Dynamic Routing and OSPF

1

• Dynamic Routing and OSPF
• (part 1)

2
IP routing

• Each router or host makes its own routing
  decisions
• Sending machine does not have to determine the
  entire path to the destination
• Sending machine just determines the next-hop
  along the path.
• This process is repeated until the destination is
  reached
• Forwarding table consulted to determine the
  next-hop

3
IP routing

• Classless routing
• route entries include
• destination
• next-hop
• mask (prefix-length) indicating size of address
  space described by the entry
• Longest match
• for a given destination, find longest prefix
  match in the routing table
• example destination is 35.35.0.0/19
• routing table entries are 35.0.0.0/8 and
  35.35.0.0/16

4
IP routing

• Default route
• where to send packets if dont have an entry for
  the destination in the routing table
• most machines have a single default route
• often referred to as a default gateway

5
Static routing

• each router manually configured with a list of
  destinations and the next hop to reach those
  destinations
• ideal for small number of destinations or stub
  networks
• stub network - network with only one or two paths
  to the rest of the network

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Dynamic Routing

• routers compute routing tables dynamically based
  on information provided by other routers in the
  network
• routers communicate topology to each other via
  different protocols
• routers then compute one or more next hops for
  each destination - trying to calculate the most
  optimal path

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Static and Dynamic Routing

• Static routing is a simplistic approach
• Shortcomings
• Cumbersome to configure
• Cannot adapt to link/node failures, addition of
  new nodes and links
• Doesn't scale to large networks
• Solution Dynamic Routing

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Desirable Characteristics

• Automatically detect and adapt to network
  topology changes
• Optimal routing
• Scalability
• Robustness
• Simplicity
• Speed of convergence
• Some control of routing choices (e.g. which links
  we prefer to use)

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Convergence - Why do I care?

• Convergence is when all the routers have the same
  routing information
• When a network is not converged, there is network
  downtime
• Packets don't get to where they are supposed to
  be going routing loops, black holes
• Occurs when there is a change in the status of a
  router or link

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Dynamic Protocols

• Metrics can be calculated based on a single
  characteristic of a path or by combining multiple
  characteristics
• Metrics commonly used
• Bandwidth
• Hop count
• Cost
• administratively defined metrics

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OSPF magic exercise

• delete your static routes
• config t
• no ip route x.x.x.x y.y.y.y z.z.z.z
• enter the following
• router ospf 1
• network x.x.x.x 0.0.0.0 area 0
• x.x.x.x ip address of your backbone interface
• redistribute connected subnets

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OSPF magic exercise

• Verify connectivity to all PCs in the network
• Do not save your config

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Dynamic Routing Protocols and OSPF (part 2)
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Types of Routing Protocols

• EGP
• Exterior Gateway Protocol
• Example BGP
• IGP
• Interior Gateway Protocol
• Example OSPF, RIP

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Types of Routing Protocols

• Link-state
• Distance-vector

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IGP

• Used within a single Autonomous System (AS)
• Within a single network

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Other Interior Gateway Protocols (IGPs)

• RIP
• Lots of scaling problems
• RIPv1 is classful and officially obsolete
• RIPv2 is classless
• EIGRP
• Proprietry (Cisco only)
• IS/IS
• The forerunner of OSPF
• Multiprotocol (OSPF is IP only)

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Distance Vector Protocols

• Listen to neighboring routes
• Install all routes in a table
• Advertise all routes in table
• Very simple
• Very Stupid
• example RIP

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RIP

• routing information protocol
• distance-vector algorithm
• cost is hop count
• broadcast information to all neighbors every 30
  seconds

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RIP
ROUTING TABLE for A A - B 1 C 2 D 3 E 2
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Why not use RIP?

• Distance Vector algorithm
• Broadcasts everything (not scalable)
• Metric is hop-count only
• Infinity of 16 (not large enough)
• Slow convergence (routing loops)
• Poor robustness

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OSPF

• Open Shortest Path First
• Dynamic IGP (Interior Gateway Protocol)
• Use within your own network
• Link state algorithm

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Shortest Path First
Metric Link Cost
3
A
B
15
4
4
C
D
7
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Link State Algorithm

• Each router maintains a database containing map
  of the whole topology
• Links
• State (including cost)
• All routers have the same information
• All routers calculate the best path to every
  destination

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Link State Algorithm (con)

• Any link state changes are flooded across the
  network
• "Global spread of local knowledge

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Link State vs. Distance vector

• Distance Vector
• views net topology from neighbors perspective
• adds distance vectors from route to router
• frequent, periodic updates slow convergence
• passes copies of routing table to neighbor routers

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Link State vs. Distance vector

• Link-State
• gets common view of entire network topology
• calculates the shortest path to other routers
• event-triggered updates faster convergence
• passes link-state routing updates to other routers

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Distance Vector and Link State Protocols

• Distance vector routers compute the best path
  from information passed to them from neighbors
• Link State routers each have a copy of the entire
  network map
• Link State routers compute best routes from this
  local map

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Note Routing is not the same as Forwarding

• Forwarding passing packets along to the next hop
• There is only one forwarding table
• Just has prefix and next-hop info
• Routing populating the forwarding table
• You might have multiple routing databases - e.g.
  both OSPF and BGP
• Routing databases have more information

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Routing and Forwarding
BGP
OSPF
Static
Forwarding Table
On Cisco, if the same prefix is received from
multiple protocols, the "administrative distance"
is used to choose between them
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OSPF

• open shortest path first
• dynamic IGP
• not distance vector
• Link-State algorithm

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OSPF How it works (1)

• "Hello" packets sent periodically on all
  OSPF-enabled interfaces
• become "neighbors"
• establishes that link can carry data
• used to determine if neighbor is up
• Adjacencies (virtual point-to-point links) formed
  between some neighbors

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How it works (2)

• Once an adjacency is established, trade
  information with your neighbor
• Topology information is packaged in a "link state
  announcement"
• Announcements are sent ONCE, and only updated if
  there's a change (or every 30 minutes)

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How it works (3)

• Each router sends Link State Announcements (LSAs)
  over all adjacencies
• LSAs describe router's links, interfaces and
  state
• Each router receives LSAs, adds them into its
  database, and passes the information along to its
  neighbors

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How it works (4)

• Each router builds identical link-state database
• Runs SPF algorithm on the database to build SPF
  tree
• Forwarding table built from SPF tree

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How it works (5)

• When change occurs
• Broadcast change
• All routers run SPF algorithm
• Install output into forwarding table

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HELLO

• Broadcast HELLO on network segment
• Receive ACK
• Establishes 2-way communication
• Repeat periodically
• Default HELLO sent every 10 seconds
• Default if no HELLO heard for 40 seconds, link
  is assumed to be dead
• Now establish adjacencies

Actually uses Multicast addresses (224.0.0.9,
224.0.0.10) so that non-OSPF devices can ignore
the packets
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The HELLO packet
HELLO
HELLO
HELLO

• Router priority
• Hello interval
• Router dead interval
• Network mask
• List of neighbors

These must match
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Neighbors

• Bi-directional communication
• Result of OSPF hello packets
• Need not exchange routing information

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Who is adjacent?

• "Adjacent" neighbors exchange routing information
• Not all neighbors are adjacent
• On a point-to-point link
• everyone
• On broadcast medium
• not everyone
• why?

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Broadcast neighbors
Order of N2 adjacencies
A
B
C
D
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Broadcast medium

• Select a neighbor Designated Router (DR)
• All routers become adjacent to DR
• Exchange routing information with the DR
• DR updates all the other neighbors
• Scales
• Adjacencies reduced from N2 to 2N
• Backup Designated Router (BDR)

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LSAs propagate along adjacencies
DR
BDR
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Other nice features of OSPF

• Authentication (optional)
• Equal-cost multipath
• more than one "best" path - share traffic
• Proper classless support (CIDR)
• Multiple areas
• For very large networks (gt150 routers)
• Aggregate routes across area boundaries
• Keep route flaps within an area
• Proper use of areas reduce bandwidth and CPU
  utilisation
• Backbone is Area 0

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Cisco OSPF commands and configuration

• show ip route
• show ip ospf neighbor
• show ip ospf database

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Configuring OSPF

• router ospf ltprocess-idgt
• network x.x.x.x m.m.m.m area ltarea-idgt
• m.m.m.m wildcard mask
• 0 dont care bit
• 1 check bit
• 0.0.0.0 mask for exact match
• network 203.167.177.10 0.0.0.0 area 0
• network 203.167.177.0 0.0.0.255 area 0

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Classroom Layout
A
B
Router
PC
Router
PC
C
D
PC
Router
PC
Router
F
E
PC
Router
PC
Router
G
H
PC
Router
PC
Router
I
J
PC
Router
PC
Router
SWITCH
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Serial Links for exercise
A
B
133.27.162.96/28
133.27.162.112/28
133.27.162.48/30
133.27.162.60/30
C
D
133.27.162.128/28
133.27.162.144/28
F
E
133.27.162.16/28
133.27.162.160/28
133.27.162.176/28
133.27.162.52/30
133.27.162.64/30
G
H
133.27.162.192/28
133.27.162.208/28
I
J
133.27.162.224/28
133.27.162.240/28
133.27.162.56/30