Slide 1Cisco IOS
Cisco technology is built around the Cisco Internetwork Operating System (IOS), which is the software that controls the routing and switching functions of internetworking devices.
*
The Purpose of Cisco IOS
*
Introduction to Routers
*
FLASH Memory- IOS Images are kept here
- Erasable reprogrammable ROM
RAM - Random Access memory
NVRAM - Start up configuration
Function
POST
Bootstrap
Maintains instructions for power-on self test (POST) diagnostics
Stores bootstrap program and basic operating system software
Mini IOS
RAM has the following characteristics and functions:
Stores routing tables
Holds ARP cache
Performs packet buffering (shared RAM)
Provides temporary memory for the configuration file of the router while the router is powered on
Loses content when router is powered down or restarted
*
Retains content when router is powered down or restarted
Configuration Register – 16 bit register which decides boot sequence
*
Holds the operating system image (IOS)
Allows software to be updated without removing and replacing chips on the processor
Retains content when router is powered down or restarted
Can store multiple versions of IOS software
Is a type of electronically erasable, programmable ROM (EEPROM)
*
Connect router to network for frame entry and exit
Can be on the motherboard or on a separate module
Types of interfaces:
Based on the type we have modular and non modular
Non modular are fixed
Ethernet or FA – connecting to switch
Line – for the local configuration
Router – console port
PC- Serial Port
BRI – for ISDN WAN connectivity
Ports are numbered serially
NVRAM—Backup configurations, config register
Flash—Cisco IOS
*
Find the configuration.
Load the configuration.
Run the configured Cisco IOS software.
*
After the Post…
After the POST, the following events occur as the router initializes:
Step 1
The generic bootstrap loader in ROM executes. A bootstrap is a simple set of instructions that tests hardware and initializes the IOS for operation. 
Step 2
The IOS can be found in several places. The boot field of the configuration register determines the location to be used in loading the IOS.
Step 3
Step 4
The configuration file saved in NVRAM is loaded into main memory and executed one line at a time. The configuration commands start routing processes, supply addresses for interfaces, and define other operating characteristics of the router.
Step 5
*
From Flash Memory
The flash memory file is decompressed into RAM.
Note: The 2500 series routers do not operate this way. The 2500 series routers normally run Cisco IOS from Flash. The Cisco IOS in Flash is not compressed but it is relocatable. Relocatable means the Cisco IOS image can be run from Flash or from RAM.
The 2500 can run from RAM if you use the boot system tftp command to boot the Cisco IOS image.
*
If no configuration is present in NVRAM, enter setup mode.
*
*
*
*
Establishing a
HyperTerminal Session
Take the following steps to connect a terminal to the console port on the router:
First, connect the terminal using the RJ-45 to RJ-45 rollover cable and an RJ-45 to DB-9 or RJ-45 to DB-25 adapter.
*
Router LED Indicators
*
*
The Cisco command-line interface (CLI) uses a hierarchical structure. This structure requires entry into different modes to accomplish particular tasks.
Each configuration mode is indicated with a distinctive prompt and allows only commands that are appropriate for that mode.
*
CLI Command Modes
All command-line interface (CLI) configuration changes to a Cisco router are made from the global configuration mode. Other more specific modes are entered depending upon the configuration change that is required.
Global configuration mode commands are used in a router to apply configuration statements that affect the system as a whole.
The following command moves the router into global configuration mode
Router#configure terminal (or config t)
Router(config)#
When specific configuration modes are entered, the router prompt changes to indicate the current configuration mode.
*
IOS (tm) 2500 Software (C2500-JS-L), Version 12.0(3), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-1999 by cisco Systems, Inc.
Compiled Mon 08-Feb-99 18:18 by phanguye
Image text-base: 0x03050C84, data-base: 0x00001000
ROM: System Bootstrap, Version 11.0(10c), SOFTWARE
BOOTFLASH: 3000 Bootstrap Software (IGS-BOOT-R), Version 11.0(10c), RELEASE SOFTWARE(fc1)
wg_ro_a uptime is 20 minutes
System restarted by reload
(output omitted)
Purpose: This slide presents the show version command.
Emphasize: Point out that this command is useful when troubleshooting problems because it gives the versions of the various software components and files. It also displays how long the router has been in operation and where it obtained the image file.
*
Viewing the Configuration
Emphasize: When you exit the setup mode, the configuration can be saved to RAM and NVRAM at the same time.
*
!
Displays the current and saved configuration
Purpose: This slide shows the format and output of the show running-config and show startup-config commands, which display the active and backup configuration files, respectively.
Emphasize: We put these two commands on the same page because it is easy to confuse the two. The show running-config command displays the configuration information in memory, while the show startup-config command displays the backup file.
Often in class someone will enter commands and then say that the router did not accept them. This scenario might indicate that the person entered the commands to modify the configuration information in memory, and then entered a show startup-config (show config) to look at the backup file that has not yet been updated to reflect the changes. You must use another command to update the backup file.
Default parameters do not display in the running configuration.
*
Configurations in two locations - RAM and NVRAM.
The running configuration is stored in RAM.
Any configuration changes to the router are made to the running-configuration and take effect immediately after the command is entered.
The startup-configuration is saved in NVRAM and is loaded into the router's running-configuration when the router boots up.
To save the running-configuration to the startup configuration, type the following from privileged EXEC mode (i.e. at the "Router#" prompt.)
Router# copy run start
Configuring a Router’s Name
A router should be given a unique name as one of the first configuration tasks.
This task is accomplished in global configuration mode using the following commands:
Router(config)#hostname Gates
Gates(config)#
*
Message Of The Day (MOTD)
A message-of-the-day (MOTD) banner can be displayed on all connected terminals.
Enter global configuration mode by using the command config t
Enter the command
Save changes by issuing the command copy run start
*
# terminal history size 25
As its name suggests, the most common use of the show version command is to determine which version of the Cisco IOS a device is running. However, this command does much more than that—it actually offers several different uses. Let David Davis introduce you to the many uses of Cisco's show version command, and see how its output varies according to which device you're using.
1. The version of the IOS operating system
2. The version of the ROM bootstrap
3. The version of the boot loader
4. How someone last powered on the device (In addition to powering on in the usual manner, you can also power on a device with a system reset (i.e., warm reboot) or by a system panic.)
5. The time and date the system last started
6. The "uptime" for the system (i.e., how much time has passed since the last power-on)
7. The image file that the device last started (i.e., the actual path to the IOS software)
8. How much RAM the device has
9. The processor board ID, which you can use to determine the version of the device's motherboard
10. The number and type of each interface on the device (e.g., Qty 2 Ethernet, Qty 6 Serial, etc.)
11. The number of terminal lines on the router if a router has asynchronous serial lines attached
12. The amount of nonvolatile RAM (NVRAM), used to hold the SAVED version of the configuration file, also known as the startup-configuration
13. The amount and type of Flash on the device (except on a switch), used to hold the operating system when it isn't in use (Think of it as the equivalent to a hard drive on a PC.)
14. The configuration register on the device, which is a hexadecimal number used to tell the device what to do when it boots. (Typically, this only changes when you need to bypass the configuration file because of a lost password, but you can also change it for other special cases.)
15. The hostname of the device
SHOW HISTORY will give all commands available in history bufer
By default the buffer size is 10, this can be seen by SHOW TERMINA: Command
The terminal history size can be increased by
#Terminal History size 25
Passwords restrict access to routers.
Passwords should always be configured for virtual terminal lines and the console line.
*
Privileged Mode Password – 2
Connecting to the Auxiliary Port
When a modem is connected to the auxiliary port, a remote user can dial in to the router and configure it. Use the light blue console cable and the DB-9-to-DB-25 connector adapter that came in the router accessory kit. To connect a modem to the router, follow these steps:
Step 1- Connect the RJ-45 end of the adapter cable to the black AUX port on the router.
Step 2-Connect the DB-9 end of the console cable to the DB-9 end of the modem adapter.
*
Configuring a Telnet Password
A password must be set on one or more of the virtual terminal (VTY) lines for users to gain remote access to the router using Telnet.
*
Gates(config)# password cisco
Only the enable secret password is encrypted by default
Need to manually configure the user-mode and enable passwords for encryption
To manually encrypt your passwords, use the service password-encryption command
Router#config t
Router(config)#service password-encryption
Gates(config)# no password
AUI to Host
*
Setting descriptions on an interface is helpful to the administrator
Only locally significant
R1(config)#int e0
*
Configuring Interfaces
An interface needs an IP Address and a Subnet Mask to be configured.
All interfaces are “shutdown” by default.
The DCE end of a serial interface needs a clock rate.
R1#config t
R1(config)#int e0
R1(config-if)#no shutdown
R1(config-if)# bandwidth 64
R1(config-if)#no shutdown
R1#
On new routers, Serial 1 would be just Serial 0/1 and e0 would be f0/0.
s = serial e = Ethernet f = fast Ethernet
*
#Show controllers s 0
#Show IP interface brief
# copy run startup-config
Objectives
Upon completion of this chapter, you will be able to complete the following tasks:
Distinguish the use and operation of static and dynamic routes
Configure and verify a static route
Identify how distance vector IP routing protocols such as RIP and IGRP operate on Cisco routers
Enable Routing Information Protocol (RIP)
Enable Interior Gateway Routing Protocol (IGRP)
Verify IP routing with show and debug commands
Purpose: this figure states the chapter objectives.
*
Routing
The process of transferring data from one local area network to another
Layer 3 devices
Routed protocol Enables to forward packet from one router to another – Ex – IP, IPX
Routing protocol sends and receives routing information packets to and from other routers – Ex -RIP, OSPF , IGRP
Routing protocols gather and share the routing information used to maintain and update routing tables.
That routing information is in turn used to route a routed protocol to its final destination
In internetworking , the process of moving a packet of data from source to destination . Routing is usually performed by a dedicated device called a router . Routing is a key feature of the Internet because it enables messages to pass from one computer to another and eventually reach the target machine. Each intermediary computer performs routing by passing along the message to the next computer. Part of this process involves analyzing a routing table to determine the best path. Routing is often confused with bridging, which performs a similar function. The principal difference between the two is that bridging occurs at a lower level and is therefore more of a hardware function whereas routing occurs at a higher level where the software component is more important. And because routing occurs at a higher level, it can perform more complex analysis to determine the optimal path for the packet.
A routing protocol sends and receives routing information packets to and from other routers. A routed protocol can be routed by a router, which means that it can be forwarded from one router to another. Yes, there are protocols that can't be routed, such as NetBEUI (Network Basic Input Output System Extended User Interface).
That a routed protocol can be routed may seem obvious, but unless you know how to differentiate it from a routing protocol, you may have trouble with the wording for some questions on the exam.
A protocol is a set of rules that defines how two devices communicate with one another. It also defines the format for the packets used to transmit data over communications lines. A routed protocol contains the data elements required for a packet to be sent outside its host network or network segment. In other words, a routed protocol can be routed. Protocols used to communicate routing information between routers within an autonomous system are Interior Gateway Protocols (IGP), which are routing protocols, but not routed protocols.
Routing protocols gather and share the routing information used to maintain and update routing tables. That routing information is in turn used to route a routed protocol to its final destination. Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP) are the routing protocols you need to know for the exam. If you can remember what the abbreviations mean, you'll remember that they are routing protocols because they have routing in their names. Remember, too, that they are not routed protocols.
*
Destination addresses
Possible routes
Best route
172.16.1.0
10.120.2.0
Purpose: This figure introduces students to routing. The router must accomplish the items listed in the figure for routing to occur.
Emphasize:
Path determination occurs at Layer 3, the network layer. The path determination function enables a router to evaluate the available paths to a destination and to establish the best path.
Routing services use network topology information when evaluating network paths. This information can be configured by the network administrator (static routes) or collected through dynamic processes (routing protocols) running in the network.
Transition:
*
172.16.1.0
10.120.2.0
E0
S0
Purpose: This figure explains that routers must learn about paths not directly connected.
*
Static routing - network administrator configures information about remote networks manually. They are used to reduce overhead and for security.
*
IP Routing Process
Step-by-step what happens when Host A wants to communicate with Host B on a different network
A user on Host A pings Host B’s IP address.
E0
E1
10.0.0.1
10.0.0.2
A
B
20.0.0.2
20.0.0.1
1. Internet Control Message Protocol (ICMP) creates an echo request payload (which is just the alphabet in the data field).
2. ICMP hands that payload to Internet Protocol (IP), which then creates a packet.
3. Once the packet is created, IP determines whether the destination IP address is on the local network or a remote one.
4. Since IP determines that this is a remote request, the packet needs to be sent to the default gateway so the packet can be routed to the remote network. The Registry in Windows is parsed to find the configured default gateway.
5. The default gateway of host 172.16.10.2 (Host_A) is configured to 172.16.10.1. To be able to send this packet to the default gateway, the hardware address of the router’s interface Ethernet 0 (configured with the IP address of 172.16.10.1) must be known.
6. Next, the ARP cache is checked to see if the IP address of the default gateway has already been resolved to a hardware address:
If the hardware address isn’t already in the ARP cache of the host, an ARP broadcast is sent out onto the local network to search for the hardware address of 172.16.10.1. The router responds to the request and provides the hardware address of Ethernet 0, and the host caches this address. The router also caches the hardware address of Host_A in its ARP cache.
7. Once the packet and destination hardware address are handed to the Data Link layer, the LAN driver is used to provide media access via the type of LAN being used (in this example, Ethernet). A frame is then generated, encapsulating the packet with control information. Within that frame are the hardware destination and source addresses, plus, in this case, an Ether-Type field that describes the Network layer protocol that handed the packet to the Data Link layer—in this instance, IP. At the end of the frame is something called a Frame Check Sequence (FCS) field that houses the result of the cyclic redundancy check (CRC).
8. Once the frame is completed, it’s handed down to the Physical layer to be put on the physical medium (in this example, twisted-pair wire) one bit at a time.
_
If the CRC matches, then the hardware destination address is checked to see if it matches, too (which, in this example, is the router’s interface Ethernet 0). If it’s a match, then the Ether-Type field is checked to find the protocol used at the Network layer.
10. The packet is pulled from the frame, and what is left of the frame is discarded. The packet is handed to the protocol listed in the Ether-Type field—it’s given to IP.
11. IP receives the packet and checks the IP destination address. Since the packet’s destination address doesn’t match any of the addresses configured on the receiving router itself, the router will look up the destination IP network address in its routing table.
12. The routing table must have an entry for the network 172.16.20.0, or the packet will be discarded
Immediately and an ICMP message will be sent back to the originating device with a “destination network unreachable” message.
13. If the router does find an entry for the destination network in its table, the packet is switched to the exit interface—in this example, interface Ethernet 1.
14. The router packet-switches the packet to the Ethernet 1 buffer.
15. The Ethernet 1 buffer needs to know the hardware address of the destination host and first checks the ARP cache. If the hardware address of Host_B has already been resolved, then the packet and the
hardware address are handed down to the Data Link layer to be framed. If the hardware address has not already been resolved, the router sends an ARP request out E1 looking for the hardware address of 172.16.20.2. Host_B responds with its hardware address, and the packet and destination hardware address are both sent to the Data Link layer for framing.
16. The Data Link layer creates a frame with the destination and source hardware address, Ether-Type field, and FCS field at the end of the frame. The frame is handed to the Physical layer to be sent out on the physical medium one bit at a time.
17. Host_B receives the frame and immediately runs a CRC. If the result matches what’s in the FCS field, the hardware destination address is then checked. If the host finds a match, the Ether-Type field is then checked to determine the protocol that the packet should be handed to at the Network layer—IP, in this example.
18. At the Network layer, IP receives the packet and checks the IP destination address. Since there’s finally a match made, the protocol field is checked to find out whom the payload should be given to.
19. The payload is handed to ICMP, which understands that this is an echo request. ICMP responds to this by immediately discarding the packet and generating a new payload as an echo reply.
20. A packet is then created including the source and destination address, protocol field, and payload. The destination device is now Host_A.
21. IP then checks to see whether the destination IP address is a device on the local LAN or on a remote network. Since the destination device is on a remote network, the packet needs to be sent to the default gateway.
22. The default gateway IP address is found in the Registry of the Windows device, and the ARP cache is checked to see if the hardware address has already been resolved from an IP address.
_
_ The Ether-Type field with 0x0800 (IP) in it
_ The FCS field with the CRC result in tow
25. The frame is now handed down to the Physical layer to be sent out over the network medium one bit at a time.
26. The router’s Ethernet 1 interface receives the bits and builds a frame. The CRC is run, and the FCS field is checked to make sure the answers match.
27. Once the CRC is found to be okay, the hardware destination address is checked. Since the router’s interface is a match, the packet is pulled from the frame and the Ether-Type field is checked to see what protocol at the Network layer the packet should be delivered to.
28. The protocol is determined to be IP, so it gets the packet. IP runs a CRC check on the IP header first, and then checks the destination IP address.
29. But the router does know how to get to network 172.16.10.0—the exit interface is Ethernet 0—so the packet is switched to interface Ethernet 0.
30. The router checks the ARP cache to determine whether the hardware address for 172.16.10.2 has already been resolved.
31. Since the hardware address to 172.16.10.2 is already cached from the originating trip to Host_B, the hardware address and packet are handed to the Data Link layer.
32. The Data Link layer builds a frame with the destination hardware address and source hardware address, and then puts IP in the Ether-Type field. A CRC is run on the frame, and the result is placed in the FCS field.
33. The frame is then handed to the Physical layer to be sent out onto the local network one bit at a time.
34. The destination host receives the frame, runs a CRC, checks the destination hardware address, and looks in the Ether-Type field to find out whom to hand the packet to.
35. IP is the designated receiver, and after the packet is handed to IP at the Network layer, it checks the protocol field for further direction. IP finds instructions to give the payload to ICMP, and ICMP determines the packet to be an ICMP echo reply.
*
AUI to Host
*
AUI to Host
*
Host A can ping router R1 and R2
*
Static routing
Default routing
Dynamic routing
No bandwidth usage between routers
Adds security
Administrator must really understand the internetwork
If a network is added to the internetwork, the administrator has to add a route to it on all routers
Not feasible in large networks
*
R1(config)#ip route network [mask]
{address | interface}[distance] [permanent]
Purpose: This figure describes the command syntax used to establish an IP static route.
Emphasize: A static route allows manual configuration of the routing table. No dynamic changes to this table entry will occur as long as the path is active. Routing updates are not sent on a link that is only defined by a static route; hence, conserving bandwidth.
The ip route field descriptions are as follows:
network—Destination network or subnet
mask—Subnet mask
address—IP address of next-hop router
interface—Name of the interface to use to get to the destination network
*
ip route The command used to create the static route.
destination_network The network you’re placing in the routing table.
mask The subnet mask being used on the network.
next-hop_address The address of the next-hop router that will receive the packet and forward it to the remote network. This is a router interface that’s on a directly connected network.
exitinterface You can use it in place of the next-hop address if you want, but it’s got to be on a point-to-point link, such as a WAN
administrative_distance By default, static routes have an administrative distance of 1 (or even 0 if you use an exit interface instead of a next-hop address)
permanent If the interface is shut down, or the router can’t communicate to the next-hop router, the route will automatically be discarded from the routing table. Choosing the permanent option keeps the entry in the routing table no matter what happens.
ip route [destination_network] [mask] [next-hop_address or exitinterface]
[administrative_distance] [permanent
R1(config)#ip route 30.0.0.0 255.0.0.0 20.0.0.2
Purpose: This figure describes the command syntax used to establish an IP static route.
Emphasize: A static route allows manual configuration of the routing table. No dynamic changes to this table entry will occur as long as the path is active. Routing updates are not sent on a link that is only defined by a static route; hence, conserving bandwidth.
Describe the The ip route field descriptions:
network—destination network or subnet
mask—subnet mask
address—IP address of next hop router
interface—name of interface to use to get to destination network.
*
R2# config t
R3# config t
The router Serial to serial
AUI to Host
*
Verifying Static
Route Configuration
After static routes are configured it is important to verify that they are present in the routing table and that routing is working as expected.
The command show running-config is used to view the active configuration in RAM to verify that the static route was entered correctly.
*
R2# config t
R3# config t
The router Serial to serial
AUI to Host
*
Can only use default routing on stub networks
Stub networks are those with only one exit path out of the network
The only routers that are considered to be in a stub network are R1 and R3
S0
S0
E0
E0
10.0.0.1
10.0.0.2
40.0.0.2
20.0.0.1
20.0.0.2
30.0.0.1
S0
S1
30.0.0.2
40.0.0.1
A
B
172.16.2.1
SO
172.16.1.0
B
172.16.2.2
Network
A
B
This route allows the stub network to reach all known networks beyond router A.
10.0.0.0
Purpose: This figure gives an example of a default route configuration.
*
Configuring Default Routes
Default routes are used to route packets with destinations that do not match any of the other routes in the routing table.
A default route is actually a special static route that uses this format:
ip route 0.0.0.0 0.0.0.0 [next-hop-address | outgoing interface]
This is sometimes referred to as a “Quad-Zero” route.
Example using next hop address:
Router(config)#ip route 0.0.0.0 0.0.0.0 172.16.4.1
Example using the exit interface:
Router(config)#ip route 0.0.0.0 0.0.0.0 s0/0
*
R3# config t
R2# config t
A
B
AUI to Host
*
Routing protocols are
routing tables.
Once the path is determined a router can route a routed protocol.
Network
Protocol
Destination
Network
Connected
RIP
IGRP
10.120.2.0
172.16.2.0
172.17.3.0
Purpose: This figure introduces students to routing protocols and compares routing protocols to routed protocols.
Emphasize: If network 10.120.2.0 wants to know about network 172.16.2.0, it must learn it from its S0 (or possibly S1) interface.
If you have100’s of routers it become difficult to configure static routes
Static has less overhead,
Autonomous System
AS 2000
AS 3000
AS 1000
An Autonomous System (AS) is a group of IP networks, which has a single and clearly defined routing policy.
Group of routers which can exchange updates
AS are identified by numbers
Fig. 48 IGP and EGP (TI1332EU02TI_0004 The Network Layer, 67)
All Routing protocols are categorized as IGP or EGP
Routing Categories
AS 1000
AS 2000
AS 3000
Fig. 49 The use of IGP and EGP protocols (TI1332EU02TI_0004 The Network Layer, 67)
Routing Categories
Exterior Gateway
Protocol (EGP)
Autonomous Systems: Interior or Exterior Routing Protocols
An autonomous system is a collection of networks under a common administrative domain.
IGPs operate within an autonomous system.
EGPs connect different autonomous systems.
Purpose: This figure discusses autonomous systems, IGPs and EGPs.
Emphasize: Introduce the interior/exterior distinctions for routing protocols, as follows:
Interior routing protocols are used within a single autonomous system
Exterior routing protocols are used to communicate between autonomous systems
The design criteria for an interior routing protocol require it to find the best path through the network. In other words, the metric and how that metric is used is the most important element in an interior routing protocol.
Exterior protocols are used to exchange routing information between networks that do not share a common administration. IP exterior gateway protocols require the following three sets of information before routing can begin:
A list of neighbor (or peer) routers or access servers with which to exchange routing information
A list of networks to advertise as directly reachable
The autonomous system number of the local router
*
Purpose: This figure introduces the three classes of routing protocols.
Emphasize: There is no single best routing protocol.
*
Distance Vector
RIP V1
Classful Routing Overview
Classful routing protocols do not include the subnet mask with the route advertisement.
Within the same network, consistency of the subnet masks is assumed.
Summary routes are exchanged between foreign networks.
Examples of classful routing protocols:
RIP Version 1 (RIPv1)
Classless routing protocols include the subnet mask with the route advertisement.
Classless routing protocols support variable-length subnet masking (VLSM) and subnetting
Examples of classless routing protocols:
RIP Version 2 (RIPv2)
routers and accumulate distance vectors.
Distance vector algorithms do not allow a router to know the exact topology of an internetwork.
This information is somewhat same to the information found on signs at a highway intersection. A sign points toward a road leading away from the intersection and indicates the distance to the destination.
Further down the highway, another sign also points toward the destination, but now the distance to the destination is shorter.
Distance is interm of HOPS
Vector is direction
Exchange entire routing tables with all neighbors at regular intervals
More BW consumed
*
Uses Bellman Ford Algorithm
It needs to find out the shortest path from one network to other
How to determine which path is best?
192.168.10.1
192.168.20.1
Basis for All other routing protocol Algorithm
Think that you are going for a trip from Mumbai to Delhi, there are two paths one 1500 Kms and another 1200Kms
The routing table contains information about two routes
*
There are two Distance Vector Protocol, Both uses different metric
RIP – Hops
IGRP - Composite
*
RIP uses only Hop count
RI routing table metric for 192.168.20.1 network will be
3
2
192.168.20.1
0
1
1
2
2
3
R1
192.168.10.1
RI routing table metric for 192.168.20.1 network will be
30
60
192.168.20.1
Routing Loops
A network problem in which packets continue to be routed in an endless circle
When Network 5 fails, Router E tells Router C. This causes Router C to stop routing to Network5 through Router E. But Routers A, B, and D don’t know about Network 5 yet, so they keep sending out update information. Router C will eventually send out its update and cause B to stop routing to Network 5, but Routers A and D are still not updated. To them, it appears that Network 5 is still available through Router B with a metric of 3.
*
destinations from each neighbor.
Layer 3 of 3
*
Inconsistent Routing Entries
Each node maintains the distance from itself to each possible destination network.
Slide 1 of 4
Purpose: This figure describes the first of the general problems that a distance vector protocol could face without the corrective influence of some countermeasure.
*
Slide 2 of 4
*
Inconsistent Routing Entries (Cont.)
Router C concludes that the best path to network 10.4.0.0 is through router B.
Slide 3 of 4
*
Inconsistent Routing Entries (Cont.)
Router A updates its table to reflect the new but erroneous hop count.
Slide 4 of 4
Emphasize: In Layer 4, router A receives the new routing table from router B, detects the modified distance vector to network 10.4.0.0, and recalculates its own distance vector to network 10.4.0.0 as 3.
If all routers in an internetwork do not have up-to-date, accurate information about the state of the internetwork, they might use incorrect routing information to make a routing decision.
*
Hop count for network 10.4.0.0 counts to infinity.
Purpose: This figure describes another of the general problems that a distance vector protocol could face without the corrective influence of some countermeasure.
Emphasize: Both routers conclude that the best path to network 10.4.0.0 is through each other and continue to bounce packets destined for network 10.4.0.0 between each other, incrementing the distance vector by 1 each time.
This condition, called count to infinity, continuously loops packets around the network, despite the fundamental fact that the destination network 10.4.0.0 is down. While the routers are counting to infinity, the invalid information allows a routing loop to exist.
*
Routing Loops
Packets for network 10.4.0.0 bounce (loop) between routers B and C.
Slide 4 of 4
Emphasize: In Layer 4, router A receives the new routing table from router B, detects the modified distance vector to network 10.4.0.0, and recalculates its own distance vector to network 10.4.0.0 as 3.
If all routers in an internetwork do not have up-to-date, accurate information about the state of the internetwork, they might use incorrect routing information to make a routing decision.
*
Defining a Maximum
Define a limit on the number of hops to prevent infinite loops.
Purpose: This figure describes a corrective measure that attempts to solve the routing loop problems that a distance vector protocol could face.
Emphasize: Routing loops occur only when routing knowledge being propagated has not yet reached the entire internetwork—when the internetwork has not converged after a change. Fast convergence minimizes the chance for a routing loop to occur, but even the smallest interval leaves the possibility open.
To avoid prolonging the count-to-infinity time span, distance vector protocols define infinity as some maximum number. This number refers to a routing metric, such as a hop count.
*
Maximum Hop Count
One way of solving routing loop problem is to define a maximum hop count.
RIP permits a hop count of up to 15, so anything that requires 16 hops is deemed unreachable
*
Split Horizon
It is never useful to send information about a route back in the direction from which the original information came.
Purpose: This figure introduces the corrective measure known as “split horizon.” The split horizon technique attempts to solve routing loops.
Emphasize: The split horizon technique attempts to eliminate routing loops and speed up convergence. The rule of split horizon is that it is never useful to send information about a route back in the direction from which the original packet came. In the example:
Router C originally announced a route to network 10.4.0.0 to router B. It makes no sense for router B to announce to router C that router B has access to network 10.4.0.0 through router C.
Given that router B passed the announcement of its route to network 10.4.0.0 to router A, it makes no sense for router A to announce its distance from network 10.4.0.0 to router B.
Because router B has no alternative path to network 10.4.0.0, router B concludes that network 10.4.0.0 is inaccessible.
In its basic form, the split-horizon technique simply omits from the message any information about destinations routed on the link. This strategy relies either on routes never being announced or on old announcements fading away through a timeout mechanism.
Split horizon also improves performance by eliminating unnecessary routing updates. Under normal circumstances, sending routing information back to the source of the information is unnecessary.
* Splithorizon - the routing protocol advertises routes out an interface only if they were
not learned from updates entering that interface.
*
Solution to the Routing Loop problem
Split Horizon is a rule that routing information cannot be sent back in the direction from which it was received
Had split horizon been used in our example, Router B would not have included information about network 10.4.0.0 in its update to Router C.
*
Route Poisoning. Usually used in conjunction with split horizon
Route poisoning involves explicitly poisoning a routing table entry for an unreachable network
*
Triggered Updates
New routing tables are sent to neighboring routers on a regular basis.
RIP updates occur every 30 seconds
However a triggered update is sent immediately in response to some change in the routing table.
The router that detects a topology change immediately sends an update message to adjacent routers that, in turn, generate triggered updates notifying their adjacent neighbors of the change.
Triggered updates, used in conjunction with route poisoning, ensure that all routers know of failed routes.
*
Holddowns
Holddowns are a technique used to ensure that a route recently removed or changed is not reinstated by a routing table update from another route
Holddown prevents regular update messages from reinstating a route that is going up and down (called flapping)
Holddowns prevent routes from changing too rapidly by allowing time for either the downed route to come back up
Holddowns make a router wait a period of time before accepting an update for a network whose status or metric has recently changed
*
Solution: Holddown Timers
RIP versions 1 and 2 also use the concept of hold timers. When a destination has become
unreachable (or the metric has increased enough to cause poisoning), the destination goes
into "holddown". During this state, no new path will be accepted for the same destination
for this amount of time. The hold time indicates how long this state should last.
* Hold-down timer - After finding out that a router to a subnet has failed, a router waits a
*
192.168.10.1
192.168.20.1
1Mbps
1Mbps
56kbps
56kbps
RIP uses only hop count to determine the best path to a network. If RIP finds more than one link to the same remote network with the same hop count, it will automatically perform a round-robin load balancing. RIP can perform load balancing for up to six equal-cost links (four by default).
However, a problem with this type of routing metric arises when the two links to a remote network are different bandwidths but the same hop count. Figure 5.9, for example, shows two links to remote network 172.16.10.0.
*
RIP Timers
Route update timer Sets the interval (typically 30 seconds) between periodic routing updates
Route invalid timer Determines the length of time (180 seconds) before a router determines that a route has become invalid
Holddown timer This sets the amount of time during which routing information is suppressed. This continues until either an update packet is received with a better metric or until the holddown timer expires. The default is 180 seconds
*
Routing Information Protocol (RIP) is a true distance-vector routing protocol.
It sends the complete routing table out to all active interfaces every 30 seconds
RIP only uses hop count to determine the best way to a remote network
It has a maximum allowable hop count of 15
AD is 120
Bellman-ford algorithm
Works well in small networks, but it’s inefficient on large networks
*
The router command starts a routing process.
The network command is required because it enables the routing process to determine which interfaces participate in the sending and receiving of routing updates.
An example of a routing configuration is:
Gates(config)#router rip
Gates(config-router)#network 172.16.0.0
*
AUI to Host
*
IP Routing Table
Purpose: This figure displays the show ip route command, which displays the contents of the router’s IP routing table.
Emphasize: Discuss the IP routing table in detail. Show the locations of the hop count (metric) and the administrative distance (120).
Discuss the following fields:
R—Refers to routes learned from RIP.
via—Refers to the router that informed us about this route.
00:00:07 timer value—RIP updates are every 30 seconds. Ask, “How long until the next update?”
The interfaces used for the best path
*
Emphasize: Explain that debug commands also provide information for monitoring IP.
*
Passive Interface
Passive-interface command prevents RIP update broadcasts from being sent out a defined interface, but same interface can still receive RIP updates
R1#config t
R1(config)#router rip
R1(config-router)#network 192.168.10.0
Passive-interface command depends upon the routing protocol
RIP router with a passive interface will still learn about the networks advertised by other routers
EIGRP, a passive-interface will neither send nor receive updates.
152-5
S0
S0
E0
E0
192.168.0.16/29
S0
S1
192.168.0.4/30
192.168.0.8/30
192.168.0.32/28
1. Find out the IP Address and SNM of each interfaces
A
B
S0
S0
E0
E0
192.168.0.18
255.255.255.248
S0
S1
192.168.0.17
255.255.255.248
192.168.0.5
255.255.255.252
192.168.0.6
255.255.255.252
192.168.0.9
255.255.255.252
192.168.0.10
255.255.255.252
192.168.0.33
255.255.255.240
192.168.0.34
255.255.255.240
A
B
S0
S0
E0
E0
192.168.0.16/29
S0
S1
192.168.0.4/30
192.168.0.8/30
192.168.0.32/28
*
Enabling IGRP
Purpose: This chapter introduces the Cisco IOS™ CLI on the Catalyst® 1900 switch and router.
Timing: This chapter should take about 2 hours to present.
Note: The Catalyst 1900 switch only has a subset of the router Cisco IOS commands available.
Contents:
Introduction to Cisco IOS. Explain to the student what is IOS?
Cisco Device startup procedures in general.
IOS configuration source.
Cat 1900 switch startup procedures.
Intro to Cat 1900 CLI. This part covers the basic configuration on the switch, like setting the IP address and hostname. More details about the various Cat 1900 switch configuration commands are explained in Chapter 6 and 7.
Router startup procedures. More details on the router startup process is discussed in chapter 5.
Router IOS CLI.
Sophisticated metric
Purpose: The figure introduces the IGRP routing protocol. IGRP is a sophisticated distance vector routing protocol.
Emphasize: The Interior Gateway Routing Protocol (IGRP) is a dynamic distance-vector routing protocol designed by Cisco in the mid-1980s for routing in an autonomous system that contains large, arbitrarily complex networks with diverse bandwidth and delay characteristics. Historically, IGRP became one of the success factors for the early Cisco IOS software capabilities because of its superiority to RIP version 1.
The important IGRP characteristics are as follows:
More scalability than RIP
Sophisticated metric
Multiple-path support
Bandwidth
Delay
Reliability
Load
MTU
Purpose: This figure presents the IGRP metric with its five possible components.
Emphasize : Bandwidth and delay are the two metrics that are most commonly used. They also comprise the default metric.
Note: Changing IGRP metrics can have great impact on network performance.
Describe the IGRP 24-bit metric field, as follows:
Bandwidth—Minimum bandwidth on the route, in kilobits per second.
Delay—Route delay, in tens of microseconds.
Reliability—Likelihood of successful packet transmission, expressed as an integer from 0 to 255.
Loading—Effective bandwidth of path.
MTU—Minimum MTU in path, expressed in bytes.
The following equation calculates the metric. It is presented for instructors and is not required to be taught:
metric = [k1 x bandwidth + (k2 x bandwidth) / (256 - load) + k3 x delay]
If k5 does not equal 0, an additional operation is done:
metric = metric x (k5/(reliability + k4))
The default constant values are k1 = k3 = 1 and k2 = k4 = k5 = 0.
Again, if default values are set, metric = bandwidth + delay.
*
Some of the IGRP key design characteristics emphasize the following:
It is a distance vector routing protocol.
Routing updates are broadcast every 90 seconds.
Bandwidth, load, delay and reliability are used to create a composite metric.
The main difference between RIP and IGRP configuration is that when you configure IGRP, you supply the autonomous system number. All routers must use the same number in order to share routing table information.
Can explain pin hole congestion how RIP and IGRP Handles (Load Balancing)
IGRP – Hop count 25, default - 100
*
Update timers these specify how frequently routing-update messages should be sent. The default is 90 seconds.
Invalid timers These specify how long a router should wait before declaring a route invalid if it doesn’t receive a specific update about it. The default is 3*90 = 270.
Holddown timers These specify the holddown period. The default is three times the update timer period plus 10 seconds. 280 seconds
*
AUI to Host
*
LabA#sh ip route
I 192.168.50.0 [100/170420] via 192.168.20.2, Serial0/0
I 192.168.40.0 [100/160260] via 192.168.20.2, Serial0/0
I 192.168.30.0 [100/158360] via 192.168.20.2, Serial0/0
C 192.168.20.0 is directly connected Serial0/0
C 192.168.10.0 is directly connected, FastEthernet0/0
The I means IGRP-injected routes. The 100 in [100/160360] is the administrative distance of IGRP. The 160,360 is the composite metric. The lower the composite metric, the better the route.
To delete all routes
debug ip igrp events Command
summary of the IGRP routing information that is running on the network.
debug ip igrp transactions Command
shows message requests from neighbor routers asking for an update and the broadcasts sent from your router toward that neighbor router.
no debug all – to turn off all debug