CCNA – Semester 2
Chapter 4 –
Distance Vector Routing Protocols
CCNA Exploration version 4.0
2
Objectives
• Identify the characteristics of distance vector routing
protocols.
• Describe the network discovery process of distance
vector routing protocols using Routing Information
Protocol (RIP).
• Describe the processes to maintain accurate routing
tables used by distance vector routing protocols.
• Identify the conditions leading to a routing loop and
explain the implications for router performance.
• Recognize that distance vector routing protocols are in
use today
3
Introduction to Distance Vector
Routing Protocols
4
Distance Vector Routing Protocols
• Dynamic routing protocols help the network administrator
overcome the time-consuming and exacting process of
configuring and maintaining static routes.
• 28 routers shown in the figure? What happens when a link goes
down? How do you ensure that redundant paths are available?
Dynamic routing is the most common choice for large networks
like the one shown.
5
Distance Vector Routing Protocols
– Routing Information Protocol (RIP): RFC 1508
• Hop count is used as the metric
• Max hop is 15
• Routing updates are broadcast or multicast every 30 seconds
– Interior Gateway Routing Protocol (IGRP)
• Proprietary protocol developed by Cisco.
• Bandwidth, delay, load and reliability are used to create a composite metric.
• Routing updates are broadcast every 90 seconds,
• IGRP is the predecessor of EIGRP and is now obsolete
– Enhanced Interior Gateway Routing Protocol (EIGRP)
• It can perform unequal cost load balancing.
• It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path.
• Routing updates are sent only when there is a change in the topology.
6
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
7
Distance Vector Technology
• Characteristics of Distance Vector routing protocols:
Periodic updates
Neighbors: Routing by rumor
Broadcast updates 255.255.255.255
Entire routing table is included with routing update
8
Routing Protocol Algorithms
• At the core of the distance vector protocol is the algorithm. The
algorithm is used to calculate the best paths and then send that
information to the neighbors.
• Routing Protocol Algorithm:
- Defined as a procedure for accomplishing a certain task
9
Routing Protocol Characteristics
• Routing Protocol Characteristics
• Criteria used to compare routing protocols includes
Time to convergence
Scalability
Classless (use of VLSM) or Classful
Resource usage
Implementation & maintenance
10
Routing Protocol Characteristics
11
Distance Vector Routing Protocols
• 4.1.4.2
12
Network Discovery
13
Cold Start
• Router initial start up
After a cold start and before the exchange of
routing information, the routers initially discover
their own directly connected networks and subnet
masks.
14
Exchange of Routing Information
Initial Exchange:
• 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:
– Metric is updated
– 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
15
Initial Exchange of Routing Information
10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0
10.1.0.0 0
Routing Table
10.2.0.0 0
10.2.0.0 0
Routing Table
10.3.0.0 0
10.3.0.0 0
Routing Table
10.4.0.0 0
10.3.0.0 1 10.1.0.0 1 10.2.0.0 1
10.4.0.0 1
10.4.0.0 2 10.1.0.0 2
16
Convergence
• Convergence must be
reached before a network is
considered completely
operable
• Speed of achieving
convergence consists of 2
interdependent categories
– Speed of broadcasting
routing information
update
– Speed of calculating
routes
17
Routing Table Maintenance
18
Periodic Updates : RIPv1 & IGRP
• Periodic updates are time intervals in which a router sends out its entire routing table.
• Changes may occur for several reasons, including:
– Failure of a link
– Introduction of a new link
– Failure of a router
– Change of link parameters
19
Periodic Updates : RIPv1 & IGRP
• RIP uses 4 timers
– Update timer 30s
– Invalid timer 180s
– Holddown timer 180s
– Flush timer 240s
20
Bounded Updates: EIGRP
• EIRPG routing updates are
– Partial updates
– Triggered by topology changes
– Bounded: meaning the propagation of partial updates
are automatically bounded so that only those routers
that need the information are updated.
– Non periodic
21
Triggered Updates
• Conditions in which triggered updates are sent
– Interface changes state
– Route becomes unreachable
– Route is placed in routing table
22
Random Jitter
• Synchronized updates
– 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
Used of random variable called RIP_JITTER, 0% to 15%
of the specified update interval (25 to 30 seconds for the
default 30-second interval)
23
Routing Loop
24
Definitions & Implications
• Routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its destination.
• The IP protocol has its own mechanism to prevent the possibility of a packet traversing the network endlessly. IP has a Time-to-Live (TTL) field and its value is decremented by 1 at each router. If the TTL is zero, the router drops the packet.
25
Definitions & Implications
• Routing loops may be caused by:
– Incorrectly configured static routes
– Incorrectly configured route redistribution
– 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
– Routing updates may be lost or not processed in a timely manner
26
Count to Infinity
• This is a routing loop whereby packets bounce infinitely
around a network.
4.4.2.1
27
Setting a maximum
• Distance Vector routing protocols set a specified
metric value to indicate infinity
– Once a router “counts to infinity” it marks the route
as unreachable
28
Preventing Routing Loops with Holddown
Timers
• Network is unstable: a interface resets as up, then down,
then up again in rapid succession. The route is flapping.
Using triggered updates, the routers might react too quickly
and unknowingly create a routing loop. A routing loop could
also be created by a periodic update that is sent by the
routers during the instability. Holddown timers prevent
routing loops from being created by these conditions.
Holddown timers also help prevent the count to infinity
condition.
• Holddown timers allow a router to not accept any changes
to a route for a specified period of time
• Point of using holddown timers
– Allows routing updates to propagate through network
with the most current information.
29
Holddown timers
Holddown timers work in the following way:
1. A router receives an update from a neighbor indicating
that a network that previously was accessible is now no
longer accessible.
30
Holddown timers
2. The router marks the network as possibly down and starts
the holddown timer.
31
Holddown timers
3. If an update with a better metric for that network is
received from any neighboring router during the holddown
period, the network is reinstated and the holddown timer is
removed.
32
Holddown timers
4. If an update from any other neighbor is received during
the holddown period with the same or worse metric for
that network, that update is ignored. Thus, more time is
allowed for the information about the change to be
propagated.
33
Holddown timers
5. Routers still forward packets to destination networks that
are marked as possibly down. This allows the router to
overcome any issues associated with intermittent
connectivity. If the destination network truly is unavailable
and the packets are forwarded, black hole routing is
created and lasts until the holddown timer expires.
34
Split Horizon Rule
• Split Horizon rule:
– A router should not advertise a network through the
interface from which the update came.
4.4.5.1
35
Split horizon with Poison Reverse or Route
Poisoning
• Route poisoning is used to mark the route as unreachable in a routing update that is sent to other routers.
• Split horizon with poison reverse:
– The rule states that once a router learns of an unreachable route through an interface, advertise it as unreachable back through the same interface
36
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
37
IP & TTL
• 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
38
Distance Vector Routing Protocols today
39
RIP and EIGRP
• Factors used to determine whether to use RIP or EIGRP
include
– Network size
– Compatibility between models of routers
– Administrative knowledge
40
RIP
• RIP:
– Supports split horizon & split horizon with poison reverse
– Capable of load balancing
– Easy to configure
– Works in a multi vendor router environment
• RIPv2 introduced the following improvements to RIPv1:
– 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.
41
EIGRP
• Features:
– Triggered updates
– EIGRP hello protocol used to establish neighbor adjacencies
– Supports VLSM & route summarization
– Use of topology table to maintain all routes
– Classless distance vector routing protocol
– Cisco proprietary protocol
42
Distance Vector Routing Protocols Compared
4.5.1.1
43
Summary
44
Top Related