Sybex CCNA 640-802

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Sybex CCNA 640-802 Chapter 6 IP Routing
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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.

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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

A
C
B
D
4
Basic Path Selection

• On what interface will the router send out a
  packet if it has destination address of
  10.10.10.18?

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Simple IP Routing
gtping 172.16.1.2
172.16.1.0
172.16.2.0
172.16.3.1
172.16.3.2
e0
e0
s0
B
A
B
s0
172.16.2.2
Host A
172.16.1.1
172.16.2.1
172.16.1.2
Host B
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Routing/PDU ExampleHost 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)
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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

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Routing table

• Tells where to send datagram for a particular
  network

Network Next-Hop
Port Metric
194.181.200.0 194.181.208.1 Eth0
1 193.2.1.0
194.181.208.320 Eth1 14 153.5.0.0
194.181.214.25 Fddi0
8 0.0.0.0 194.181.210.1
S0 5

• Next-Hop routers must be directly reachable

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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!

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Routing Algorithm

• Extract destination IP address from datagram
• Extract network address from the IP address
• If destination network equals my network
• Send directly to destination using physical
  network
• Else If destination address matches a
  host-specific route in the routing table
• Send to the router specified in the routing table

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Routing Algorithm (cont.)

• Else if destination network matches a network in
  the routing table
• Send to the router specified in the routing entry
• Else If there is a default route in the routing
  table
• Send to the router specified in the default route
  entry
• Else
• Send a No route to host message to the source

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Step-by-Step IP Routing Process (book, pp
331-36)

• The IP routing process is fairly simple and
  doesnt change, regardless of the size of your
  network.
• For an example, well use Figure 6.2 to describe
  step-by-step what happens when Host_A wants to
  communicate with Host_B on a different network

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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

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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
  reachedin this example, ICMP.

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Step 3

• Once the packet is created, IP determines whether
  the destination IP address is on the local
  network or a remote one.

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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.

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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 routers 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 routers
  interface thats connected to the 172.16.10.0
  network.
• Because hosts only communicate via hardware
  addresses on the local LAN, its 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.

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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\gtarp -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 isnt 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.

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• 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
  layerin 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_As
  hardware (MAC) address and the destination
  hardware address of the default gateway. It does
  not include the remote hosts MAC
  addressremember that!

FIGURE 6 . 3 Frame used from Host_A to the Lab_A
router when Host_B is pinged
Destination MAC Source MAC Ether-Type field Packet   FCS (CRC)
(routers E0 MAC address) (Host_A MAC address) Ether-Type field Packet   FCS (CRC)
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Step 7
FIGURE 6 . 3 Frame used from Host_A to the Lab_A
router when Host_B is pinged
Destination MAC Source MAC Ether-Type field Packet   FCS (CRC)
(routers E0 MAC address) (Host_A MAC address) Ether-Type field Packet   FCS (CRC)
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Step 8
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Step 9
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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., its 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.

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Step 11

• IP receives the packet and checks the IP
  destination address.
• Since the packets destination address doesnt
  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.

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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 couldnt be set up on 2
  networks. But this network is subnetted, i.e.,
  the subnet mask is 255.255.255.0.

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Step 13

• If the router does find an entry for the
  destination network in its table, the packet is
  switched to the exit interfacein this example,
  interface Ethernet 1.
• The output below (next slide) displays the Lab_A
  routers routing table. The C means directly
  connected.
• No routing protocols are needed in this network
  since all (both) networks are directly connected.

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Step 13 (continued)

• Lab_Agtsh 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

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Step 14

• The router packet-switches the packet to the
  Ethernet 1 buffer.
• OK, ready to go out to Host_B, but first

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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 routers ARP cache,
  then the packet and the hardware address are
  handed down to the Data Link layer to be framed.
• Lets take a look at the ARP cache on the Lab_A
  router by using the show ip arp command
• Lab_Ash 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.

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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.

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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

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Step 17

• Host_B receives the frame and immediately runs a
  CRC. finally!!
• If the result matches whats 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

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Step 18

• At the Network layer, IP receives the packet and
  checks the IP destination address.
• Since theres finally a match made, the Protocol
  field is checked to find out to whom the payload
  should be given.

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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.

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Step 20

• A packet is then created, including the
• source and destination addresses,
• Protocol field, and
• payload.
• The destination device is now Host_A

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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.

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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

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Step 22 (continued)
Above is a view of my home computers Registry
settings HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\
Services\longkey\Parameters\Tcpip
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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.

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Step 24

• The Data Link layer frames the packet of
  information and includes the following in the
  header
• The destination source hardware addresses
• The Ether-Type field with 0x0800 (IP) in it
• The FCS field with the CRC result in tow

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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.

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Step 26

• The routers 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.

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Step 27

• Once the CRC is found to be okay, the hardware
  destination address is checked.
• Since the routers interface is a match, the
  packet is pulled from the frame and the
  Ether-Type field is checked to see to what
  protocol at the Network layer the packet should
  be delivered.

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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 doesit only checks the header for errors.

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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.

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Step 30

• The router checks the ARP cache to determine
  whether the hardware address for 172.16.10.2 has
  already been resolved.

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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.

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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.

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Step 33

• The frame is then handed to the Physical layer to
  be sent out onto the local network one bit at a
  time.

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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.

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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.

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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

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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 routers Mac address to the
  nexts.
• 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.
• I.e., in a local LAN, you always use a Mac
  addrss, not IP.

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• 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)

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• 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. Heres an
  example of a serial interface on a 2811
• Todd(config)interface serial ?
• lt0-2gt Serial interface number
• Todd(config)interface serial 0/0/?
• lt0-1gt Serial interface number
• Todd(config)interface serial 0/0/0
• Todd(config-if)

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• Notes (continued)
• You should always view a running-config output
  first so you know what interfaces you have to
  deal with. Heres 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

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• 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

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• 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
• ltinterface-namegt slot/port is used whenever the
  interfaces are native on the network module.
• ltinterface-namegt 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.

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• Notes (continued)
• Pg 346-47 Just a command idiosyncrasy
• With ISR routers you cant use erase start, you
  must enter erase startup-config
• This is so even though no other command begins
  with S
• Eg Routererase s?
• startup-config
• So under the normal rules of the Cisco IOS,
  erase s should work exactly like erase
  startup-config, but it doesnt.
• 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.

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• 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
  SDMs wizard through page 359.

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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
• Static Routing
• Default Routing
• Dynamic Routing

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Static Routes
Stub Network
172.16.1.0
172.16.2.0
SO
SO
A
A
B
B
172.16.3.2
172.16.3.1
Routes must be unidirectional
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Static Route Configuration
ip route remote network mask
addressinterface 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

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Static Route Example
Stub Network
172.16.2.0
172.16.1.0
SO
SO
A
B
B
172.16.3.2
172.16.3.1
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
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Default Routes
Stub Network
172.16.1.0
172.16.2.0
SO
SO
creates a wireless network that hosts can connect
to.
A
B
B
172.16.3.2
172.16.3.1
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 As
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

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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 routes 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.

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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 weve 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.

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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
  dont

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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

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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

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Administrative Distance
Default Administrative Distance Directly
Connected 0 Static Route 1 RIP
120 IGRP 100 EIGRP 90 OSPF 110
Router B
Router A
IGRPAdministrative Distance100
RIPAdministrative Distance120
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 its a tie, load
balancing is used.
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78
Distance Vector
DistanceHow farVectorIn which direction
A
C
B
D
Routing Table
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.
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Discovering Routes
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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.
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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

82
Meaning of Distance Vector (2/2)
83
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.

84
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

85
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.

86
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.

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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.

88
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.

89
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.

90
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.

91
Distance Vector Routing Protocols
92
Comparison of Routing Protocol
93
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.

94
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

95
Routing Loop (3/6)
96
Routing Loop (4/6)
97
Routing Loop (5/6)
98
Routing Loop (6/6)
99
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 its
  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.

99 / 380
100
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.

101
Count to infinity (2/5)
102
Count to infinity (3/5)
103
Count to infinity (4/5)
104
Count to infinity (5/5)
105
Routing Loops

• Split Horizon
• Routing information cannot be sent back in the
  direction from which it was received.

105 / 380
106
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.

107
Split Horizon Rules (2/5)
108
Split Horizon Rules (3/5)
109
Split Horizon Rules (4/5)
110
Split Horizon Rules (5/5)
111
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

111 / 380
112
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.

113
Route Poisoning (2/4)
114
Route Poisoning (3/4)
115
Route Poisoning (4/4)
116
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.

117
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.

118
Split Horizon with Poison reverse (3/5)
119
Split Horizon with Poison reverse (4/5)
120
Split Horizon with Poison reverse (5/5)
121
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 thats 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

122
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.

123
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.

124
RIP Timers (3/3)
125
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

126
RIP Routing Configuration
Router(config)router rip
Router(config-router)network network-number
192.168.10.0
10.3.5.0
172.16.10.0
Network is a classful network address. Every
device on network uses the same subnet mask
127
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
128
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
129
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
130
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 update per 90 sec Full route table update per 30 sec
AD 100 AD 120
Uses bandwidth and delay of the line as metric, maximum hop count 255 Uses only hop count to determine the best path to a remote network, max 15
131
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.

132
Passive Interface

• Maybe you dont 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

X
Updates
Internet
S0
Gateway
This allows a router to receive route updates on
an interface, but not send updates via that
interface
133
Verifying RIP

• Routershow ip protocols
• Routershow ip route
• Routerdebug ip rip
• Routerundebug all (un all)

134
Summary

• Open your books and go through all the written
  labs and the review questions.
• Review the answers in class.

134