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Published byDrusilla Flowers Modified over 9 years ago
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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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8 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 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.
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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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Routing Protocol Distance Vector Classless Routing Protocol Uses Hold- Down Timers Use of Split Horizon or Split Horizon w/ Poison Reverse Max Hop count = 15 Auto Summary Support CIDR Supports VLSM Uses Authen - tication RIPv1YesNoYes No RIPv2Yes
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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
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