CIT 384: Network Administration

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CIT 384: Network Administration

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1

CIT 384: Network Administration

Routing ][

CIT 384: Network Administration

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2

Topics

1.
Switch Forwarding Process

2.
IP Routing Process

3.
DNS

4.
Connected and Static Routes

5.
Classful Routing

6.
VLSM

7.
Summarization

CIT 384: Network Administration

Slide #
3

Switch Forwarding Process

1.
Determine the VLAN in which the frame should be forwarded as
follows:

a.
If frame arries on an access interface, use the interface’s access VLAN.

b.
If frame arrives on a trunk interface, use VLAN listed in trunking header.

2.
If incoming interface is in STP Learning or Forwarding State in that
VLAN, add source MAC address to MAC address table with
incoming interface and VLAN ID.

3.
If incoming interface is not in an STP Forwarding State for that
VLAN, discard the frame.

4.
Look for destination MAC address of frame in MAC address table for
VLAN identified in step 1. If address is

a.
Found
: Forward frame out the only interface listed in the matched
address table entry.

b.
Not found
: Flood frame out all other access ports in same VLAN and out
all trunk ports that support this VLAN.

CIT 384: Network Administration

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4

IP Routing Process: Edge Hosts

1.
Compare destination IP of packet to local
subnet and netmask.

2.
If destination on same subnet as host, send
packet directly to host. ARP is used to find
destination MAC address.

3.
If destination host not on same subnet, then
send packet directly to gateway router.
ARP is used to find router’s MAC address.

CIT 384: Network Administration

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5

IP Routing Process: Routers

1.
Check errors using data link FCS. If error, discard frame.

2.
Check frame’s destination MAC address, and process
only if directed to this router or to a broadcast/multicast
address.

3.
Discard frame’s old data
-
link header and trailer, leaving
IP packet.

4.
Compare destination IP address to routing table, finding
route that matches destination address. Route identifies
outgoing interface of router and possibly next
-
hop router.

5.
Determine destination MAC address.

6.
Encapsulate packet inside new data link header and
trailer, appropriate for outgoing interface, and fwd packet.

CIT 384: Network Administration

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6

IP Routing Example

PC1 sends packet to PC3 (172.16.3.3)


PC1 configuration


ifconfig eth0 172.16.1.1 netmask 255.255.255.0


route add default gw 172.16.1.251


Compares 172.16.3.3 to local net:


Network address: 172.16.1.0


Subnet mask: 255.255.255.0


Since 172.16.3.3 is not in address range,


Sends to default route 172.16.1.251

CIT 384: Network Administration

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7

CIT 384: Network Administration

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8

IP Routing Example: R1

1.
R1 checks FCS, finds no errors.

2.
R1 finds its own fa0/0 MAC as dest MAC, so it
will process the frame.

3.
R1 discards old data
-
link header and trailer.

4.
R1 compares dest IP (172.16.3.3) to routing table,
finding a route for 172.16.3.0 with netmask
255.255.255.0.

5.
R1 looks up MAC address of route destination
(172.16.2.252) in ARP table.

6.
R1 encapsulates IP packet in new Ethernet frame,
with R1’s fa0/1 MAC address as destination.

CIT 384: Network Administration

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9

IP Routing Example: R2

1.
R2 checks FCS, finds no errors.

2.
R2 finds own fa0/0 MAC address as destination
MAC, so it will process frame.

3.
R2 discards old data
-
link header and trailer.

4.
R2 compares destination IP (172.16.3.3) to
routing table, finding matching route with
outgoing interface fa0/1.

5.
R2 looks up destination IP’s MAC address.

6.
R2 encapsulates IP packet in new Ethernet frame,
with R2’s fa0/1 MAC as source and PC3’s MAC
as destination. R2 sends the frame.

CIT 384: Network Administration

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10

Routing Example with PPP

CIT 384: Network Administration

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11

Matching the Most Specific Route

R1#show ip route rip



172.16.0.0/16 is variably subnetted, 5 subnets, 4 masks

R 172.16.1.1/32 [120/1] via 172.16.25.2, 00:00:04, Serial0/1/1

R 172.16.1.0/24 [120/2] via 172.16.25.129, 00:00:09, Serial0/1/0

R 172.16.0.0/22 [120/1] via 172.16.25.2, 00:00:04, Serial0/1/1

R 172.16.0.0/16 [120/2] via 172.16.25.129, 00:00:09, Serial0/1/0

R 0.0.0.0/0 [120/3] via 172.16.25.129, 00:00:09, Serial0/1/0

R1#show ip route 172.16.4.3


Routing entry for 172.16.0.0/16


Known via "rip", distance 120, metric 2


Redistributing via rip


Last update from 172.16.25.129 on Serial0/1/0, 00:00:19 ago


Routing Descriptor Blocks:


* 172.16.25.129, from 172.16.25.129, 00:00:19 ago, via Serial0/1/0


Route metric is 2, traffic share count is

CIT 384: Network Administration

Slide #
12

Ranges defined by each route

172.16.1.1

172.16.1.0


172.16.1.255

172.16.0.0


172.16.3.255

172.16.0.0


172.16.255.255

0.0.0.0


255.255.255.255

CIT 384: Network Administration

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13

Matching the Most Specific Route

172.16.1.1
: Matches all 5 routes

Longest prefix is /32, route to 172.16.1.1/32

172.16.1.2
: Matches last 4 routes

Longest prefix is /24, route to 172.16.1.0/24

172.16.2.3
: Matches last 3 routes

Longest prefix is /22, route to 172.16.0.0/22

172.16.4.3
: Matches last 2 routes

Longest prefix is /16, route to 172.16.0.0/16

CIT 384: Network Administration

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14

Why DNS?


HOSTS.TXT original flat text file mapping


Problems


Load
: everyone had to d/l from one server.


Collisions
: no two hosts can have the same
name in a flat namespace


Consistency
: by the time a host file had reached
every host, it was already out of date when
compared to the master copy on the server.


CIT 384: Network Administration

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15

DNS

Domain Name System


Released in 1984 with RFCs 882 and 883.


Nameservers provide name/IP mappings.


Resolvers on clients query nameservers.

Distributed database of name/IP mappings.


Tree structure prevents collisions.


Each domain controls its own nameservers, which
contain db for names in that domain.


Nameservers can query other
-
domain nameservers when
resolvers ask for names outside the local domain.

CIT 384: Network Administration

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The DNS Namespace

CIT 384: Network Administration

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17

Domains are Subtrees


Domains are
subtrees of the
DNS tree.


Similar to subtrees
of directory tree.


Does not indicate
anything about
subnet addresses.


Can be
administered
independently.

CIT 384: Network Administration

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18

Delegation of Domains


Top Level Domains
delegate subdomains to
organizations.


Organizations can
delegate subdomains of
their domain too.

CIT 384: Network Administration

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19

Name Resolution Process

1.
Resolver on host translates
hostnames for apps.

2.
Resolver asks nameserver to
translate for it.

3.
Nameserver asks root server,
with refers it to au svr.

4.
Au server refers to gov.au
server.

5.
Gov.au server refers to
gbrmpa.gov.au server.

6.
Gbrmpa.gov.au server returns
IP address to nameserver.

7.
Nameserver may cache
translation for future use.

8.
Nameserver sends IP address
to resolver.

CIT 384: Network Administration

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20

Mapping Addresses to Names

Use in
-
addr.arpa domain.


Nodes are labeled by
an octet of IP address.


in
-
addr has 256
subtrees, each
representing the first
octet of an IP address.


Four
-
level nodes map
back to hostnames.

CIT 384: Network Administration

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21

Maximum Transmission Unit (MTU)

router(config)# interface fa0/1

router(config
-
if)# mtu 1000

router(config
-
if)# end

CIT 384: Network Administration

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22

Connected Routes

Routes to networks where rtr has interface.


Router determines these routes from interface IP
address and subnet.


Listed in route table with a C prefix.

How to deliver packets to non
-
connected nets?


Needs routes for those subnets, or


A default route for all other networks.

CIT 384: Network Administration

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23

show ip route

Router
-
251>show ip route

Codes: C
-

connected, S
-

static, R
-

RIP, M
-

mobile, B
-

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


i
-

IS
-
IS, su
-

IS
-
IS summary, L1
-

IS
-
IS level
-
1, L2
-

IS
-
IS level
-
2


ia
-

IS
-
IS inter area, *
-

candidate default, U
-

per
-
user static route


o
-

ODR, P
-

periodic downloaded static route


Gateway of last resort is 10.11.241.1 to network 0.0.0.0



10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C

10.11.241.0/24 is directly connected, FastEthernet0/0

C

10.251.0.0/16 is directly connected, FastEthernet0/1

S*

0.0.0.0/0 [1/0] via 10.11.241.1


CIT 384: Network Administration

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24

show ip route
address

Router
-
251>show ip route 10.11.241.9

Routing entry for 10.11.241.0/24


Known via "connected", distance 0, metric 0 (connected, via
interface)


Routing Descriptor Blocks:


* directly connected, via FastEthernet0/0


Route metric is 0, traffic share count is 1


Router
-
251>show ip route 144.144.144.144

% Network not in table

CIT 384: Network Administration

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25

Static Routes

Routes entered by network administrator.


Listed in route table with S prefix.


Easy to understand network behavior.


Avoids problems with routing protocols.


Requires extensive planning for large networks.


Does not adapt to changing networks.


Does not scale to large networks.

CIT 384: Network Administration

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Creating a Static Route

ip route
network subnet destination

ip route 10.1.0.0 255.255.0.0 10.1.0.1

ip route 10.6.9.0 255.255.255.0 10.6.9.95

ip route 10.2.0.0 255.255.0.0 fa0/1

CIT 384: Network Administration

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Default Routes

What happens if no route matches an IP?


Packet is discarded by router.


Unless there is a default route.

Default route matches all destination addrs.


Listed with S* prefix in routing table.

Configuring a default route

ip route 0.0.0.0 0.0.0.0 10.9.3.1

Configuring a default route for routing protocols

ip default
-
network 10.0.0.0

(a route to 10.x.y.z must already exist)

CIT 384: Network Administration

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Classful and Classless Routing

Classless routing:

When a packet’s
destination only matches default route,
forward packet using default route.



ip classless

Classful routing:

When a packet’s destination
only matches default route, only use the
default route if this router does not know any
routes in the classful network in which the
destination IP address resides.



no ip classless

CIT 384: Network Administration

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Classful Routing


R3 will use its default route only if it does


not have another route for that address class;


a single class
-
B 168.13 route will prevent


use of the default route for any 168.13 network.

CIT 384: Network Administration

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Classful and Classless Routing

R3#show ip route

Gateway of last resort is 168.13.100.1 to network 0.0.0.0


168.13.0.0/24 is subnetted, 4 subnets

R 168.13.1.0 [120/1] via 168.13.100.1, 00:00:13, Serial0.1

C 168.13.3.0 is directly connected, Ethernet0

R 168.13.2.0 [120/1] via 168.13.100.2, 00:00:06, Serial0.1

C 168.13.100.0 is directly connected, Serial0.1

R3#ping 10.1.1.1

Type escape sequence to abort.

Sending 5, 100
-
byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round
-
trip min/avg/max = 84/89/114 ms

R3#ping 168.13.200.1

Type escape sequence to abort.

Sending 5, 100
-
byte ICMP Echos to 168.13.200.1, timeout is 2 seconds:

.....

Success rate is 0 percent (0/5)

CIT 384: Network Administration

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31

Extended Ping


Ping comes from interface closest to dest.


Extended ping is configurable.


Alter source address to use different interface.


Can also modify


Number of pings sent.


Size of ping packets.


Data included.


Don’t fragment flag.


Type of Service


CIT 384: Network Administration

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Extended Ping

Albuquerque#show ip route

10.0.0.0/24 is subnetted, 5 subnets

S 10.1.3.0 [1/0] via 10.1.130.253

S 10.1.2.0 [1/0] via 10.1.128.252

C 10.1.1.0 is directly connected, Ethernet0

C 10.1.130.0 is directly connected, Serial1

C 10.1.128.0 is directly connected, Serial0

Albuquerque#ping 10.1.2.252

Type escape sequence to abort.

Sending 5, 100
-
byte ICMP Echos to 10.1.2.252, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round
-
trip min/avg/max = 4/4/8 ms

Albuquerque#ping

Protocol [ip]:

Target IP address: 10.1.2.252
(Ethernet0 on Yosemite)

Extended commands [n]: y

Source address or interface: 10.1.1.251
(Ethernet0)

Data pattern [0xABCD]:

Sending 5, 100
-
byte ICMP Echos to 10.1.2.252, timeout is 2 seconds:

. . . . .

Success rate is 0 percent (0/5)

CIT 384: Network Administration

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VLSM

Variable Length Subnet Masking


Using more than one subnet mask in a single
classful network.


Not supported by old protocols RIPv1, IGRP.

CIT 384: Network Administration

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34

Overlapping Subnets

Subnets must not have overlapping addresses.


Results in overlapping router table entries.


Routing is not predictable in such a situation.


Some hosts may only be accessible from some parts of
the network.

Determining overlap

1.
Calculate subnet number and broadcast address of each
subnet; this gives you the address range.

2.
Compare address ranges of all subnets, looking for
overlaps.

CIT 384: Network Administration

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35

Overlapping Subnets Example

Subnet

Subnet Number

1
st

Address

Last Address

Broadcast

R1 LAN

172.16.2.0

172.16.2.1

172.16.3.254

172.16.3.255

R2 LAN

172.16.4.0

172.16.4.1

172.16.5.254

172.16.5.255

R3 LAN

172.16.5.0

172.16.5.1

172.16.5.254

172.16.5.255

R1
-
R2

172.16.9.0

172.16.9.1

172.16.9.2

172.16.9.3

R1
-
R3

172.16.9.4

172.16.9.5

172.16.9.6

172.16.9.7

CIT 384: Network Administration

Slide #
36

Designing a Subnetting Scheme


Three subnets with prefix /24 (255.255.255.0)


Three subnets with prefix /26 (255.255.255.192)


Four subnets with prefix /30 (255.255.255.252)

CIT 384: Network Administration

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37

Route Summarization

Combining routes to same destination.


Routes must be to adjacent subnets.


Reduces routing table size, improves perf.

Manual


Admin does with
ip summary
-
address

Automatic


Handled by routing protocol.


Can break routing in discontiguous networks.

CIT 384: Network Administration

Slide #
38

Creating Summary Routes

Seville(config)#interface serial 0/0

Seville(config
-
if)#
ip summary
-
address

eigrp 1 10.3.0.0 255.255.0.0

Seville#show ip route

Codes: C
-

connected, S
-

static, D
-

EIGRP, EX
-

EIGRP external ...

Gateway of last resort is not set


10.0.0.0/8 is variably subnetted, 9 subnets, 3 masks

D 10.2.0.0/16 [90/2684416] via 10.1.6.1, 00:00:36, Serial0/0

D 10.3.0.0/16 is a summary, 00:00:38, Null0

D 10.1.1.0/24 [90/2195456] via 10.1.6.1, 00:00:36, Serial0/0

C 10.3.5.0/24 is directly connected, Loopback5

C 10.3.4.0/24 is directly connected, FastEthernet0/0

C 10.1.6.0/30 is directly connected, Serial0/0

C 10.3.7.0/24 is directly connected, Loopback7

D 10.1.4.0/30 [90/2681856] via 10.1.6.1, 00:00:36, Serial0/0

C 10.3.6.0/24 is directly connected, Loopback

CIT 384: Network Administration

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39

Creating a Manual Summary

1.
List all subnets you want to summarize.

2.
Find first N bits of subnet numbers that are the
same for all subnets.

3.
Summary route subnet number is the in
-
common
bits followed by all 0s.

4.
Summary route subnet mask is N 1s followed by
32
-
N 0s.

5.
Check work by comparing IP address range of
summary subnet with summarized subnet ranges.

CIT 384: Network Administration

Slide #
40

Manual Summary Example

0000 1010 0000 0010 0000 0 001 0000 0000 10.2.1.0

0000 1010 0000 0010 0000 0 010 0000 0000 10.2.2.0

0000 1010 0000 0010 0000 0 011 0000 0000 10.2.3.0

0000 1010 0000 0010 0000 0 100 0000 0000 10.2.4.0


Subnet Address: 0’s replace non
-
shared bits

0000 1010 0000 0010 0000 0 000 0000 0000 10.2.0.0


Broadcast Address: 1’s replace subnet address bits

1111 1111 1111 1111 1111 1 000 0000 0000 255.255.248.0

CIT 384: Network Administration

Slide #
41

Autosummarization

Classful routing protocols use static masks.


Cannot use route to 10.3.4.0/24.


Can only use 10.0.0.0/8 route.

Albuquerque will send all 10 net traffic to Seville.

CIT 384: Network Administration

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42

Discontiguous Networks

Albuquerque ends up with 2 10.0.0.0/8 routes


Balances packets between S0/0 and S0/1


Applications lose ~50% of packets through A.

CIT 384: Network Administration

Slide #
43

Discontiguous Networks

Albuquerque#show ip route

Codes: C
-

connected, S
-

static, I
-

IGRP, R
-

RIP, M
-

mobile, B
-

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 inter area


*
-

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, 3 subnets

C 172.16.1.0 is directly connected, Ethernet0/0

C 172.16.2.0 is directly connected, Serial0/0

C 172.16.3.0 is directly connected, Serial0/1

R 10.0.0.0/8 [120/1] via 172.16.3.3, 00:00:13, Serial0/1


[120/1] via 172.16.2.2, 00:00:04, Serial0/0

CIT 384: Network Administration

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44

Autosummarization Support


RIPv1 always uses autosummarization.


RIPv2 and EIGRP use autosummary default


Turn off with
no auto
-
summary


OSPF does not use autosummarization.

CIT 384: Network Administration

Slide #
45

References

1.
Paul Albitz and Cricket Liu,
DNS and BIND, 5
th

edition
, O’Reilly, 2006.

2.
James Boney,
Cisco IOS in a Nutshell, 2
nd

edition
, O’Reilly, 2005.

3.
Cisco, Cisco Connection Documentation,
http://www.cisco.com/univercd/home/home.htm

4.
Cisco, Internetworking Basics,
http://www.cisco.com/univercd/cc/td/doc/cisintw
k/ito_doc/introint.htm

5.
Wendell Odom,
CCNA Official Exam
Certification Library, 3
rd

edition
, Cisco Press,
2007.