Cisco
CHAPTER 11 Layer 3 Protocols
11.1 Layer 3 Devices
What devices use Layer 2 addresses to segment networks?
What do bridges and switches use to make their decisions?
What device uses Layer 3 or logical addresses to segment networks?
What device use used to connect more than one network (or subnetwork) together?
How does a router recognize one network from another?
What is an interface on a router? How is each one identified? 11.1.4
11.2 Network to Network Communications
Static v. Dynamic assignment of IP addresses
Static - If you assign IP addresses statically, you must go to each individual device and
configure it with an IP address.
Dynamic - 2 example methods
1. RARP - Reverse address resolution protocol - if sending host does NOT know
its own IP address - it sends out a broadcast asking the RARP server to
respond. The RARP server maintains a table matching MACs to Ips - so it
will respond and tell the source host the correct IP
2. DHCP - Dynamic host configuration protocol - All that is required using DHCP is a
defined range of IP addresses on a DHCP server. As hosts come online they contact the
DHCP server and request an address. The DHCP server chooses an address and allocates it
to that host. With DHCP, the entire computer’s configuration can be obtained in one message
(e.g. along with the IP address, the server can also send a subnet mask).
Question: Which protocols are used to dynamically assign IP addresses?
When you send a message to another user, you need FOUR addresses before you can
send the message - what are they?
1.
3.
2.
4.
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If you don't know your own IP address, what do you do?
What if you don't know the destination MAC address?
What is ARP? What does it do? ARP finds MAC addresses given an IP address.
In order for devices to communicate, the sending devices need both the IP addresses
and the MAC addresses, of the destination devices. When they try to communicate with
devices whose IP addresses they know, they must determine the MAC addresses. The
TCP/IP suite has a protocol, called ARP; it can "automatically" obtain the MAC address.
ARP enables a computer to find the MAC address of the computer that is associated
with an IP address.
Address Resolution Protocol. If a sending device knows the IP address of the
destination host but not the MAC address, then ARP finds the MAC address that
matches the IP address. It then sends this information back to the device that needed
the information.
What is an ARP table?
Routers and hosts on a network build these tables. They map MAC addresses to
IP addresses. They run in RAM of the device. In routers, ARP tables can contain
MAC and IP addresses of devices on other networks, as well as on their own.
How does a router keep track of MAC addresses?
It builds an ARP table that resolves MAC address to IP addresses. ARP tables are
also maintained by other devices on the network, too, including hosts.
What if the MAC address is not found in the device's ARP table?
What is the structure of an ARP request (for a message destined to a host on the SAME
network/sub-network as the sender)?
Three parts:
MAC header – has the destination’s MAC address designated as FF-FF-FF-FF-FFFF (broadcast) and the source’s MAC address (which it knows)
IP header – has the destination host IP address and the source host IP address
specified
ARP Request Message – asks for the MAC address of the destination
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This message goes out to all devices on the network. The device whose IP
address matches the IP header portion of the ARP request, sends back its MAC
address to the sending host (ARP reply). This host then resolves the MAC
address with the IP address, puts it in its ARP table, and correctly addresses the
packet and sends it on its way.
What is RARP? What does it do?
Reverse Address Resolution Protocol. This is the opposite of ARP. In ARP, the
sending device has the destination’s IP, but not its MAC. With RARP, the sending
device has the destination’s MAC not its IP address. So RARP resolves MAC
addresses with known IP addresses. This information goes into the ARP table of
the device.
11.3 Advanced ARP Concepts
What is a default gateway?
The IP address of the router interface that connects to the network segment that
the source host is located on. When a host's ARP request fails to match the IP
address to a MAC, then it must have the MAC address of the default gateway (the
router interface) so the message can leave the network segment. Why does the
message have to leave the segment?
How could a host determine, before it ARPs, if the destination MAC is on the same
network/sub-net segment?
What if no default gateway is provided to a host - who can that host communicate with?
What is proxy ARP?
A variation of ARP. It uses an intermediate device (such as a router) to send an
ARP reply on behalf of the destination host to the source host. A router running
proxy ARP responds to ARP replies with its own (the router’s) MAC address if the
ARP request is for a destination host that isn’t on the router’s local subnet. A
proxy ARP would be used in an environment when a router acts as a default
gateway.
Create a flowchart for the ARP and the DHCP process: 11.3.5
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11.4 Routable (or routed) Protocols
What is a routable protocol?
Examples of routed or routable protocols
The focus of this course is on the most commonly used routable protocol, which
is IP. Even though you will concentrate on IP, it is important to know that there
are other routable protocols. Two of them are IPX/SPX (Novell) and AppleTalk.
Which address(es) go in an "IP header"?
What about "non-routable" protocols? What would make a protocol "non-routable"?
Example of a "non-routable" protocol:
There are protocols that do not support Layer 3; these are classed as non-routable
protocols. The most common of these non-routable protocols is NetBEUI. NetBEUI is a
small, fast, and efficient protocol that is limited to running on one segment.
11.5 Routing Protocols
Define:
Routing protocols (Note: Do not confuse with routed protocols.) determine the
paths that routed protocols follow to their destinations.
What do routing protocols enable routers to do?
Place an “x” in the appropriate column of the following table to identify each protocol:
Protocol
Routed Routing
Appletalk
RIP
OSPF
IPX
EIGRP
SPX
IGRP
IP
Why are two of these protocols different colors than the others?
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What is the most popular routing protocol used to transfer info between routers on the
same network?
What DO we call a routing protocol that exchanges info between routers on the same
network?
What is a routing metric?
The way a router determines the best path for a packet to take. Examples: hop
count, bandwidth, load, delay, reliability
11.6 Other Network Layer Services
What is the difference between connection-oriented and connectionless systems?
A connection-oriented system has error checking built into it (Phone system). It
also establishes a connection with the receiving host before transmitting any
data. A connectionless system does neither. It is a best-effort delivery system.
(Postal system)
What is another name for a connectionless system?
packet switched
What is another name for a connection-oriented system?
What does IP rely on to determine whether packets have been lost and to request
retransmission?
A Layer 4 connection-oriented protocol (like TCP) Ex: IP is a connectionless system; it
treats each packet independently. For example, if you use an FTP program to download
a file, IP does not send the file in one long stream of data. It treats each packet
independently. Each packet can travel different paths. Some may even get lost. IP relies
on the transport layer protocol to determine whether packets have been lost, and to
request retransmission. The transport layer is also responsible for reordering the
packets.
11.7 ARP Tables
Does a device have information on devices on other segments in its ARP table? Why or
why not?
Routers do, but hosts don’t. Hosts can only “see” devices on the same
network/subnet that the host is on. Routers, however, exchange tables of
information with other routers so they can “see” devices on other networks.
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Hosts can get information on devices on other networks by going through a
proxy ARP (e.g., router).
How does a device find the MAC address of another device on a different subnet?
It has to go through a proxy ARP device.
As a packet moves through the network, going from device to device and router to
router, what piece of addressing information never changes?
What happens if a packet comes into a router that’s not directly connected to the
destination network, and the router has no knowledge of the network?
11.8 Interior Gateway Protocol (IGP) and Exterior Gateway Protocol
(EGP)
Define
Exterior Gateway Protocols route data between autonomous systems. An example of
an EGP is BGP (Border Gateway Protocol
Interior Gateway Protocols route data within an autonomous system. These are what
make up the main part of our study. Examples of IGPs are:
 RIP - Routing Information Protocol
 IGRP - Interior Gateway Routing Protocol
 EIGRP - Enhanced Interior Gateway Routing Protocol
 OSPF - Open Shortest Path First
IGP's are the primary focus RIP is a non-proprietor - "generic" - weakest - provides least info from router to router
(bases decisions only on hop counts - 15 hop max is its range) 11.8.3
IGRP and EIGRP are proprietor (Cisco developed) - superior info flow
IGRP - distance vector (like RIP's hop counts) but when determining the best path, it
also takes into consideration such things as bandwidth, load, delay, and reliability.
EIGRP - distance vector and link state (what is the status of the different links)
What is the difference between dynamic and static routes?
Dynamic routes are those that routers learn automatically by exchanging routing
tables. Routers are set up to exchange this information on a regular basis (e.g.,
every 30 seconds, 90 seconds, whatever). Static routes are entered into the
router’s configuration file by the network administrator. Therefore, the only way a
static route can be updated is if the administrator manually changes it in the
router’s configuration file.
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Check out example on 11.8.9 of packet routing using RIP.
What is a stub network? Would it be routed statically or dynamically?
It’s a route to a network that only has one path to get there. It would normally be
configured statically by the administrator.
What is one advantage of dynamically maintained networks VS statically maintained
ones?
One advantage is that routers automatically update themselves. It takes little
administrative overhead.
What is one disadvantage?
One disadvantage is security. Static routes are only known to the administrator(s)
of the network. It’s difficult for outsiders to get this information.
Place an “x” in the appropriate column of the following table to identify each type of
routing protocol:
Protocol
RIP
IGRP
EIGRP
OSPF
DistanceVector
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Link- Interior
State Gateway
Exterior
Gateway
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Dynamic Static
Proprietary Routing Routing
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