Monday, February 21, 2011

Distance Vector Routing Protocols


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.

RIP uses hop count as the metric for path selection.
If hop count for a network is greater then 15, RIP cannot supply a route to that network.
IGRP/EIGRP use Bandwidth, delay, load and reliability for path selection.
EIGRP can perform unequal cost load balancing.
EIGRP uses Diffusing Update Algorithm (DUAL) to calculate the shortest path.
EIGRP only sends updates when there is a change in the topology.

Characteristics of Distance Vector routing protocols:
Periodic updates – RIP 30sec, IGRP – 90sec.
Neighbours, directly connected routers , (Routers using distance vector routing are not aware of the nework topology.
Broadcast updates –
Entire routing table is included with routing updates.

Routing Protocol algorithms
Defined as a procedure for accomplishing a certain task.
Purpose of Routing Algorithms:
1. Send and Receive Updates.
2. Calculate best path, install routes.
3. Detect and react to topology changes.

Routing Protocol Characteristics:
Criteria used to compare routing protocols includes.
Time to convergence.
Classless (Use of VLSM) or Classful.
Resource usage.
Implementation and maintenance.

Advantages & Disadvantages of Distance Vector Routing Protocols:

Network Discovery:
Router initial start up (Cold Starts):
Initial network discovery.
Directly connected networks are initially placed in routing table.

Initial Exchange of Routing Information:
If a routing protocol is configured then.
Routers will exchange routing information.

Routing updates received from other routers:
Router checks update for new information.
If there is new information:
1. Metric is updated.
2. New information is stored in routing table.

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.

Convergence must be reached before a network is considered completely operable.
Speed of achieving convergence consists of 2 interdependent categories.
How quickly the routers propagates a change in the topology in a routing update to it s neighbour, (Speed of broadcasting routing information).
Speed of calculating best path routes.

Routing Table Maintenance:
Periodic Updates: RIPv1 and RIPv2.
These are time intervals in which a router sends out its entire routing table.

RIP uses 4 timers:
Update timer – 30 sec.
Invalid timer – 180 sec, (if update has not been received to refresh).
Holddown timer – 180 sec.
Flush timer – 240 sec.

Bounded Updates : EIGRP.
EIGRP routing updates are. (uses reliable exchanges, waits for a confirmation)
Partial updates, sent (triggered) only when there is a change in topology that influences routing info.
Bounded, propagation of partial updates are automatically bounded so that only those routers that need the information are updated.
Non periodic, because they are not sent out on a regular basis.

Triggered Updates
: (Routing table update that is sent immediately in response to a routing change).
Conditions in which triggered updates are sent:
Interface changes state, (up or down).
Route has entered (or exited) the “unreachable” state.
Route is placed in routing table.

Random (RIP) Jitter :
Synchronized updates, (between routers):
Adds a variable amount of time to the update interval for each router in the network.
This random jitter, or variable amount of time, ranges from 0% to 15% of the specified update interval.
In this way, the update interval varies randomly in a range from 25 to 30 seconds for the default 30-second interval.

A condition where multiple routers on multi access LAN segments transmit routing updates at the same time:
Problems with synchronized updates :
Bandwidth consumption.
Packet collisions.
Solution to problems with synchronized updates:
Use a random variable, called RIP_JITTER.

Routing Loops:

Routing loops are :
A condition in which a packet is continuously transmitted within a series of routers without ever reaching its destination.

Routing loops may be caused by :
Incorrectly configured static routes.
Incorrectly configured route redistribution. (more then one Routing protocol)
Slow convergence.
Incorrectly configured discard routes.

Routing loops can create the following issues :
Excess use of bandwidth.
CPU resources may be strained.
Network convergence is degraded.
Lost routing updates lead to more loops, which lead to black holes.

Count to Infinity :
This is a routing loop where packets bounce infinitely around the network.
A condition that exists when inaccurate routing updates increase the metric value to “infinity” for a network that is no longer reachable.

Setting a maximum (to prevent loops???).
Distance Vector routing protocols set a specified metric value to indicate infinity.
Once a router “counts to infinity” it marks the route as unreachable:
RIP defines infinity as 16 hops.
An “unreachable” metric. Once the routers “count to infinity”, they mark the route as unreachable..

Holddown Timers
Allow a router to not accept any changes to a route for a specified period of time.
Allows routing updates to propagate through network with the most current information.
RIP set at 180sec.

The Split Horizon Rule is used to prevent routing loops :
Split Horizon rule:
A router should not advertise a network through the same interface from which the update came.

Split horizon with poison reverse (or Route Poinsoning) :
Once a router learns of an unreachable route, advertise it back through the same interface as unreachable. (setting the “count to infinity” as described above)

Speeds up convergence.
AKA route poisoning.

The TTL field in a IP header is used to prevent packets from endlessly traveling on a network.
TTL value is decreased by one by every router on the route to the destination.
If the value reaches 0 then Packet is discarded.

Factors used to determine whether to use RIP or EIGRP include :
Network size. (RIP has a max of 15 hops in between 2 PC’s)
Compatibility between models of routers.
Administrative knowledge.

Features of RIP: (RIPv1 & RIPv2)
Supports split horizon and poison reverse.
Capable of load balancing, (up to 6 and by default is set to 4).
Easy to configure.
Works in a multi vendor router environment.
Does support Triggered updates.

Additional for RIPv2 :
Includes the subnet mask in the routing updates, making it a classless routing protocol.
Has authentication mechanism to secure routing table updates.
Supports variable length subnet mask (VLSM).
Uses multicast addresses instead of broadcast.
Supports manual route summarization.

Features of EIGRP :
Triggered updates only, (no periodic updates).
EIGRP hello protocol – neighbour adjacencies.
Supports VLSM and route summarization.
Use of topology table to maintain all routes, (unlike RIPv2).
Classless – supports VLSM.
Cisco proprietary protocol.

Advantages :
Metric based on minimum BW and cumulative delay of the path rather then hop count.
Fast convergence due to DUAL, which allows the insertion of backup routes into the EIGRP topology table.
Bounded updates mean that it uses less bandwidth.
Supports multiple network layer protocols through Protocol Dependent Modules, which include support for IP, IPX and AppleTalk.

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