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Dynamic Routing Protocols
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Function(s) of Dynamic Routing Protocols:
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Dynamically share information between routers (Discover
remote networks).
Automatically update routing table when topology changes
(Maintaining up-to-date routing information).
Determine best path to a destination networks.
Ability to find a new best path if the current path is no
longer available
Dynamic Routing Protocols
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Components of a routing protocol
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Algorithm
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In the case of a routing protocol algorithms are used for
facilitating routing information and best path determination
Routing protocol messages
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These are messages for discovering neighbors and exchange of
routing information
Classifying Routing Protocols
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Dynamic routing protocols are grouped according to
characteristics. Examples include:
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RIP
IGRP
EIGRP
OSPF
IS-IS
BGP
Autonomous System is a group of routers under the control of a
single authority.
Classifying Routing Protocols
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Types of routing protocols:
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Interior Gateway Protocols (IGP)
Exterior Gateway Protocols (EGP)
Classifying Routing Protocols
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Interior Gateway Routing Protocols (IGP)
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Used for routing inside an autonomous system & used to
route within the individual networks themselves
Examples: RIP, EIGRP, OSPF
Exterior Routing Protocols (EGP)
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Used for routing between autonomous systems
Example: BGPv4
Classifying Routing Protocols
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Distance Vector & Link State Routing Protocols
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Distance vector
Routes are advertised as vectors
of distance & direction
Incomplete view of network
topology
Generally, periodic
updates
Link state
Complete view of network
topology is created
Updates are not
periodic
Distance Vector Routing Protocols
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Distance Vector Technology - the Meaning of
Distance Vector
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A router using distance vector routing protocols knows
2 things:
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Distance to final destination
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Vector, or direction, traffic should be directed
Distance Vector Routing Protocols :
- each router detects its direct connected networks and form its initial
routing table
- routers pass periodic copies of routing table to neighbor routers
and learn the best paths to all networks
( the paths with
the least metric ) and form the final routing table (convergence)
- after convergence periodic updates (full routing table) are sent to
indicate any change in the topology .
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Distance Vector Routing Protocols
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Characteristics of Distance Vector routing
protocols:
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Periodic updates
Neighbors
Broadcast updates
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Entire routing table is included with routing update
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Distance Vector Routing Protocols
 Routing Protocol Algorithm:
– Defined as a procedure for accomplishing a certain task
Network Discovery
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Router initial start up (Cold Starts)
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Initial network discovery
• Directly connected networks are initially placed in
routing table
Network Discovery
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Initial Exchange of Routing Information
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Routers will exchange routing information
Routing updates received from other routers
Router checks update for new information
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If there is new information:
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Metric is updated
New information is stored in routing table
Network Discovery
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Exchange of Routing Information
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Router convergence is reached when
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All routing tables in the network contain the same network
information
Routers continue to exchange routing information
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If no new information is found then Convergence is reached
Routing loops
Count to infinity problem
Count to infinity is a condition that exists when inaccurate
routing updates increase the metric value to "infinity" for a
network that is no longer reachable. The animation shows
what happens to the routing tables when all three routers
continue to send inaccurate updates to each other.
Routing loops
Solution: Setting a maximum
To eventually stop the incrementing of the metric, "infinity" is
defined by setting a maximum metric value. For example,
RIP defines infinity as 16 hops - an "unreachable" metric.
Once the routers "count to infinity," they mark the route as
unreachable.
Routing loops
Route Poisoning , Poison Reverse :
Route poisoning is used to mark the route as unreachable
in a routing update that is sent to other routers.
Unreachable is interpreted as a metric that is set to the
maximum. For RIP, a poisoned route has a metric of 16.
Routing loops
- When network 10.0.0.0 fails , router A will mark its metric by 16 (a
max. hop count value to avoid counting to infinity) and send its routing
table to B after the periodic interval.
- Before B sends its periodic update to C , router C sent its routing table
to B containing a path to 10.0.0.0 with a better metric so B think that
10.0.0.0 can be reached by C while C depends on B for that so loop
occurs
Routing loops Solutions
1- Split Horizon :
- Route learned from an interface can not be sent back on
the same interface.
- Split horizon with poison reverse: the split horizon rule is
violated in case of poison reverse only.
Routing loops Solutions
2- Hold-down Timers :
Router that informed with a failed route don’t accept any update about it
for a time equal to the hold down timer so by the end of the timer all
routers would know that route failed ( it is useful in flapping networks ).
- hold finish if :
The hold-down timer expires.
Another update is received with a better metric.
Routing loops Solutions
3- Triggered Updates :
Instead of sending updates after a time interval , router
sends the update as soon as a route fails or any change
occurs so other routers immediately modify their routing
tables ( this is the most used solution ).
Routing loops Solutions
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IP & TTL
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Purpose of the TTL field
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The TTL field is found in an IP header and is used to prevent
packets from endlessly traveling on a network
How the TTL field works
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TTL field contains a numeric value
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The numeric value is decreased by one by every router on the
route to the destination
If numeric value reaches 0 then Packet is discarded
Distance Vector Routing Protocols
RIPv1
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RIP Characteristics
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A classful, Distance Vector (DV) routing protocol
Metric = hop count
Routes with a hop count > 15 are unreachable
Updates are broadcast every 30 seconds
RIPv1
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RIP Message Format
RIP header - divided
into 3 fields
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Command field
Version field
Must be zero
Route Entry composed of 3 fields
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Address family
identifier
IP address
Metric
RIPv1
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RIP Operation
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RIP uses 2 message types:
• Request message
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This is sent out on startup by each RIP enabled interface
Requests all RIP enabled neighbors to send routing table
Response message
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Message sent to requesting router containing routing table
RIPv2
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Difference between RIPv1 & RIPv2
• RIPv1
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A classful distance vector routing protocol
Does not support discontiguous subnets
Does not support VLSM
Does not send subnet mask in routing update
Routing updates are broadcast
RIPv2
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A classless distance vector routing protocol that
is an enhancement of RIPv1’s features
Next hop address is included in updates
Routing updates are multicast
The use of authentication is an option
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Similarities between RIPv1 & RIPv2
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Use of timers to prevent routing loops
Use of split horizon or split horizon with poison reverse
Use of triggered updates
Maximum hop count of 15
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Comparing RIPv1 & RIPv2 Message Formats
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RIPv2 Message format is similar to RIPv1 but has 2
extensions
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1st extension is the subnet mask field
2nd extension is the addition of next hop address
Routing
Protocol
Distance
Vector
Classless
Routing
Protocol
Uses
HoldDown
Timers
Use of
Split
Horizon
or
Split
Horizon
w/
Poison
Reverse
Max
Hop
count
= 15
Auto
Summary
Support
CIDR
Supports
VLSM
Uses
Authen
tication
RIPv1
Yes
No
Yes
Yes
Yes
Yes
No
No
No
RIPv2
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
RIP
distance vector routing protocol
- Administrative Distance = 120
- metric is hop count , metric 16 means unreachable
- full routing tables are flooded in the network till convergence
occurs (use Bellman Ford algorithm)
- After convergence , periodic updates are sent every 30 seconds ,
hold down timer = 180 sec
- at change , triggered update is sent
- support load balancing if many paths to the same network exist
with an equal metric
(maximum is six
paths , default = 4)
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- classful
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IGRP
- Distance vector routing protocol
- Cisco proprietary
- Administrative Distance is 100
- Metric is composite (bandwidth , delay by default) while (load , reliability ,
MTU) can be used , maximum metric is 255
- Full routing tables are flooded in the network till convergence occurs (use
Bellman Ford algorithm)
- After convergence , periodic updates are sent every 90 seconds , hold down
timer = 270 sec
- At change , triggered update is sent
- support load balancing if many paths to the same network exist even if they
have unequal metric (maximum is six paths , default = 4)
- Classful
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