Chapter 4 Distance Vector Routing Protocols CIS 82 Routing Protocols and Concepts Rick Graziani Cabrillo College [email protected] Last Updated: 3/9/2009

Chapter 4Distance Vector Routing Protocols

CIS 82 Routing Protocols and Concepts

Rick Graziani

Cabrillo College

[email protected]

Last Updated: 3/9/2009

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Note This presentation will be updated prior to March. 25, 2008 The audio of the lecture for this presentation will be available on

my web site after March. 25, 2008

This presentation contains additional information in notes section.

My web site is www.cabrillo.edu/~rgraziani. For access to these PowerPoint presentations and other

materials, please email me at [email protected].

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For further information This presentation is an

overview of what is covered in the curriculum/book.

For further explanation and details, please read the chapter/curriculum.

Book: Routing Protocols

and Concepts By Rick Graziani and

Allan Johnson ISBN: 1-58713-206-0 ISBN-13: 978-58713-

206-3

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Topics Introduction to Distance Vector

Routing Protocols Distance Vector Technology Routing Protocol Algorithms Routing Protocol

Characteristics Network Discovery

Cold Start Initial Exchange of Routing

Information Exchange of Routing

Information Routing Table Maintenance

Periodic Updates Bounded Updates Triggered Updates Random Jitter

Routing Loops Defining a Routing Loop Implications of Routing Loops Count-to-Infinity Condition Preventing Routing Loops by

setting a Maximum Metric Value

Preventing Routing Loops with Hold-down Timers

Preventing Routing Loops with the Split Horizon Rule

Preventing Routing Loops with IP and TTL

Distance Vector Routing Protocols Today RIP EIGRP

Introduction to Distance Vector Routing Protocols

Distance Vector Technology Routing Protocol Algorithms Routing Protocol Characteristics

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Introduction to Distance Vector Routing Protocols

Understanding the operation of distance vector routing is critical to enabling, verifying, and troubleshooting these protocols.

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Configuring and maintaining static routes for a large network would be overwhelming.

What happens when that link goes down at 3:00 a.m.?

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RIP: Routing Information Protocol originally specified in RFC 1058. IGRP: Interior Gateway Routing Protocol - Cisco proprietary EIGRP: Enhanced IGRP – Cisco proprietary IGRP and EIGRP: Cisco never submitted RFCs to IETF for these protocols.

Why did Cisco develop IGRP/EIGRP?

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Meaning of Distance Vector

Distance vector (repeat) Routes are advertised as vectors of

distance and direction. Direction is simply the:

nexthop router or exit interface.

Routing protocol Does not know the topology of an

internetwork. Only knows the routing information

received from its neighbors.

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Meaning of Distance Vector

I can get to 172.16.3.0/24 in one hop out my

S0/0/0.

What is the Distance to 172.16.3.0/24? 1 hop

What is the Direction: S0/0/0

Does R1 have a topology map? No, it only knows distance and direction!

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Operation of Distance Vector Routing Protocols

Periodic updates Some distance vector routing protocols periodically broadcast the entire

routing table to each of its neighbors. (RIP and IGRP) 30 seconds for RIP

Inefficient: Bandwidth and CPU resources Always sent, even no changes

Timer Expires

Timer Expires

Timer Expires

Timer Expires

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What are Neighbors? Neighbors are routers that:

Share a link Use the same routing protocol.

What are the only addresses routers know about before there is any routing knowledge? Network addresses of its own interfaces Network addresses of its neighbors.

R1 is unaware of R3 and its networks

Neighbor of R1

Neighbor of R1

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Routing Protocols use Broadcast updates (255.255.255.255) Multicast updates

Neighbor routers will process the updates. What will other devices on that link do if they receive a routing update but

are not running that routing protocol including hosts? They will process the update up to Layer 3 (Multicast update) or Layer 4

(Broadcast update) before discarding it.

Timer Expires

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Routing Protocol Algorithms

The routing protocol algorithm used by a particular routing protocol is responsible for building and maintaining the router’s routing table.

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The algorithm sends and receives updates.

Update

Update

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The algorithm on each router: Independently makes calculations updates its routing table

Calculating best paths and installing new routes

Update

Update

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The algorithm on each router: Detect and react to topology changes.

Detecting and reacting to topology change

UpdateX

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Routing Protocol Characteristics

Time to convergence: Faster the better.

Scalability: How large a network the routing protocol can handle.

Classless (use of VLSM) or classful: Support VLSM and CIDR

Resource usage: Routing protocol usage of RAM, CPU utilization, and link bandwidth

utilization. Implementation and maintenance:

Level of knowledge that is required for a network administrator.

More later

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Advantages and Disadvantages of Distance Vector Routing Protocols

Advantages: Simplicity Low resource requirements Minimum link bandwidth

Disadvantages: Slow convergence Limited scalability Potential for routing loops

(coming)

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Comparing Routing Protocol Features

Note: Some of this is relative such as Resource usage and Implementation and Maintenance.

Network Discovery

Cold Start Initial Exchange of Routing Information Exchange of Routing Information

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Cold Start

Network discovery is part of the process of the routing protocol algorithm that enables routers to first learn about remote networks.

First: Only knows directly connected networks.

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Initial Exchange of Routing Information

R1: Sends an update about network 10.1.0.0 out the Serial 0/0/0 interface with a

metric of 1 Sends an update about network 10.2.0.0 out the FastEthernet 0/0 interface with

a metric of 1 Receives an update from R2 about network 10.3.0.0 on Serial 0/0/0 with a metric

of 1 Stores network 10.3.0.0 in the routing table with a metric of 1

Update

Update

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R2: Sends an update about network 10.3.0.0 out the Serial 0/0/0

interface with a metric of 1 Sends an update about network 10.2.0.0 out the Serial 0/0/1

interface with a metric of 1

Update

Update

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R3: Sends an update about network 10.4.0.0 out the Serial 0/0/1

interface with a metric of 1 Sends an update about network 10.3.0.0 out the FastEthernet

0/0 interface with a metric of 1

Update

Update

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Have we reached convergence? No

What needs to still be learned? R1 does not have knowledge of 10.4.0.0 R3 does not have knowledge of 10.1.0.0

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Next Exchange of Routing Information

R1: Sends out complete routing table.

Does R2 learn anything new? No

Update

Update

Thanks, but nothing new

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Does R1 Learn anything new? Yes, 10.4.0.0

Does R3 Learn anything new? Yes, 10.1.0.0

Update

Update

S0/0/1

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Does R2 learn anything new? No

Update

Update

S0/0/1

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Note on Split Horizon

Distance vector routing protocols typically implement a technique known as split horizon. Prevents information from being sent out the same interface from which

it was received. More later

10.1.0.0 Update

10.1.0.0 UpdateX10.1.0.0 Update

S0/0/1

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Convergence The amount of time it takes

for a network to converge is directly proportional to the size of that network.

Routing protocols are compared based on how fast they can propagate this information—their speed to convergence.

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2

3

4

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Routing Table Maintenance

Periodic Updates Bounded Updates Triggered Updates Random Jitter

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Periodic Updates

Depending on the routing protocol, routers must maintain the routing tables so that they have the most current routing information.

Some distance vector routing protocols use periodic updates. RIP and IGRP

Sent even when there is no new information.

Periodic Update

Periodic Update

Periodic Update

Periodic Update

Periodic Update

Periodic Update

S0/0/1

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Periodic Updates

Routing update may contain a topology change. What might those changes be?:

Failure of a link Introduction of a new link Failure of a router Change of link parameters

Periodic Updates

Periodic Update

Periodic Update

Periodic Update

Periodic Update

Periodic Update

Periodic Update

S0/0/1

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RIP Timers

IOS implements three additional timers for RIP. Update timer: 30 seconds. Invalid Timer: If an update has not been received in 180 seconds (the

default), the route is marked as invalid by setting the metric to 16. Route still is in routing table.

Flush Timer: 240 seconds (default) When the flush timer expires, the route is removed from the routing

table. Hold-down Timer: 180 seconds (default)

Later in this chapter.

Periodic Update

Periodic Update

No update for 10.4.0.0 from R3 received, mark route as

“possibly down”, but leave in routing table.

Still no update for 10.4.0.0 from R3 received. Remove this route

from the routing table.

S0/0/1

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RIP timer values can be verified with two commands: show ip route and show ip protocols.

R1# show ip route

10.0.0.0/16 is subnetted, 4 subnetsC 10.2.0.0 is directly connected, Serial0/0/0R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:04, Serial0/0/0C 10.1.0.0 is directly connected, FastEthernet0/0R 10.4.0.0 [120/2] via 10.2.0.2, 00:00:04, Serial0/0/0

Elapsed time since the last update, expressed in seconds

R1# show ip protocolsRouting Protocol is “rip” Sending updates every 30 seconds, next due in 13 seconds Invalid after 180 seconds, hold down 180, flushed after 240<output omitted>Routing Information Sources: Gateway Distance Last Update 10.3.0.1 120 00:00:27

RIP Timers

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EIGRP does not send periodic updates. EIGRP sends bounded updates about a route when a path changes or the

metric for that route changes. Note: More in Chapter 9 EIGRP.

Bounded Updates

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A triggered update is a routing table update that is sent immediately in response to a routing change.

Triggered updates do not wait for update timers to expire.

What is the advantage to a triggered update? Speeds up convergence.

Triggered Updates

X

Update timer not yet expired

Triggered Update

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Random Jitter

To prevent the synchronization of updates between routers, Cisco IOS uses a random variable, called RIP_JITTER, which subtracts a variable amount of time to the update interval for each router in the network. Ranges from 0 to 15 percent of the specified update interval. 25.5 to 30 seconds for the default 30-second interval.

Collision! We will randomize our updates between 25.5

and 30 seconds so collisions don’t happen.

Routing Loops

Defining a Routing Loop Implications of Routing Loops Count-to-Infinity Condition Preventing Routing Loops by setting a Maximum Metric Value Preventing Routing Loops with Hold-down Timers Preventing Routing Loops with the Split Horizon Rule Preventing Routing Loops with IP and TTL

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Defining a Routing Loop

A routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its intended destination network. Can occur when two or more routers have inaccurate routing

information to a destination network. Issue with distance vector routing protocols but not link-state.

The loop can be a result of: Incorrectly configured static routes Incorrectly configured route redistribution (CCNP-level courses) Inaccurate routing because of slow convergence in a changing network

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Implications of Routing Loops

What might a some problems with a routing loop? A routing loop can create the following conditions:

Link bandwidth – looping traffic Router’s CPU - Burdened with useless packet forwarding Routing updates might get lost or not processed in time. Packets might get lost in “black holes”.

A routing loop can have a devastating effect on a network.

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Assuming no split horizon, what if 10.4.0.0 network goes down?

Is there a potential for a problem here?

Let’s see…

Periodic Update 10.4.0.0 2 hops thru me

X

This is great, I now have a route to 10.4.0.0 again!

10.4.0.0 S0/0/1 2

IP Packet: DA 10.4.1.1Loop until TTL is 0

S0/0/1

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Mechanisms available to eliminate routing loops: Defining a maximum metric to prevent count to infinity Hold-down timers Split horizon Route poisoning or poison reverse Triggered updates (covered previously)

I mistakenly believe I have a route to 10.4.0.0.

S0/0/1

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Count-to-Infinity Condition

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.

Each protocol defines infinity at a different value.


X

10.4.0.0 S0/0/1 2



3


4

S0/0/1

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Count-to-Infinity Condition

This count continues indefinitely, each router thinking the other router has a route to 10.4.0.0.

To eventually stop the incrementing of the metric, “infinity” is defined by setting a maximum metric value.

RIP defines infinity as 16 hops — an “unreachable” metric. When the routers “count to infinity,” they mark the route as unreachable.


X

10.4.0.0 S0/0/1 10




12

1213

14

15


16


16 means “network unreachable” in RIP

16 means “network unreachable” in RIP 16

S0/0/1

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Preventing Routing Loops with Hold-Down Timers

A routing loop could also be created by a periodic update that is sent by the routers during the instability.

Hold-down timers: Prevent routing loops from being created by these conditions.

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Network 10.4.0.0 attached to R3 goes down. R3 sends a triggered update.

X

Update timer not yet expired

Triggered Update

S0/0/1

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R2 receives the update from R3 indicating that network 10.4.0.0 is now no longer accessible.

R2 marks the network as possibly down and starts the hold-down timer.

X

Possibly down - Start Hold-down Timer

Triggered Update

S0/0/1

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If an update with a better metric for that network is received from any neighboring router during the hold-down period, R2 will reinstate the network and the hold-down timer will be removed.

Note: In this example their can’t be a better metric than 1 hop.

X


Triggered Update

Update with better metric

S0/0/1

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If an update from any other neighbor is received during the hold-down 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.

X


Update with worse metric: 10.4.0.0 3 hops

Same or worse metric - Still possibly down - Keep Hold-down Timer going

S0/0/1

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R1 and R2 still forward packets to 10.4.0.0, even though it is marked as possibly down.

This allows the router to overcome any issues associated with intermittent connectivity.

If the destination network is truly unavailable and the packets are forwarded, black-hole routing is created and lasts until the hold-down timer expires.

X

Possibly down Possibly down

IP Packet: DA 10.4.1.1

S0/0/1

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When the hold-down timers expire on R1 and R2, 10.4.0.0 is removed from the routing table.

No traffic to 10.4.0.0 will be routed – packets dropped by each router.

X

Possibly down Possibly down

ExpiresExpires

S0/0/1

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Split horizon rule says that a router should not advertise a network through the interface from which the update came. Helps prevent routing loops caused by slow convergence.

What network(s) will R1 NOT include in its routing updates to R2? 10.3.0.0 and 10.4.0.0

What network(s) will R2 NOT include in its routing updates R1? R3? R1: 10.1.0.0 R3: 10.4.0.0

What network(s) will R3 NOT include in its routing updates R2? 10.1.0.0 and 10.2.0.0

S0/0/1

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1. R3 advertises the 10.4.0.0 network to R2.2. R2 receives the information and updates its routing table.3. R2 then advertises the 10.4.0.0 network to R1 out S0/0/0.

R2 does not advertise 10.4.0.0 to R3 out S0/0/1, because the route originated from that interface.

4. R1 receives the information and updates its routing table.5. Because of split horizon, R1 also does not advertise the information about

network 10.4.0.0 back to R2.

Periodic Update: 10.4.0.0

Periodic Update: 10.4.0.0 XPeriodic Update:

10.4.0.0 X

S0/0/1

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What networks does R1 advertise to R2? R1 advertises network 10.1.0.0 to R2.

What networks does R2 advertise to R1? R2 advertises networks 10.3.0.0 and 10.4.0.0 to R1.

What networks does R2 advertise to R3? R2 advertises networks 10.1.0.0 and 10.2.0.0 to R3.

What networks does R3 advertise to R2? R3 advertises network 10.4.0.0 to R2.

Notice that each router increments the hop count before sending the update.

Split horizon can be disabled by an administrator to achieve the proper routing under certain conditions.

S0/0/1

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Route Poisoning

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.

Route poisoning speeds the convergence process.

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Split Horizon with Poison Reverse

Split horizon with poison reverse The concept of split horizon with poison reverse is that explicitly

telling a router to ignore a route is better than not telling it about the route in the first place.

Periodic Update: 10.4.0.0 = 16

S0/0/1

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The Time to Live (TTL) is an 8-bit field in the IP header that limits the number of hops a packet can traverse through the network before it is discarded.

The TTL is decreased by 1 by every router on the route to its destination.

If the TTL field reaches 0 before the packet arrives at its destination, the packet is discarded and the router sends an Internet Control Message Protocol (ICMP) error message back to the source of the IP packet.

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Situation where the routing tables do not have accurate information about the downed 10.4.0.0 network.

Even in the case of this routing loop, packets will not loop endlessly in the network.

Eventually the TTL value will be decreased to 0 and the packet will be discarded by the router.


X

10.4.0.0 S0/0/1 2

IP Packet: DA 10.4.1.1Loop until TTL is 0

S0/0/1

Distance Vector Routing Protocols Today

RIP EIGRP

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Distance Vector Routing Protocols Today

Although link-state routing protocols have several advantages over distance vector routing protocols, distance vector routing protocols are still in use today.

Link-state routing protocols will be discussed later.

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RIP and EIGRP

For distance vector routing protocols, there really are only two choices: RIP or EIGRP.

The decision about which routing protocol to use in a given situation is influenced by a number of factors, including Size of the network Compatibility between models of routers Administrative knowledge required

RIPv1: First Look

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RIPv1: First Look

Download Packet Tracer Topology: cis82-RIPv1-A-student.pkt

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Specifying Networks

Use the network command for each directly connected network.

R1(config)# router rip

R1(config-router)# network directly-connected-classful-network-address

R1(config-router)# ...







Only directly connected classful network addresses!

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Specifying Networks

If you enter a subnet or host IP address, IOS automatically converts it to a classful network address.

For example, if you enter the command network 192.168.1.32, the router will convert it to network 192.168.1.0.


R1(config-router)# network 192.168.1.0









Only directly connected classful network addresses!

Verify with the commands: show ip route show ip protocols

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Verifying RIP: show ip route Command

R1# show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile,

<output omitted>

Gateway of last resort is not set

R 192.168.4.0/24 [120/1] via 192.168.2.2, 00:00:02, Serial0/0/0

R 192.168.5.0/24 [120/2] via 192.168.2.2, 00:00:02, Serial0/0/0

C 192.168.1.0/24 is directly connected, FastEthernet0/0

C 192.168.2.0/24 is directly connected, Serial0/0/0

R 192.168.3.0/24 [120/1] via 192.168.2.2, 00:00:02, Serial0/0/0

An R in the output indicates RIP routes. Because this command displays the entire routing table, including

directly connected and static routes, it is normally the first command used to check for convergence.

Routes might not immediately appear when you execute the command because networks take some time to converge..

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R2# show ip route


<output omitted>



R 192.168.5.0/24 [120/1] via 192.168.4.1, 00:00:12, Serial0/0/1

R 192.168.1.0/24 [120/1] via 192.168.2.1, 00:00:24, Serial0/0/0



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R3# show ip route


<output omitted>




R 192.168.1.0/24 [120/2] via 192.168.4.2, 00:00:08, Serial0/0/1

R 192.168.2.0/24 [120/1] via 192.168.4.2, 00:00:08, Serial0/0/1

R 192.168.3.0/24 [120/1] via 192.168.4.2, 00:00:08, Serial0/0/1

Chapter 4Distance Vector Routing Protocols

CIS 82 Routing Protocols and Concepts

Rick Graziani

Cabrillo College

[email protected]

Last Updated: 3/9/2009

Documents

Chapter 4 Distance Vector Routing Protocols CIS 82 Routing Protocols and Concepts Rick Graziani Cabrillo College [email protected] Last Updated: 3/9/2009