Ch' 67 Routing Theory Part 3

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• Ch. 6-7 Routing Theory Part 3
• CCNA Semester 2
• Originally by Rick Graziani, Instructor
• Modified by Prof. Yousif

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• Link-State Routing Protocols
• The first type of routing protocol we discussed
  was distance vector.
• The second type of routing protocol that we will
  examine is link-state.
• In this presentation we will only examine the
  very basic concepts of link-state routing
  protocols.
• In CCNP Advanced Routing we examine the link
  state routing protocol OSPF in detail.
• I have added a presentation, Introduction to
  OSPF, which we will discuss at the end of this
  semester.

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• Distance Vector Routing Protocols
• Distance vector routing protocols like RIP and
  IGRP do not know the exact topology of a network.
• All distance vector routing decisions are made
  from information from neighboring routers
  routing by rumor.
• The only information the router has about a route
  is how far away the network is in hops or using
  another cost (distance) and which interface to
  send forward the packet out of (vector).
• The router has no way to make its own decision on
  which direction is ultimately the best way to
  send the packets.

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• Link-State Routing Protocols - History
• An IETF working group designed a routing protocol
  specifically for IP routing, OSPF (Open Shortest
  Path First).
• For most network administrators they had two
  open-standard routing protocols to choose from
  RIP, simple but very limited, or OSPF, robust but
  more sophisticated to implement.
• IGRP and EIGRP are Cisco proprietary
• IS-IS is used in IP networks, but not as common
  as OSPF

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• Theory of Link-State Routing Protocols
• In this presentation we will examine some of
  the theory behind link-state routing protocols.
• This will only be a brief introduction to the
  link-state theory, requiring much more time and
  perhaps even some requisite knowledge of
  algorithms.
• At the end of this presentation will be some
  suggested resources for leaning more about the
  theory of link-state routing and Dijkstras
  algorithm.

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• Mathematical point of view
• Link-state routing is not based on IP addresses,
  subnets and network information!
• Link-state routing has a mathematical point of
  view, looking at the network as nothing more than
  a graph with vertices and the costs to these
  vertices.
• Okay, Im losing you and I said I wouldnt get
  mathematical.
• Link-state routing is based on a very simple
  algorithm known as Dijkstrass algorithm,
  invented by Edsger Wybe Dijkstra
• This algorithm can and has been used in many
  areas of human activity, not just for routing.

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1 Flooding of link-state information

• Link-State Theory
• The network is viewed as a graph, showing the
  complete topology of the network.
• How do routers build this topology?
• 1 Flooding of link-state information
• The first thing that happens is that each node,
  router, on the network announces its own piece of
  link-state information to other all other routers
  on the network who their neighboring routers are
  and the cost of the link between them.
• Example Hi, Im RouterA, and I can reach
  RouterB via a T1 link and I can reach RouterC via
  an Ethernet link.
• Each router sends these announcements to all of
  the routers in the network.

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1 Flooding of link-state information
3 SPF Algorithm
2 Building a Topological Database

• 2. Building a Topological Database
• Each router collects all of this link-state
  information from other routers and puts it into a
  topological database.
• 3. Shortest-Path First (SPF), Dijkstras
  Algorithm
• Using this information, the routers can recreate
  a topology graph of the network.
• Believe it or not, this is actually a very simple
  algorithm and I highly suggest you look at it
  some time, or even better, take a class on
  algorithms. (Radia Perlmans book,
  Interconnections, has a very nice example of how
  to build this graph she is one of the
  contributers to the SPF and Spanning-Tree
  algorithms.)

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1 Flooding of link-state information
5 Routing Table
3 SPF Algorithm
2 Building a Topological Database
4 SPF Tree

• 4. Shortest Path First Tree
• This algorithm creates an SPF tree, with the
  router making itself the root of the tree and the
  other routers and links to those routers, the
  various branches.
• Note Just a reminder that the link-state
  algorithm and graph it creates is mathematically
  based and although we are mentioning routers and
  their links, it has nothing to do with IP
  addresses or other network information.
• 5. Routing Table
• Using this information, the router creates a
  routing table.
• I bet you can create this tree given the
  link-state information!

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• Exercise From link-state flooding to routing
  tables - Lets try it
• For this exercise we will not worry about the
  individual, leaf, networks attached to each node
  or router (shown as a blank line), but focus on
  how the topology is built to find the the
  shortest path between each router.
• In order to keep it simple, we will take some
  liberties with the actual process and algorithm,
  but you will get the basic idea!
• You are RouterA and you have a link to RouterB
  with a cost of 15, a link to RouterC with a cost
  of 2, a link to RouterD with a cost of 5, and a
  leaf network apple.
• This is your own link-state information, which
  you will flood to all other routers so they can
  do the same thing we will be doing for RouterA.

Leaf network apples
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• We now get the following link-state information
  from RouterB
• RouterB has a link to RouterA with a cost of 15.
• RouterB has a link to RouterE with a cost of 2.
• And information about its own leaf network
  bananas.

bananas
Now lets attach the two graphs


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• We now get the following link-state information
  from RouterC
• RouterC has a link to RouterA with a cost of 2.
• RouterC has a link to RouterD with a cost of 2.
• And information about its own leaf network
  cherries.

cherries
Now lets attach the two graphs


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• We now get the following link-state information
  from RouterD
• RouterD has a link to RouterA with a cost of 5.
• RouterD has a link to RouterC with a cost of 2.
• RouterD has a link to RouterE with a cost of 10.
• And information about its own leaf network
  donuts.

donuts
Now lets attach the two graphs


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• We now get the following link-state information
  from RouterE
• RouterE has a link to RouterB with a cost of 2.
• RouterE has a link to RouterD with a cost of 10.
• And information about its own leaf network
  eggs.

eggs
Now lets attach the two graphs and we have all
the nodes, their links between them and their and
leafs!


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• Topology
• Using the topological information we listed,
  RouterA has now built a complete topology of the
  network.
• The next step is for the link-state algorithm to
  find the best path to each node and leaf network.

bananas
eggs
cherries
apples
donuts
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• Choosing the best path
• Using the link-state algorithm RouterA can now
  proceed to find the shortest path to each leaf
  network.
• Try doing it on your own!

bananas
eggs
cherries
apples
donuts
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• Choosing the best path
• Now RouterA knows the best path to each network.

bananas
eggs
cherries
apples
donuts
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• Creating the Routing Table
• RouterA can now enter these paths into its
  routing table, with network numbers, exit
  interfaces and costs to each network.

Network interface cost Apples i0
conn. Bananas i1 15 Cherries i2
2 Donuts i2 4 Eggs
i2 14 Other directly connected
networks
bananas
i1
eggs
cherries
apples
i2
i0
i3
i interface
donuts
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1 Flooding of link-state information
5 Routing Table
3 SPF Algorithm
2 Building a Topological Database
4 SPF Tree

• And now you have seen and done the process!
• All of the routers in the network go through this
  same process.

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• Link-State Routing Protocols Hello Messages and
  LSAs
• First of all small Hello messages are exchanged
  between routers to find out who their neighbors
  are. This is known as forming adjacencies.
• Once a link-state router knows who their adjacent
  neighbors are, the actual information exchanged
  between the routers are known as LSAs (Link State
  Advertisements) to build and maintain their link
  state databases. (Topological database).
• There are different types of LSAs for different
  types of information and different situations
  all of which is discussed in CCNP Advanced
  Routing.
• Once the routing tables are built and the network
  is converged, routers do not exchange routing
  tables periodically.
• Instead, routers using link-state routing
  protocols exchange periodic Hello messages
  between immediate neighbors, to make sure they
  are still there and the link between them is
  still up.

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• Link-State Routing Protocols Topology Change
• When there is a change in the network, link going
  down, new link coming up, etc., the router(s)
  attached to that link floods out LSAs to all
  other routers in the network, containing only the
  changed link information.
• All other routers enter this new information into
  their topological database, re-run the SPF
  algorithm, come up with a new SPF tree, and
  eventually a new routing table with possible new
  best paths to some networks.

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• From on-line curriculum
• Running link-state routing protocols in most
  situations requires that routers use more memory
  and perform more processing than distance-vector
  routing protocols.
• For link-state routing, their memory must be able
  to hold information from various databases, the
  topology tree, and the routing table.
• Using Dijkstra's algorithm to compute the SPF
  requires a processing task proportional to the
  number of links in the internetwork, multiplied
  by the number of routers in the internetwork.

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• From on-line curriculum
• During the initial discovery process, all routers
  using link-state routing protocols send LSA
  packets to all other routers.
• This action floods the internetwork as routers
  make their en masse demand for bandwidth, and
  temporarily reduce the bandwidth available for
  routed traffic that carries user data.
• After this initial flooding, link-state routing
  protocols generally require only minimal
  bandwidth to send infrequent or event-triggered
  LSA packets that reflect topology changes. (and
  Hello messages)

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• Just a few notes on this table
• When a link-state router boots up it will most
  likely need to exchange complete database
  information with neighboring routers in order to
  synchronize their databases. (CCNP Advanced
  Routing)
• Distance vector routing protocols can also use
  triggered updates.

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Interconnections Bridges and Routers by Radia
Perlman
Cisco IP Routing Packet Forwarding
Intra-domain Routing Protocols by Alex Zinin
Routing TCP/IP Volume I by Jeff Doyle
OSPF, Anatomy of an Internet Routing Protocol by
John Moy (creator of OSPF)

• For more information on OSPF, link-state routing
  protocol, Dijkstras algorithm and routing in
  general, check out these sources.

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Topics (Continued)

• Part II. Routing Theory and Dynamic Routing
  Operations (continued)
• Hybrid Routing Protocols
• Concepts
• EIGRP (not IS-IS)
• Path Switching
• Example Host X to Host Y (with three routers in
  between)
• LAN-to-LAN Routing
• LAN-to-WAN Routing
• Cisco Router Configuration
• Summary
• Topics (Review)

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• The balanced hybrid approach combines aspects of
  the link-state and distance-vector algorithms.
• These are really distance-vector routing
  protocols which apply some of the advantages of a
  link-state routing protocols, and also known as
  advanced-distance-vector routing protocols.
• EIGRP is known as balanced hybrid routing
  protocol.
• EIGRP is covered in CCNP Advanced Routing but it
  uses many of the concepts from IGRP which we
  discuss this semester.
• In the curriculum, IS-IS is described as a
  balanced hybrid, but it is more often regarded as
  a link-state routing protocol.
• Examples of hybrid protocols are OSI's IS-IS
  (Intermediate System-to-Intermediate System), and
  Cisco's EIGRP (Enhanced Interior Gateway Routing
  Protocol). (On-line curriculum)

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• I also disagree with the following information in
  the on-line curriculum
• Balanced-hybrid routing protocols use distance
  vectors with more accurate metrics to determine
  the best paths to destination networks. However,
  they differ from most distance-vector protocols
  by using topology changes to trigger routing
  database updates.
• Balanced hybrid routing protocols dont
  necessarily use more accurate metrics than a
  distance vector routing protocol. EIGRPs
  metrics are more accurate than RIP, but not
  necessarily more accurate than IGRP.
• RIP and IGRP both use triggered updates during
  topology changes to speed up network convergence,
  the same as a balanced hybrid.
• The real difference is that a hybrid routing
  protocol like EIGRP does not pass entire routing
  table information periodically like RIP or IGRP
  and uses other mechanisms for loop free routing.
• EIGRP also uses the DUAL algorithm which
  guarantees loop-free path selection.

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Topics (Continued)

• Part III. Routing Theory and Dynamic Routing
  Operations (continued)
• Hybrid Routing Protocols
• Concepts
• EIGRP (not IS-IS)
• Path Switching
• Example Host X to Host Y (with three routers in
  between)
• LAN-to-LAN Routing
• LAN-to-WAN Routing
• Cisco Router Configuration
• Summary
• Topics (Review)

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Path Switching
Y
X
Data Link Header
IP (Network layer) Packet
Data Link Frame Data Link Header IP Packet

• Path Switching
• Host X has a packet(s) to send to Host Y
• A router generally relays a packet from one data
  link to another, using two basic functions
• 1. a path determination function - Routing
• 2. a switching function Packet Forwarding
• Lets go through all of the stages these routers
  use to route and switch this packet.
• See if you can identify these two functions at
  each router.
• Note Data link addresses have been abbreviated.

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00-10 0A-10
192.168.4.10 192.168.1.10

• From Host X to Router RTA
• Host X begins by encapsulating the IP packet into
  a data link frame (in this case Ethernet) with
  RTAs Ethernet 0 interfaces MAC address as the
  data link destination address.
• How does Host X know to forward to packet to RTA
  and not directly to Host Y? How does Host X know
  or get RTAs Ethernet address?
• Remember, it looks at the packets destination ip
  address does an AND operation and compares it to
  its own ip address and subnet mask.
• It determines if the two ip addresses are on the
  same subnet or not.
• If they are on the same subnet, it looks for the
  destination MAC address of the packet in its ARP
  cache. sending out an ARP request if it is not
  there.
• If they are on different subnets, it looks for
  the MAC address of the default gateway in its ARP
  cache sending out an ARP request if it is not
  there.

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0B-31 00-20
192.168.4.10 192.168.1.10
1
3
2

• RTA to RTB
• 1. RTA looks up the IP destination address in
  its routing table.
• 192.168.4.0/24 has next-hop-ip address of
  192.168.2.2 and an exit-interface of e1.
• Since the exit interface is on an Ethernet
  network, RTA must resolve the next-hop-ip
  address with a destination MAC address.
• 2. RTA looks up the next-hop-ip address of
  192.168.2.2 in its ARP cache.
• If the entry was not in the ARP cache, the RTA
  would need to send an ARP request out e1. RTB
  would send back an ARP reply, so RTA can update
  its ARP cache with an entry for 192.168.2.2.

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0B-31 00-20
192.168.4.10 192.168.1.10
1
3
2

• RTA to RTB (continued)
• 3. Data link destination address and frame
  encapsulation
• After finding the entry for the next-hop-ip
  address 192.168.2.2 in its ARP cache, RTA uses
  the MAC address for the destination MAC address
  in the re-encapsulated Ethernet frame.
• The frame is now forwarded out Ethernet 1 (as
  specified in RTAs routing table.
• Notice, that the IP Addresses did not change.
• Also notice that the Routing table was used to
  find the next-hop ip address, used for the data
  link address and exit interface, to forward the
  packet in a new data link frame.

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FFFF
192.168.4.10 192.168.1.10
1
2

• RTB to RTC
• 1. RTB looks up the IP destination address in
  its routing table.
• 192.168.4.0/24 has next-hop-ip address of
  192.168.3.2 and an exit-interface of s0 (serial
  0).
• Since the exit interface not on an Ethernet
  network, RTA does not need to resolve the
  next-hop-ip address with a destination MAC
  address.
• Remember, serial interfaces do not have MAC
  addresses.

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FFFF
192.168.4.10 192.168.1.10
1
2

• RTB to RTC
• 2. Data link destination address and frame
  encapsulation.
• When the interface is a point-to-point serial
  connection, the Routing Table process does not
  even look at the next-hop IP address.
• Remember, a serial link is like a pipe - only
  one way in and only one way out.
• RTA now encapsulates the IP packet into the
  proper data link frame, using the proper serial
  encapsulation (HDLC, PPP, etc.).
• The data link destination address is set to a
  broadcast, since there is only one other end of
  the pipe and the frame is now forwarded out
  serial 0.

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0B-20 0C-22
192.168.4.10 192.168.1.10
1
3
2

• RTC to Host Y
• 1. RTC looks up the IP destination address in
  its routing table.
• 192.168.4.0/24 is a directly connected network
  with an exit-interface of e0.
• RTC realizes that this destination ip address is
  on the same network as one of its interfaces and
  it can sent the packet directly to the
  destination and not another router.
• Since the exit interface is on an directly
  connected Ethernet network, RTC must resolve the
  destination ip address with a destination MAC
  address.
• 2. RTC looks up the destination ip address of
  192.168.4.10 in its ARP cache.
• If the entry was not in the ARP cache, the RTC
  would need to send an ARP request out e0. Host Y
  would send back an ARP reply, so RTC can update
  its ARP cache with an entry for 192.168.4.10.

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0B-20 0C-22
192.168.4.10 192.168.1.10
1
3
2

• RTC to Host Y (continued)
• 3. Data link destination address and frame
  encapsulation
• After finding the entry for the destination ip
  address 192.168.4.10 in its ARP cache, RTC uses
  the MAC address for the destination MAC address
  in the re-encapsulated Ethernet frame.
• The frame is now forwarded out Ethernet 0 (as
  specified in RTAs routing table.

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• From Cisco on-line curriculum
• When the router checks its routing table entries,
  it discovers that the best path to destination
  Network 2 uses outgoing port To0, the interface
  to a token-ring LAN.
• Although the lower-layer framing must change as
  the router passes packet traffic from Ethernet on
  Network 1 to token-ring on Network 2, the Layer 3
  addressing for source and destination remains the
  same.
• In the Figure, the destination address remains
  Network 2, Host 5, regardless of the different
  lower-layer encapsulations.

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• From Cisco on-line curriculum
• Routers enable LAN-to-WAN packet flow by keeping
  the end-to-end source and destination addresses
  constant while encapsulating the packet in data
  link frames, as appropriate, for the next hop
  along the path.
• NOTE
• Remember, when the interface is a point-to-point
  serial connection, the Routing Table process does
  not even look at the next-hop IP address in the
  routing table, only the exit-interface.

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Topics (Continued)

• Part II. Routing Theory and Dynamic Routing
  Operations (continued)
• Hybrid Routing Protocols
• Concepts
• EIGRP (not IS-IS)
• Path Switching
• Example Host X to Host Y (with three routers in
  between)
• LAN-to-LAN Routing
• LAN-to-WAN Routing
• Cisco Router Configuration
• Summary
• Topics (Review)

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Topics (Continued)

• Part II. Routing Theory and Dynamic Routing
  Operations (continued)
• Hybrid Routing Protocols
• Concepts
• EIGRP (not IS-IS)
• Path Switching
• Example Host X to Host Y (with three routers in
  between)
• LAN-to-LAN Routing
• LAN-to-WAN Routing
• Cisco Router Configuration
• Summary
• Topics (Review)

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• Summary
• We have covered a lot of topics and a lot of new
  concepts.
• These topics will be reinforced when we discuss
  the specific routing protocols Understanding
  these concepts is necessary to understand to be
  able to design, implement, and troubleshoot
  networks.
• As we will see, anyone can type in a few commands
  to enable routing on a router, but if you do not
  understand these concepts we discussed at best
  you may not be optimally routing packets in your
  network and at worst you may be creating routing
  loops, blackholes, and unreachable networks.
• Understanding these concepts will also better
  prepare you for the CCNP Advanced Routing class.
• Even if you do not take the CCNP Advanced Routing
  class, these concepts will help you general
  understanding networking and routing protocols.

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Cabrillo College

• Ch. 11 Routing Basics End of Part 3
• CCNA Semester 2
• Rick Graziani, Instructor