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  • pg 1
									Routing Fundamentals and Subnets
        (Module 10 – Part 3)
Routable and routed protocols:

  A protocol is a set of rules that determines how computers
  communicate with each other across networks.
  A protocol describes the following:
     – The format that a message must conform to
     – The way in which computers must exchange a message within
       the context of a particular activity
  A routed protocol allows the router to forward data between
  nodes on different networks.
IP as a routed protocol:

Routable and routed protocols:
As a packet travels through an internetwork to
its final destination, the Layer 2 frame headers
and trailers are removed and replaced at every
Layer 3 device.
This is because Layer 2 data units, frames, are
for local addressing.
Layer 3 data units, packets, are for end-to-end

                   encapsulation           de-encapsulation
             Router Protocol Stripping

The Data Frames are transmitted on the Ethernet segment
All stations pink up the frame and bring it to the Data Link layer to
check if the Frame is for them
All devices except the Router will discard the Frame
Routable and routed protocols:
 As a frame is received at a router interface, the destination MAC
 address is extracted.
 The address is checked to see if the frame is directly addressed to the
 router interface, or if it is a broadcast.
 In either of these two cases, the frame is accepted. Otherwise, the frame
 is discarded since it is destined for another device on the collision
 The accepted frame has the Cyclic Redundancy Check (CRC)
 information extracted from the frame trailer, and calculated to verify that
 the frame data is without error.
 If the check fails, the frame is discarded. If the check is valid, the frame
 header and trailer are removed and the packet is passed up to Layer 3.
 The packet is then checked to see if it is actually destined for the router,
 or if it is to be routed to another device in the internetwork.
 If the destination IP address matches one of the router ports, the Layer 3
 header is removed and the data is passed up to the Layer 4.
Routable and routed protocols:
Routable and routed protocols:
 Some protocols, such as IPX, require only a network number because
 these protocols use the host's MAC address for the host number.
 Other protocols, such as IP, require a complete address consisting of a
 network portion and a host portion.
 These protocols also require a network mask in order to differentiate
 the two numbers.
 The network address is obtained by ANDing the address with the
 network mask.
 Consider following address & SNM:
        IP:                  11000000. 10101000.00011001.01001111
        SNM:                 11111111.11111111.11111111.11100000
        SubNet Addr:         11000000. 10101000.00011001.01000000 (Subnet for the IP
Routable and routed protocols:
• The most common routable protocol is IP.
• Other examples of routable protocols include:
   – IPX/SPX
   – AppleTalk
   – DECnet:
      • Group of communications products (including a protocol suite)
        developed and supported by Digital Equipment Corporation
   – XNS:
      • Xerox Network Systems
      • A protocol suite originally designed by Palo Alto Research Center
• These protocols provide Layer 3 support.
• Non-routable protocols do not provide Layer 3
   – The most common non-routable protocol is NetBEUI.
     NetBEUI is a small, fast, and efficient protocol that is
     limited to frame delivery within one segment.
Anatomy of an IP packet:

The IP header consists of the following:
   Version – Indicates the version of IP currently used; four bits. If the
   version field is different than the IP version of the receiving device, that
   device will reject the packets.
   IP header length (HLEN) – Indicates the datagram header length in
   32-bit words. This is the total length of all header information,
   accounting for the two variable-length header fields.
   Type-of-service (TOS) – Specifies the level of importance that has
   been assigned by a particular upper-layer protocol, eight bits.
Anatomy of an IP packet:

 The IP header consists of the following:
    Total length – Specifies the length of the entire packet in bytes,
    including data and header, 16 bits. To get the length of the data
    payload subtract the HLEN from the total length.
    Identification – Contains an integer that identifies the current
    datagram, 16 bits. This is the sequence number.
    Flags – A three-bit field in which the two low-order bits control
    fragmentation. One bit specifies whether the packet can be
    fragmented, and the other specifies whether the packet is the last
    fragment in a series of fragmented packets.
Anatomy of an IP packet:

 The IP header consists of the following:
    Fragment offset – Used to help piece together datagram fragments,
    13 bits. This field allows the previous field to end on a 16-bit boundary.
    Time-to-live (TTL) – A field that specifies the number of hops a packet
    may travel. This number is decreased by one as the packet travels
    through a router. When the counter reaches zero the packet is
    discarded. This prevents packets from looping endlessly.
    Protocol – indicates which upper-layer protocol, such as TCP or UDP,
    receives incoming packets after IP processing has been completed,
    eight bits.
    Header checksum – helps ensure IP header integrity, 16 bits.
Anatomy of an IP packet:

 The IP header consists of the following:
    Source address – specifies the sending node IP address, 32 bits.
    Destination address – specifies the receiving node IP address, 32
    Options – allows IP to support various options, such as security,
    variable length.
    Padding – extra zeros are added to this field to ensure that the IP
    header is always a multiple of 32 bits.
    Data – contains upper-layer information, variable length up to 64 Kb.
The Network Layer
 Routing is an OSI Layer 3 function.

 The following are the two key functions of a router:
   – Routers must maintain routing tables and make sure
      other routers know of changes in the network topology.
      This function is performed using a routing protocol to
      communicate network information with other routers.
   – When packets arrive at an interface, the router must use
      the routing table to determine where to send them. The
      router switches the packets to the appropriate interface,
      adds the necessary framing information for the interface,
      and then transmits the frame.

 A router is a network layer device that uses one or more
 routing metrics to determine the optimal path along which
 network traffic should be forwarded.

 Routing metrics are values used in determining the advantage
 of one route over another.
 Routing protocols use various combinations of metrics for
 determining the best path for data.
 Routed protocols transport data across a network.
 Routing protocols allow routers to choose the best path for
 data from source to destination.
 A routing protocol functions includes the following:
    Provides processes for sharing route information
    Allows routers to communicate with other routers to update
    and maintain the routing tables
Routing algorithms and metrics:

 Metrics can be based on a single characteristic of a path, or
 can be calculated based on several characteristics. The
 following are the metrics that are most commonly used by
 routing protocols:
 Hop count:
    The number of routers that a packet must travel through
    before reaching its destination. Each router the data must
    pass through is equal to one hop. A path that has a hop
    count of four indicates that data traveling along that path
    would have to pass through four routers before reaching its
    final destination. If multiple paths are available to a
    destination, the path with the least number of hops is
Routing algorithms and metrics:

   The data capacity of a link. Normally, a 10-Mbps Ethernet link is
   preferable to a 64-kbps leased line.
   The length of time required to move a packet along each link from
   source to destination. Delay depends on the bandwidth of intermediate
   links, the amount of data that can be temporarily stored at each router,
   network congestion, and physical distance.
   The amount of activity on a network resource such as a router or a link.
   Usually a reference to the error rate of each network link.
Routing algorithms and metrics:

   The delay on a data link using IBM PC clock ticks. One tick
   is approximately 1/18 second.
   Cost is an arbitrary value, usually based on bandwidth,
   monetary expense, or other measurement, that is assigned
   by a network administrator.
Routing Protocols:
 Two families of routing protocols are Interior Gateway
   Protocols (IGPs) and Exterior Gateway Protocols (EGPs).
 IGPs can be further categorized as either distance-vector or
   link-state protocols.
 Examples of distance-vector protocols include the following:
 • Routing Information Protocol (RIP) – The most common
   IGP in the Internet, RIP uses hop count as its only routing
 • Interior Gateway Routing Protocol (IGRP) – This IGP
   was developed by Cisco to address issues associated with
   routing in large networks.
 • Enhanced IGRP (EIGRP) – This Cisco-proprietary IGP
   includes many of the features of a link-state routing
   protocol. Because of this, it has been called a balanced-
   hybrid protocol, but it is really an advanced distance-vector
   routing protocol.
Link-state Routing Protocols:

 Link-state routing protocols were designed to
 overcome limitations of distance vector routing
 Link-state routing protocols respond quickly to
 network changes sending trigger updates only when
 a network change has occurred.
 Link-state routing protocols send periodic updates,
 known as link-state refreshes, at longer time
 intervals, such as every 30 minutes.
Link-state Routing Protocols:

   Open Shortest Path First
   Intermediate System-to-Intermediate System
Routing Tables:
Routers keep track of the following information in their routing
tables and routing table information to select the best path:
   Protocol type:
      • Identifies the type of routing protocol that created each entry.
   Next-hop associations:
      • Tell a router that a destination is either directly connected to the router or that it can
        be reached through another router called the next-hop on the way to the destination.
        When a router receives a packet, it checks the destination address and attempts to
        match this address with a routing table entry.
   Routing metric:
      • Different routing protocols use different routing metrics. Routing metrics are used to
        determine the desirability of a route. For example, RIP uses hop count as its only
        routing metric. IGRP uses bandwidth, load, delay, and reliability metrics to create a
        composite metric value.
   Outbound interfaces:
      • The interface that the data must be sent out of to reach the final destination.

Routers communicate with one another to maintain their routing tables
through the transmission of routing update messages.
Some routing protocols transmit update messages periodically.
Other protocols send them only when there are changes in the network
The Network Layer
Routing versus switching:

 Routing and switching might seem to perform the same function to the
 inexperienced observer.
 The primary difference is that switching occurs at Layer 2, the data link
 layer, of the OSI model and routing occurs at Layer 3.
 This distinction means routing and switching use different information in
 the process of moving data from source to destination.
Routing versus switching:

 Routing and switching might seem to perform the same function to the
 inexperienced observer.
 The primary difference is that switching occurs at Layer 2, the data link
 layer, of the OSI model and routing occurs at Layer 3.
 This distinction means routing and switching use different information in
 the process of moving data from source to destination.
Router vs Switches:

  Another difference between switched and routed networks is
  switched networks do not block broadcasts.
  As a result, switches can be overwhelmed by broadcast
  Routers block LAN broadcasts, so a broadcast storm only
  affects the broadcast domain from which it originated.
   Because routers block broadcasts, routers also provide a
  higher level of security and bandwidth control than switches.
              Chapter #10

Finish Labs:
  – 10.3.5b (Subnetting a Class A Network)
  – 10.3.5c (Subnetting a Class B Network)
  – 10.3.5d (Subnetting a Class C Network)
Practice Skills Test
Skills Test on Thursdays

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