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![Page 1: Dynamic Routing Protocols Function(s) of Dynamic Routing Protocols: – Dynamically share information between routers (Discover remote networks). – Automatically.](https://reader036.fdocuments.net/reader036/viewer/2022081513/56649e455503460f94b392c8/html5/thumbnails/1.jpg)
Dynamic Routing Protocols
Function(s) of Dynamic Routing Protocols:– 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
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Dynamic Routing Protocols
Components of a routing protocol– Algorithm
• In the case of a routing protocol algorithms are used for facilitating routing information and best path determination
– Routing protocol messages• These are messages for discovering neighbors and
exchange of routing information
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Classifying Routing Protocols
Dynamic routing protocols are grouped according to characteristics. Examples include:
– RIP– IGRP– EIGRP– OSPF– IS-IS– BGP
Autonomous System is a group of routers under the control of a single authority.
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Classifying Routing Protocols
Types of routing protocols:– Interior Gateway Protocols (IGP)– Exterior Gateway Protocols (EGP)
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Classifying Routing Protocols
Interior Gateway Routing Protocols (IGP)
– Used for routing inside an autonomous system & used to route within the individual networks themselves
– Examples: RIP, EIGRP, OSPF Exterior Routing Protocols (EGP)
– Used for routing between autonomous systems– Example: BGPv4
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Classifying Routing Protocols
Distance Vector & Link State Routing Protocols
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
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Distance Vector Routing Protocols
Distance Vector Technology - the Meaning of Distance Vector
– A router using distance vector routing protocols knows 2 things:
• Distance to final destination
• Vector, or direction, traffic should be directed
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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 Characteristics of Distance Vector
routing protocols:– Periodic updates– Neighbors – Broadcast updates– 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
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Network Discovery
Router initial start up (Cold Starts)– Initial network discovery
• Directly connected networks are initially placed in routing table
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Network Discovery
Initial Exchange of Routing Information• Routers will exchange routing information• Routing updates received from other routers
Router checks update for new information– If there is new information:
• Metric is updated• New information is stored in routing table
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Network Discovery
Exchange of Routing Information– Router convergence is reached when
• All routing tables in the network contain the same network information
– Routers continue to exchange routing information
• If no new information is found then Convergence is reached
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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.
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Routing loops
Solution: Setting a maximumTo 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.
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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.
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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
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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.
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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.
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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 ).
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Routing loops Solutions
IP & TTL– Purpose of the TTL field
• 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– TTL field contains a numeric value
• 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
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RIPv1 RIP Characteristics
– A classful, Distance Vector (DV) routing protocol
– Metric = hop count– Routes with a hop count > 15 are
unreachable– Updates are broadcast every 30
seconds
Distance Vector Routing Protocols
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RIPv1
RIP Message Format RIP header - divided
into 3 fields– Command
field– Version field – Must be zero
Route Entry - composed of 3 fields
– Address family identifier
– IP address– Metric
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RIPv1
RIP Operation– RIP uses 2 message types:
• Request message– This is sent out on startup by each RIP enabled
interface– Requests all RIP enabled neighbors to send routing
table
• Response message– Message sent to requesting router containing routing
table
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RIPv2
– Difference between RIPv1 & RIPv2• RIPv1
– 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– 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– 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
– RIPv2 Message format is similar to RIPv1 but has 2 extensions
• 1st extension is the subnet mask field• 2nd extension is the addition of next hop address
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RoutingProtocol
DistanceVector
ClasslessRoutingProtocol
UsesHold-DownTimers
Use ofSplit
Horizon or
Split Horizon
w/Poison
Reverse
MaxHop
count= 15
AutoSummary
SupportCIDR
SupportsVLSM
Uses Authen
-tication
RIPv1 Yes No Yes Yes Yes Yes No No No
RIPv2 Yes Yes Yes Yes Yes Yes Yes Yes Yes
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RIP
29
- 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)
- classful
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IGRP
30
- 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