: to reach F go to D
Node D: to reach F go to E
Node E: go directly to F
Distance vector routing makes
poor routing decisions if
directions are not completely
correct
(e.g., because a node is down).
If parts of the directions incorrect, the routing may be incorrect until therouting algorithms has re-converged.
A B C
D E F
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Distance Vector vs. Link State Routing
In link state routing, each node has a complete map of the
topology
If a node fails, each
node can calculate
the new route
Difficulty: All nodes need to
have a consistent view of the
network
A B C
D E F
A B C
D E F
A B C
D E F
A B C
D E F
A B C
D E F
A B C
D E F
A B C
D E F
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Link State Routing: Properties
Each node requires complete topology information
Link state information must be flooded to all nodes
Guaranteed to converge
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Link State Algorithm
Each router contains a database containing a map ofthe whole topology Links
Their state (including cost) All routers have the same information
All routers calculate the best path to every destination
Any link state changes are flooded across the
network ´Global spread of local knowledgeµ
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Link State Routing
Each router Distributes LSA (Link StateAnnouncement/Advertisement) to other routers
Each router computes its best path to everydestination
On network failure
New LSAs are flooded
All routers recompute routing table Only changes are propagated
R1
LSA
X
LSA
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OSPF: How it works
Hello Protocol
Responsible for establishing and maintaining neighbour
relationships
Hello Packets sent periodically on all OSPF enabledinterfaces
Contains information like
Hello Interval
A list of known neighbours Router Dead Interval
Hello
HelloHello
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1 ± Flooding of link-state
information
2 ± Building a
Topological
Database
3 ± SPF Algorithm
4 ± SPF Tree
5 ± Routing Table
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OSPF: How it works
Trade Information using LSAs
Each router builds an identical link state database
SPF algorithm run on the database Forwarding table built from the SPF tree
When change occurs:
Announce the change to all OSPF neighbours
All routers run the SPF algorithm on the revised
database
Install any change in the forwarding table
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OSPF (Open Shortest Path First)
An interior routing protocol
Dividing an AS into areas
to handle routing efficiently and in a timely manner
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OSPF
Areas
is a collection of networks, hosts, and routers in AS
AS can be divided into many different areas.
Routers inside an area flood the area with routing information.
Area Border Router
summarizing the information about the area and send it to otherareas
Backbone
All of the areas inside an AS must be connected to the backbone
serving as a primary area
consisting of backbone routers
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OSPF Cost
Cost is the OSPF metric used in path selection
Cost is based on bandwidth
Default bandwidth is 1.544Mbps ² cost is 64
Cost can be 1 (100Mbps) to 65535
Cost is 108÷ bandwidth
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OSPF
Advantages
Changes in an OSPF network arepropagated quickly.
OSPF is hierarchical, using area 0 as thetop as the hierarchy.
OSPF is a Link State Algorithm.
OSPF supports Variable Length SubnetMasks (VLSM).
OSPF uses multicasting within areas.
After initialization, OSPF only sendsupdates on routing table sections whichhave changed; it does not send the entirerouting table.
Using areas, OSPF networks can belogically segmented to decrease the sizeof routing tables. Table size can befurther reduced by using routesummarization.
OSPF is an open standard, not related toany particular vendor.
Disadvantages
OSPF is very processor intensive.
OSPF maintains multiple copies of routinginformation, increasing the amount ofmemory needed.
OSPF is not as easy to learn as someother protocols.
In the case where an entire network isrunning OSPF, and one link within it is"bouncing" every few seconds, OSPFupdates would dominate the network byinforming every other router every timethe link changed state
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Wildcard Mask
A wildcard mask is a mask of bits that indicates which parts of an IPaddress can assume any value.
In the Cisco IOS, they are used in several places, for example:
To indicate the size of a network for some routing protocols, such as OSPF.
To indicate what IP addresses should be permitted or denied in accesscontrol lists (ACLs).
A wildcard mask can be thought of as a subnet mask, with ones andzeros inverted
For example, a wildcard mask of 0.0.0.255 corresponds to a subnetmask of 255.255.255.0.