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

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

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.

Classifying Routing Protocols

Types of routing protocols:– Interior Gateway Protocols (IGP)– Exterior Gateway Protocols (EGP)

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

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

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 .

Distance Vector Routing Protocols Characteristics of Distance Vector

routing protocols:– Periodic updates– Neighbors – Broadcast updates– Entire routing table is included with routing

update

Distance Vector Routing Protocols

Routing Protocol Algorithm:– Defined as a procedure for accomplishing a certain task

Network Discovery

Router initial start up (Cold Starts)– Initial network discovery

• Directly connected networks are initially placed in routing table

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

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

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 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.

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

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

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

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

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

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

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

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

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

RIP

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- 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

IGRP

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- 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