Transcript ccna.final.2007 Power Points for Chapter #6

Sybex CCNA 640-802
Chapter 6: IP Routing
1
Chapter 6 Objectives
•
•
•
•
Understanding IP routing
Static routing
Default routing
Dynamic routing
– RIP
– RIPv2
– IGRP
– Verifying routing
–
[Oddly, the exam topics covered in this chapter (6) are listed at the beginning of the chapter.
Some of the topics listed are not really covered in this chapter at all. For example, OSPF and
EIGRP are covered in chapter 7, not chapter 6. ]
2
2
What is Routing?
In order to “route”, a router needs to know:
– Remote Networks
– Neighbor Routers
– All Possible routes to remote network
– The absolute best route to all remote networks
– Maintain and verify the routing information
– Remember: a router does not deal with hosts!
– A router only deals with networks, and the best path to them
– An IP address allows packets to move from network to network
– Hardware (Mac) addresses move the packets to specific hosts
D
3 - 329
C
B
A
Basic Path Selection
On what interface will the
router send out a
packet if it has
destination address of
10.10.10.18?
4
Simple IP Routing
>ping 172.16.1.2
172.16.2.0
172.16.1.0
172.16.3.1 172.16.3.2
e0
e0
A
B
172.16.2.2
Host A
172.16.2.1
s0
s0
B
172.16.1.1
172.16.1.2
Host B
5
Routing/PDU Example:
Host A Web browses to the HTTP Server….
1. The destination address of a frame will be the:
Host A address
2. The destination IP address of a packet will be the IP address of the: Destination Router
3. The destination port number in a
segment header will have a value of 80
(the port number used by HTTP)
6
Idea of routing (5 guest slides)
• Routers forward datagrams between connected
networks
• They need to know via which interface to send
a datagram
• Routing decisions are based on the
information stored in the routing table
Routing table
• Tells where to send datagram for a particular
network
Network
194.181.200.0
193.2.1.0
153.5.0.0
0.0.0.0

Next-Hop
194.181.208.1
194.181.208.320
194.181.214.25
194.181.210.1
“Next-Hop” routers
Port
Eth0
Eth1
Fddi0
S0
Metric
1
14
8
5
must be directly reachable
Routing table (cont.)
• Default Route - a special entry in the routing
table:
– “Pass all datagrams for unknown networks to this
router”
– Represented by the entry for network 0.0.0.0
• Routing uses network part of the address!
Step-by-Step: IP Routing Process
(book, pp 331-36)
• The IP routing process is fairly simple and
doesn’t change, regardless of the size of your
network.
• For an example, we’ll use Figure 6.2 to describe
step-by-step what happens when Host_A wants to
communicate with Host_B on a different network
12 / 331
Step 1
• Internet Control Message Protocol (ICMP)
creates an “echo request” payload (which is
just the alphabet in the data field).
– The echo request is the first part/half of what is
commonly called a “Ping”; the second part is the
echo reply, from the device being “pinged”.
•
[So, A is going to “ping” B]
13
•
•
Step 2
ICMP hands that payload to Internet Protocol
(IP), which then creates a packet.
At a minimum, this packet contains an IP source
address, an IP destination address, and a
Protocol field with 01h.
•
(Remember that Cisco likes to use 0x in front of hex
characters, so this could look like 0x01.)
• All of that tells the receiving host to whom it
should hand the payload when the destination is
reached—in this example, ICMP.
14
Step 3
• Once the packet is created, IP determines
whether the destination IP address is on the
local network or a remote one.
15
Step 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.
16
Step 5
• The default gateway of host 172.16.10.2 (Host_A) is configured
to 172.16.10.1. For this packet to be sent 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.
• Why? So the packet can be handed down to the Data Link
layer, framed, and sent to the router’s interface that’s
connected to the 172.16.10.0 network.
• Because hosts only communicate via hardware addresses on
the local LAN, it’s important to recognize that for Host_A to
communicate to Host_B, it has to send packets to the Media
Access Control (MAC) address of the default gateway.
17
Step 6
• Next, the Address Resolution Protocol (ARP) cache of the host is checked to
see if the IP address of the default gateway has already been resolved to a
hardware address. Two possibilities ensue:
• 1. If it has, the packet is then free to be handed to the Data Link layer for
framing. (The hardware destination address is also handed down with that
packet.) To view the ARP cache on your host, use the following command:
• C:\>arp -a
• Interface: 172.16.10.2 --- 0x3
• Internet Address
Physical Address
Type
• 172.16.10.1
00-15-05-06-31-b0
dynamic
• 2. 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.
18
Step 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 LAN driver provides
communication control between the NOS and NIC (network interface card).
• A frame is then generated, encapsulating the packet with control info.
• 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 that Frame Check Sequence (FCS) field that
houses the result of the cyclic redundancy check (CRC).
• The frame would look something like what is detailed in Figure 6.3. It
contains Host_A’s hardware (MAC) address and the destination hardware
address of the default gateway. It does not include the remote host’s MAC
address—remember that!
FIGURE 6 . 3
Frame used from Host_A to the Lab_A router when Host_B is pinged
Destination MAC
Source MAC
(routers E0 MAC address)
(Host_A MAC address)
Ether-Type Packet
field
FCS
(CRC)
19
Step 7
Once the packet and
destination hardware
ddress are handed to
e Data Link layer, the
LAN driver is used to
rovide media access
a the type of LAN being
used (in this example,
hernet). A frame is then
nerated, encapsulating
he 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.
The frame woul
look something li
what is detailed
Figure 6.3. It
contains Host_A
hardware (MAC
address and th
destination
hardware addre
of the default
gateway. It does
include the remo
host’s MAC
address—
remember that
At the end of the
frame is the Frame
Check Sequence
(FCS) field that
houses the result
of the cyclic
redundancy
check (CRC).
FIGURE 6 . 3
Frame used from Host_A to the Lab_A router when Host_B is pinged
Destination MAC
Source MAC
(routers E0 MAC address)
(Host_A MAC address)
Ether-Type Packet
field
FCS
(CRC)
20
Step 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.
21
Step 9
Every device in the collision
domain receives these bits and
builds the frame. They each
run a CRC and check the
answer in the FCS field.
If the answers don’t match, the
frame is discarded.
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.
22
Step 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 — i.e., it’s given to IP.
– [So now the packet is at the router, having entered at
interface E0, the default gateway for the 172.16.10.0
network.
– Next, the router will try to send the packet to its destination in
the 172.16.20.0 network.
– To do so, it will have to find this network in its routing
tables.]
23
Step 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.
24
Step 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.
– [Note that 172.16.x.x is a Class B network. .10 and
.20 would ordinarily be part of the same network
and therefore couldn’t be set up on 2 networks. But
this network is subnetted, i.e., the subnet mask is
255.255.255.0.
25
Step 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.
• The output below (next slide) displays the Lab_A
router’s routing table. The “C” means “directly
connected.”
• No routing protocols are needed in this network
since all (both) networks are directly connected.
26
Step 13 (continued)
• Lab_A>sh ip route
• Codes: C – connected , S – static , I - IGRP,R - RIP,M - mobile,
– BGP, D - EIGRP,EX - EIGRP external,O - OSPF,IA - OSPF
inter area, N1 - OSPF NSSA external type 1, N2-OSPF NSSA
external type 2, E1 - OSPF external type 1, E2 - OSPF external
type 2, E – EGP, i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia
- IS-IS intearea * - candidate default, U - per-user static route, o
– ODR P - periodic downloaded static route
• Gateway of last resort is not set
• 172.16.0.0/24 is subnetted, 2 subnets
• C 172.16.10.0 is directly connected, Ethernet0
• C 172.16.20.0 is directly connected, Ethernet1
27
Step 14
• The router packet-switches the packet to the
Ethernet 1 buffer.
– [OK, ready to go out to Host_B, but first …]
28
Step 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 and is in
the router’s ARP cache, then the packet and the hardware address are
handed down to the Data Link layer to be framed.
– Let’s take a look at the ARP cache on the Lab_A router by using the
“show ip arp” command:
• Lab_A#sh ip arp
• Protocol Address
Age(min) Hardware Addr Type
Interface
• Internet 172.16.20.1
00d0.58ad.05f4
ARPA Ethernet0
• Internet 172.16.20.2
3
0030.9492.a5dd ARPA Ethernet0
• Internet 172.16.10.1
00d0.58ad.06aa ARPA Ethernet0
• Internet 172.16.10.2 12
0030.9492.a4ac ARPA Ethernet0
– The dash (-) means that this is the physical interface on the router.
29
Step 15 (continued)
• From the output in the previous slide, we can see that the
router knows the 172.16.10.2 (Host_A) and 172.16.20.2
(Host_B) hardware addresses.
– Cisco routers will keep an entry in the ARP table for 4 hours.
• 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.
30
Step 16
• The Data Link layer creates a frame with the
destination and source hardware address,
Ether-Type field, and FCS field at the end.
– [Still a small packet – just four fields]
• The frame is handed to the Physical layer to be
sent out on the physical medium one bit at a
time.
– [Now we see packets actually going to Host_B]
31
Step 17
• Host_B receives the frame and immediately runs a
CRC. [finally!!]
• 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.
– [IP is by far the most common Layer 3 protocol.]
– [Moving up the OSI model. Data Link to Network]
m
as
s
Step 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 to whom
the payload should be given.
33
Step 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.
34
Step 20
• A packet is then created, including the
– source and destination addresses,
– Protocol field, and
– payload.
• The destination device is now Host_A
35
Step 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.
36
Step 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.
– You can search the Registry by going into the
Registry Editor (start/Run/regedit), then searching
for “DefaultGateway” (F3 – enter search parameters).
– See “Default” / “DHCP Default Gateway” next slide
37
Step 22 (continued)
Above is a view of my home computer’s Registry settings:
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\longkey\Parameters\Tcpip
38
Step 23
• Once the hardware address of the default
gateway is found, the packet and destination
hardware addresses are handed down to the
Data Link layer for framing.
39
Step 24
• The Data Link layer frames the packet of
information and includes the following in the
header:
1. The destination & source hardware addresses
2. The Ether-Type field [with 0x0800 (IP) in it]
3. The FCS field with the CRC result in tow
40
Step 25
• The frame is now handed down to the
Physical layer to be sent out over the network
medium one bit at a time.
41
Step 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.
42
Step 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 EtherType field is checked to see to what protocol
at the Network layer the packet should be
delivered.
43
Step 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.
– IP does not run a complete CRC as the Data Link
layer does—it only checks the header for errors.
44
Informational note: Between 29 and 30
Since the IP destination
address doesn’t match
any of the router’s
interfaces, the routing
table is checked to see
whether it has a route to
172.16.10.0.
If it doesn’t have a route
over to the destination
network the packet will be
discarded immediately
This is the source-point of
confusion for a lot of
administrators—when a
ping fails, most people
think the packet never
reached the destination
host.
But as we see here, that’s
not always the case!
All it takes is for just one of
the remote routers to be
lacking a route back to
the originating host’s
network and
— poof ! —
the packet is dropped on
the return trip, not on its
way to the host.
45
Troubleshooting note: Between 29 and 30
Just a quick note to mention
that if the packet is lost on
the way back to the
originating host, you will
typically see a
“request timed out”
message,
because it is an unknown
error.
If the error occurs because of
a known issue, (such as if a
route is not in the routing
table) on the way to the
destination device, you will
see a
“destination unreachable”
message.
This should help
you determine if
the problem
occurred on the way
to the destination or
on the way back.
46
Step 29
• In this case, 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.
47
Step 30
• The router checks the ARP cache to determine
whether the hardware address for 172.16.10.2
has already been resolved.
48
Step 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.
49
Step 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.
50
Step 33
• The frame is then handed to the Physical
layer to be sent out onto the local network
one bit at a time.
51
Step 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 to whom to hand the packet.
52
Step 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.
53
Step 36
• ICMP acknowledges that it has received the
reply by sending an exclamation point (!) to
the user interface.
• ICMP then attempts to send four more echo
requests to the destination host.
•The End
54
Post Script
• These steps are the basic routing process, no
matter how large the network.
– There would just be more hops in a big internetwork.
• Point to recap:
– Moving from router to router in a big internetwork, at
each hop the hardware address changes; from one
router’s Mac address to the next’s.
– But from hop to hop, the IP address remains the
same!
– This reflects the fact that hardware addresses
(Mac) are always local, while logical addresses (IP,
for example), are always remote.
55
• I.e., in a local LAN, you always use a Mac addrss, not IP.
Exercises: Test IP Routing Understanding
Key Points: pp 336 - 362
Example 1: pp 336-37 – Here, the point is that if you
have multiple hosts communicating to the server
using HTTP, they must all use a different source
port number. That is how the server keeps the data
separated at the Transport layer.
Example 2: p 337ff – Switches have nothing to do
with routing!
Example 3: p 338 – ICMP error messages are sent
by the router with the problem device, such as an
interface which is down.
56
Exercises: Test IP Routing Understanding
Key Points: pp 338-39
Look at the output of a corporate router’s routing table:
Corp#sh
ip route
[output
cut]
R
192.168.215.0
[120/2] via
via 192.168.20.2,
192.168.20.2, 00:00:23,
00:00:23, Serial0/0
Serial0/0
R
192.168.115.0
[120/1]
R
192.168.30.0
[120/1]
via
192.168.20.2,
00:00:23,
Serial0/0
192.168.20.0 isisdirectly
C 192.168.214.0
directlyconnected,
connected,Serial0/0
FastEthernet0/0
The
corporate
router
received
anshow
IP packet
with
source 192.168.22.0
IP address of(or
192.168.214.20
and
a
Since
the routing
table
doesn’t
a route
to anetwork
a default route),
the
destination
address
192.168.22.3,
do you“destination
think the Corp
router will domessage
with this back
packet?
router will discard
theofpacket
and sendwhat
an ICMP
unreachable”
out
Normally,
a
router
will
have
a
default
route
set
up,
AKA
a
“gateway
of
last
resort”.
interface FastEthernet 0/0
57
Configuring IP Routing
• This is a project that runs from pp 336 to 362.
• Setup: 5 Routers and an wireless Access Point
• Neither of our network simulators has these
routers, so all we can do is read over the
configurations.
• Notes:
– P.345: With an ISR router, no need to use the
“clock rate” command; they automatically detect it.
– P346: See the interface “serial 0/0/1”. The book
explains the way interfaces are labeled in a couple
of places:
• Pg 184 and 195: “x/y/z Slot/Subslot/Port” (brief)
58
Configuring IP Routing (continued)
• Notes: (continued)
– Page 205: Better explanation here:
– Some modular routers use three numbers
instead of two.
– The first 0 is the router itself, and then you choose
the slot, and then the port. Here’s an example of a
serial interface on a 2811:
•
•
•
•
•
•
Todd(config)#interface serial ?
<0-2> Serial interface number
Todd(config)#interface serial 0/0/?
<0-1> Serial interface number
Todd(config)#interface serial 0/0/0
Todd(config-if)#
59
Configuring IP Routing (continued)
• Notes: (continued)
– You should always view a running-config output
first so you know what interfaces you have to deal
with. Here’s a 2801 output:
–
–
–
–
–
–
–
–
–
–
–
Todd(config-if)#do show run
Building configuration...
[output cut]
!
interface FastEthernet0/0
no ip address
Shutdown
duplex auto
speed auto
!
interface FastEthernet0/1
[continued on next slide]
60
Configuring IP Routing (continued)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/0/0
no ip address
shutdown
no fair-queue
!
interface Serial0/0/1
no ip address
shutdown
!
interface Serial0/1/0
[continued in next column]
–
–
–
–
–
–
–
–
–
no ip address
shutdown
!
interface Serial0/2/0
no ip address
shutdown
clock rate 2000000
!
[output cut]
61
Configuring IP Routing (continued)
• At other times you may see a x/x/x config for
modular units (like WICs) where you have a
slot, a subslot, and a port. From Cisco.com:
– “The slot/subslot/port format only applies to WIC
interfaces. Interfaces that are native to the network
modules still use only the slot/port format. That is:
• <interface-name> slot/port is used whenever the
interfaces are native on the network module.
• <interface-name> slot/subslot/port is used whenever
the interfaces are on the WIC slot of a network module
(NM).”
• There are still more examples where the
interface is a 3-part config.
62
Configuring IP Routing (continued)
• Notes: (continued)
– Pg 346-47: Just a command idiosyncrasy:
– With ISR routers you can’t use “erase start”, you must enter
“erase startup-config”
– This is so even though no other command begins with “S”:
• Eg: Router#erase s?
•
startup-config
• So under the normal rules of the Cisco IOS, “erase s”
should work exactly like “erase startup-config”, but it
doesn’t.
– This is probably just an oversight that will be
corrected in the next IOS version. Just be aware
that you will sometimes find anomalies like this.
63
Configuring IP Routing (continued)
• Notes: (continued)
– Pg 351 ff: Wireless interfaces: 2 things unique to them:
• SSID #: “The Service Set Identifier that creates a
wireless network that hosts can connect to.”
• DHCP Pool for wireless clients: Actually just like DHCP
with wired clients. More on this in Chapter 12.
– Pg 352 ff: Author uses the SDM here – “Security Device
Manager” to configure interface R3 in the example.
• The book goes through a series of steps using the SDM’s wizard –
through page 359.
64
Configuring IP Routing in Our Network
• Even after the previous pages/slides, we still we
need to do some things to get our network up to
speed.
• 3 things to do:
1. Static Routing
2. Default Routing
3. Dynamic Routing
65/362
Static Routes
Stub Network
172.16.1.0
172.16.2.0
SO
AA
172.16.3.1
SO
172.16.3.2
B
B
Routes must be unidirectional
66 /364
Static Route Configuration
ip route remote network
[mask]
{address|interface}
[distance] - all static routes have a distance of “1”; very trustworthy
[permanent] - to keep the route in the table no matter what;
even if the interface goes down.
Router(config)#ip route remote_network mask next_hop
This means: to get here (ip address and mask) go here next (address only)
Router(config)#172.16.1.22 255.255.0.0
192.168.5.45
You can optionally add a distance if you want to change the metric of the route;
for example, you may want to prefer any dynamic route
Static Route Example
Stub Network
172.16.2.0
172.16.1.0
SO
SO
A
172.16.3.1
172.16.3.2
B
B
ip route 172.16.1.0 255.255.255.0 172.16.3.2
.
or
ip route 172.16.1.0 255.255.255.0 s0
68
Default Routes
Stub Network
172.16.1.0
172.16.2.0
SO
SO
creates a wireless
A network that hosts can B
B
172.16.3.1
172.16.3.2
connect to.
To send packets with a remote destination network not in the routing table to
the next-hop router, only used for stub networks.
ip route 0.0.0.0 0.0.0.0 172.16.3.1
ip classless
[Note: This configuration sends every packet out Router A’s 3.1 interface]
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Static Route Considerations
• When configuring static routes, consider the
following:
– By default, a static route will take precedence over
a dynamic route because of its lower
administrative distance.
– Without additional configuration, a dynamic route
to a network will be ignored if a static route is
present in the routing table for the same network.
– To reduce the number of static route entries,
define a summarized or default static route
Static Route Considerations
• The benefit of using static routes is that they do
not require the router to spend CPU cycles
and memory space to determine the best route
to a destination. The route has already been
placed in the routing table manually.
• This can work against the network, however, if
a device in the static route’s path goes down.
In this case, the packets may still attempt to use
the path (especially if the “permanent” option is
chosen), and in any event, no other route will
be chosen, as in a dynamic routing network,
because the static route has limited the choices.
71
Routing Protocols (Dynamic)
• Routing protocols are used between routers to:
– Determine the path of a packet through a network
– Maintain routing tables
– Two types:
• Interior gateway protocols (IGPs)
• exterior gateway protocols (EGPs)
• Examples:
–IGP:
RIP, IGRP, OSPF, IS-IS, EIGRP
–EGP:
Border Gateway Protocol (BGP)
• [Note: This is only one way to distinguish between routing
protocols; others include: distance vector v. link state, and
we’ve already begun to distinguish static v. dynamic]
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Routing Protocols
IGPs: RIP, IGRP
EGPs: BGP
Autonomous System 1
Autonomous System 2
• An autonomous system is a collection of networks under a
“common administrative domain”, i.e., all routers sharing the same
routing table are in the same AS.
• IGPs operate within an autonomous system.
• EGPs connect different autonomous systems.
73
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)
• IGRP
• [The problem with classful routes is that they don’t
74
Classless Routing Overview
• Classless routing protocols include the subnet
mask with the route advertisement.
– Classless routing protocols support variable-length
subnet masking (VLSM).
– Summary routes can be manually controlled within the
network.
– Examples of classless routing protocols:
•
•
•
•
RIP Version 2 (RIPv2)
EIGRP
OSPF
IS-IS
75
Classful Versus Classless
Routing Protocols
– A classful routing protocol always considers the
IP network class
• Address summarization is automatic by major
network number and discontiguous subnets are not
visible to each other
– Classless protocols transmit prefix-length or
subnet mask information with IP network
addresses.
• The IP address can be mapped so that discontinuous
subnets and VLSM are supported
76
Administrative Distance
Default Administrative Distance
Directly Connected: 0
Static Route:
1
RIP:
120
IGRP:
100
EIGRP:
90
OSPF:
110
Router B
Router A
IGRP
Administrative
Distance=100
RIP
Administrative
Distance=120
Router C
Router D
The administrative distance (AD) is used to rate the trustworthiness of routing
information received on a router from a neighbor router. An administrative distance is
an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be
passed via this route.
If a router receives two updates listing the same remote network, the first thing the
router checks is the AD. If one of the advertised routes has a lower AD than the other,
then the route with the lowest AD will be placed in the routing table.
If both advertised routes to the same network have the same AD, then routing protocol
will be used to find the best path to the remote network. The advertised route with the
lowest metric will be placed in the routing table. If it’s a tie, load balancing 77
is used.
77
Distance Vector
Distance—How far
Vector—In which direction
D
Routing
Table
C
B
A
Routing
Table
Routing
Table
Routing
Table
All routers just broadcast their entire routing table out all active interfaces on
periodic time intervals
Distance vector algorithms do not allow a router to know the exact topology of an
internetwork.
78 / 379
Discovering Routes
79
Discovering Routes: Converged Routing Tables
By “converged” we mean that each of the routers above has the same view of
the internetwork, i.e., each router sees the same number of links from one
router to any other router.
Meaning of Distance Vector (1/2)
• A router using a distance vector routing protocol
does not have the knowledge of the entire
path to a destination network.
• The router only knows
– The direction or interface in which packets should
be forwarded and
– The distance or how far it is to the destination
network
Meaning of Distance Vector (2/2)
Operation of distance vector (1/4)
• Some distance vector routing protocols call for
the router to periodically broadcast the entire
routing table to each of its neighbors.
• This method is inefficient because the updates
not only consume bandwidth but also
consume router CPU resources to process the
updates.
Operation of distance vector (2/4)
• Periodic Updates are sent at regular intervals
(30 seconds for RIP and 90 seconds for IGRP).
– Even if the topology has not changed in several
days, periodic updates continue to be sent to all
neighbors.
– Neighbors are routers that (1) share a link and are
configured to (2) use the same routing protocol.
– The router is only aware of the network addresses of
its own interfaces and the remote network
addresses it can reach through its neighbors
Operation of distance vector (3/4)
• Broadcast Updates are sent to 255.255.255.255
– Neighboring routers that are configured with the
same routing protocol will process the updates.
– All other devices will also process the update up
to Layer 3 before discarding it.
– Some distance vector routing protocols use
multicast addresses instead of broadcast
addresses.
Operation of distance vector (4/4)
• Entire Routing Table Updates are sent,
periodically to all neighbors.
– Neighbors receiving these updates must process
the entire update to find pertinent information and
discard the rest.
– Some distance vector routing protocols like EIGRP
do not send periodic routing table updates.
Routing Algorithm
• The algorithm used for the routing protocols
defines the following processes:
– Mechanism for sending and receiving routing
information.
– Mechanism for calculating the best paths and
installing routes in the routing table.
– Mechanism for detecting and reacting to topology
changes.
Routing protocol characteristics
(1/3)
• Time to Convergence - Time to convergence
defines how quickly the routers in the network
topology share routing information and
reach a state of consistent knowledge.
– The faster the convergence, the more preferable
the protocol.
– Routing loops can occur when inconsistent
routing tables are not updated due to slow
convergence in a changing network.
Routing protocol characteristics
(2/3)
• Scalability - Scalability defines how large a network
can become based on the routing protocol that is
deployed.
– The larger the network is, the more scalable the routing
protocol needs to be.
• Classless (Use of VLSM) or Classful - Classless
routing protocols include the subnet mask in the
updates.
– This feature supports the use of Variable Length Subnet
Masking (VLSM) and better route summarization.
– Classful routing protocols do not include the subnet mask
and cannot support VLSM.
Routing protocol characteristics
(3/3)
• Resource Usage - Resource usage includes the
requirements of a routing protocol such as memory
space, CPU utilization, and link bandwidth
utilization
– Higher resource requirements necessitate more powerful
hardware to support the routing protocol operation in
addition to the packet forwarding processes.
• Implementation and Maintenance - Implementation
and maintenance describes the level of knowledge
that is required for a network administrator to
implement and maintain the network based on the
routing protocol deployed.
Distance Vector Routing Protocols
Comparison of Routing Protocol
Routing Loops (1/6)
• 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.
• A routing loop can occur when two or more
routers have routing information that
incorrectly indicates that a valid path to an
unreachable destination exists.
Routing Loop (2/6)
• The loop may be a result of:
– Incorrectly configured static routes
– Incorrectly configured route redistribution
(redistribution is a process of handing the routing
information from one routing protocol to another
routing protocol)
– Inconsistent routing tables not being updated
due to slow convergence in a changing network
– Incorrectly configured or installed “discard
routes”
Routing Loop (3/6)
Routing Loop (4/6)
Routing Loop (5/6)
Routing Loop (6/6)
Routing Loops & Ways to Stop Them
• Maximum hop count, AKA, Counting to Infinity:
RIP
permits a hop count of up to 15. At 16 hops, a route is considered to be an
infinite distance away.
• This is called counting to infinity, and it’s caused by gossip (broadcasts)
and wrong information being communicated and propagated throughout
the internetwork.
• Without some form of intervention, the hop count increases indefinitely
each time a packet passes through a router.
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Count to infinity (1/5)
• 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.
Count to infinity (2/5)
Count to infinity (3/5)
Count to infinity (4/5)
Count to infinity (5/5)
Routing Loops
Split Horizon:
• Routing information cannot be sent back in the direction from
which it was received.
105 /
Split Horizon Rules (1/5)
• The split horizon rule says that a router
should not advertise a network through
the interface from which the update came.
Split Horizon Rules (2/5)
Split Horizon Rules (3/5)
Split Horizon Rules (4/5)
Split Horizon Rules (5/5)
Routing Loops
• Route poisoning:
•
•
•
Advertising the downed network as unreachable.
When one router receives a route poisoning from another, it sends an update,
called a poison reverse, back to the other router.
This ensures that all routes on the segment have received the poisoned route
information
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Route Poisoning (1/4)
• Route poisoning is yet another method
employed by distance vector routing
protocols to prevent routing loops.
• 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 (2/4)
Route Poisoning (3/4)
Route Poisoning (4/4)
Split Horizon with Poison reverse
(1/5)
• Now we can put Split Horizon together with
Route Poisoning / 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.
Split Horizon with Poison reverse
(2/5)
• The following process occurs:
• Network 10.4.0.0 becomes unavailable due to a link
failure.
• R3 poisons the metric with a value of 16 and then
sends out a triggered update stating that 10.4.0.0 is
unavailable.
• R2 processes that update, invalidates the routing
entry in its routing table, and immediately sends a
poison reverse back to R3.
Split Horizon with Poison reverse
(3/5)
Split Horizon with Poison reverse
(4/5)
Split Horizon with Poison reverse
(5/5)
Ways to Stop Router Loops
• Holddowns:
•
•
•
•
Prevents regular update messages
from reinstating a route that is going up and down
(called flapping). Typically, this happens on a serial link
that’s losing connectivity and then coming back up.
Holddown timers introduce a certain amount of
skepticism to reduce the acceptance of bad routing
information.
If the distance to a destination increases (for
example, the hop count increases from 2 to 4), the
router sets a holddown timer for that route.
Until the timer expires, the router will not accept any
new updates for the route.
This is only one type of timer used with RIP – see
next 3 slides:
RIP Timers (1/3)
• In addition to the update timer, the IOS implements
three additional timers for RIP:
• Invalid Timer. If an update has not been received to
refresh an existing route after 180 seconds (the
default), the route is marked as invalid by setting the
metric to 16.
– The route is retained in the routing table until the “flush
timer” expires.
• Flush Timer. By default, the flush timer is set for 240
seconds, which is 60 seconds longer than the
invalid timer. When the flush timer expires, the route
is removed from the routing table.
RIP Timers (2/3)
• Holddown Timer:
This timer stabilizes
routing information and helps prevent routing
loops during periods when the topology is
converging on new information.
– Once a route is marked as unreachable, it must
stay in holddown long enough for all routers in
the topology to learn about the unreachable
network.
– By default, the holddown timer is set for 180
seconds.
RIP Timers (3/3)
RIP Overview
64kbps
T1
T1
T1
–
–
–
–
Hop count metric selects the path, 16 is unreachable
Full route table broadcast every 30 seconds
Load balance maximum of 6 equal cost paths (default = 4)
RIPv2 supports VLSM and Discontiguous networks
RIP Routing Configuration
Router(config)#router rip
Router(config-router)#network network-number*
10.3.5.0
172.16.10.0
router RIP
network 172.16.0.0
network 10.0.0.0
192.168.10.0
router RIP
network 172.16.0.0
network 192.168.10.0
*Network is a classful network address.
Every device on network uses the same subnet mask
126
RIP Version 2
• Allows the use of variable length subnet masks
(VLSM) by sending subnet mask information with
each route update
• Distance Vector – same AD, and timers.
• Easy configuration, just add the command “version
2” under the router rip configuration
router rip
network 10.0.0.0
version 2
127
RIPv1 vs. RIPv2
RIPv1
RIPv2
Distance vector
Distance vector
Maximum hop count
15
Maximum hop
count 15
Classful
Classless
Broadcast based
Multicast 224.0.0.9
No support for VLSM
Supports VLSM
No authentication
MD5 authentication
No support for
discontiguous
networks
Supports
discontiguous
networks
128
Interior Gateway Routing Protocol (IGRP)
• Maximum hop count: 255 for larger network, default
100
• Composite metric: bandwidth and delay of the line.
– Those are the defaults
– Also: Load and Reliability are optionally configurable
instead
– MTU (Maximum Transmission Unit) is a “tiebreaker”
Config t
router igrp 10
129
IGRP vs. RIP
Large network
Small network
Uses AS number for
activation
Uses network
address, with all
subnet and host bits
off
Full route table
Full route table
update per 90 sec
update per 30 sec
AD 100
AD 120
Uses bandwidth and Uses only hop count
delay of the line as
to determine the best
metric, maximum hop path to a remote
count 255
network, max 15
130
Discontiguous Addressing
• Two networks of the same classful networks are separated by a
different network address
192.168.10.0/24
192.168.10.0/24
10.1.1.0/24
– RIPv1 and IGRP do not advertise subnet masks, and therefore cannot
support discontiguous subnets.
– OSPF, EIGRP, and RIPv2 can advertise subnet masks, and therefore can
support discontiguous subnets.
131
Passive Interface
Maybe you don’t want to send RIP updates out your
router interface connected to the Internet. Use the
passive-interface command:
Router(config)#router rip
Router(config-router)#passive-interface serial0
Internet
X
S0
Updates
Gateway
This allows a router to receive route updates on an interface,
but not send updates via that interface
132
Verifying RIP
Router#show ip protocols
Router#show ip route
Router#debug ip rip
Router#undebug all (un all)
133
Summary
– Open your books and go through all the written labs and
the review questions.
– Review the answers in class.
134
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