Routing Refresher

1
Routing Refresher
2
Routers and OSI Model
3
Basic Routing Process
IP on Host A determines that a packet destined to
Host B is on a remote network so sends the
packet to the default gateway.
172.130.0.1
Host A
E0 172.130.0.2
E1 220.130.0.1
Host B
220.130.0.2
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Basic Routing Process
172.130.0.1
ARP Broadcast
Host A
E0 172.130.0.2
ARP Response
E1 220.130.0.1
Host B
Host A uses ARP to determine the MAC address of
the default gateway.
220.130.0.2
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Basic Routing Process
When the router receives the packet it determines
from its routing table that network 220.130.0.0
is connected to its E1 interface so uses ARP to
get the MAC address of Host B.
172.130.0.1
Host A
ARP Broadcast
E0 172.130.0.2
Host B
E1 220.130.0.1
ARP Response
220.130.0.2
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Routing in a Large Internetwork
E0
S0
S0
E0
S1
S0
S2
S0
S1
S1
S0
TO0
E0
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Routing Process

• Learning Routes
• Routes are learned either by being entered by an
  administrator or by communicating with other
  routers
• Forwarding Packets
• Router determines where to forward packets by
  looking up destination network address in its
  route table

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Forwarding Packets

• If a route to the network is in the route table
  then the router adds the appropriate Layer 2
  header and transmits the packet out the
  appropriate interface
• If no route exists then the router either routes
  the packet to its default route (gateway of last
  resort) or drops the packet

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Route Tables

• Subnet address
• Metric that indicates the quality of the route
• Interface out which to forward packets
• Address of next router in the route
• Administrative distance of route

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Route Table Contents
75.3.0.0/24 is subnetted, 3 subnets C
75.3.0.0 is directly connected, Ethernet0 C
75.2.0.0 is directly connected, Serial0 R
129.130.1.0 120/2 via 75.2.0.1, 000004,
Serial0
Exit Interface
Metric
Destination Network
Next Hop
Administrative Distance
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Static vs. Dynamic

• Static routing
• Route table entered by administrator
• Reduces network traffic because routers dont
  have to communicate with one another
• More secure since routing updates cant be
  updated or spoofed
• Only possible with small, stable networks
• Dynamic routing
• Route table built dynamically by communicating
  with other routers
• Increases network traffic and taxes the routers
  CPU
• Allows routers to respond quickly to changes in
  the networks topology without human intervention

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(No Transcript)
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IP Route Command
ip route destination network mask
next_hop_address or exitinterface
administrative_distance permanent exitinterfa
ce must be on a point-to-point WAN
link administrative_distance used to determine
preference for routes permanent forces the
router to keep the route if the interface or the
next_hop_router goes down
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Static Route in Route Table
Router(config)ip route 10.32.0.0 255.224.0.0
75.2.0.1
75.3.0.0/24 is subnetted, 4 subnets C
75.3.0.0 is directly connected, Ethernet0 C
75.2.0.0 is directly connected, Serial0 R
129.130.1.0 120/2 via 75.2.0.1, 000004,
Serial0 S 10.32.0.0 1/0 via 75.2.0.1
Default administrative distance is 1 and metric
is 0.
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Configuring Static Routes
B
A
10.1.0.1
10.2.0.1
10.2.0.2
220.1.2.1
E0
S0
S0
E0
RouterAgtenable RouterAconfig t RouterA(Config)In
terface E0 RouterA(if-config)ip address 10.1.0.1
255.255.0.0 RouterA(if-config)no
shutdown RouterA(if-config)interface
S0 RouterA(if-config)ip address 10.2.0.1
255.255.0.0 RouterA(if-config)no
shutdown RouterA(if-config)exit
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Configuring Static Routes
B
A
10.1.0.1
10.2.0.1
10.2.0.2
220.1.2.1
E0
S0
S0
E0
RouterAshow 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, -candidate default, U per-user
static route, o ODR, P Periodic download
static route, T traffic engineered
route Gateway of last resort is not
set 10.0.0.0/16 is subnetted, 2
subnets C 10.1.0.0 is directly connected,
Ethernet 0 C 10.2.0.0 is directly connected,
Serial 0
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Configuring Static Routes
B
A
10.1.0.1
10.2.0.1
10.2.0.2
220.1.2.1
E0
S0
S0
E0
RouterAconfig t RouterA(config)ip route
220.1.2.0 255.255.255.0 10.2.0.2 RouterA(config)e
xit RouterAshow ip route Gateway of last resort
is not set 10.0.0.0/16 is subnetted, 2
subnets S 220.1.2.0 1/0 via 10.2.0.2 C 10.1.0.0
is directly connected, Ethernet 0 C 10.2.0.0 is
directly connected, Serial 0
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Default Routes
20.16.10.1
Border Router
20.32.0.2
B
10.0.0.1
20.32.0.1
20.48.0.1
20.16.0.2
C
20.48.0.2
A
Router As default route is 20.48.0.1, Bs
default route is 20.32.0.1 and Cs default routes
is 20.16.0.1. The Border Router would have
knowledge of the larger internetwork.
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Configuring the Default Route
B
A
10.1.0.1
10.2.0.1
10.2.0.2
220.1.2.1
E0
S0
S0
E0
RouterAconfig t RouterA(config)ip route 0.0.0.0
0.0.0.0 10.2.0.2 RouterA(config)ip
classless RouterA(config)exit RouterAshow ip
route Gateway of last resort is 10.2.0.2 to
network 0.0.0.0 10.0.0.0/16 is subnetted, 2
subnets S 220.1.2.0 1/0 via 10.2.0.2 C 10.1.0.0
is directly connected, Ethernet 0 C 10.2.0.0 is
directly connected, Serial 0 S 0.0.0.0/0 1/0
via 10.2.0.2
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Static Destinations

• Default gateway
• Next hop if no IP routing is enabled
• Default network
• Used by IGRP and EIGRP
• Gateway of last resort
• Default route that comes from the source with
  lowest administrative distance
• Static routes have the lowest possible
  administrative distance so will always be the
  gateway of last resort if it is configured

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Dynamic Routing Protocols

• Administrative Distances
• Single Path vs. Multipath
• Interior vs. Exterior Gateway
• Link State vs. Distance Vector

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Administrative Distances

• Each routing protocol has an associated
  administrative distance that indicates the
  relative trustworthiness of the routes learned by
  that protocol
• Lower distances are preferred when multiple
  routes exist

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Administrative Distances
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Single Path vs. Multipath

• Some routing algorithms allow the router to
  maintain multiple paths to a remote network
• Most Cisco routers allow 4 equal cost routes by
  default but this can be changed to a value from 1
  to 6
• Traffic can be balanced across all paths
  improving performance and reliability

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Interior vs. Exterior Gateway

• Exterior Gateway
• Used for routing between routing domains
• Usually operating on expensive WAN links so send
  less information to other routers
• Usually have many more routes to consider so must
  have sophisticated path determination algorithms
• Interior Gateway
• Used for routers inside routing domains
• Usually operate on LAN links so can send more
  updates
• Usually have fewer routes to consider so can be
  less sophisticated

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Link State vs. Distance Vector

• Distance Vector Protocols
• Use simple metrics such as hops to determine
  optimal routes
• Transmit entire route table at regular intervals
  to other routers
• Slow to converge and are prone to loops
• Link State Protocols
• Use more sophisticated metrics such as cost to
  determine optimal routes
• Transmit only changes to other routers
• Require more powerful routers

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IOS Commands to Know

• Show IP Route
• IP Route
• Including creating default route
• IP Classless
• Show Protocols
• Show IP Protocol

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Routersh ip protocols Routing Protocol is
"rip" Sending updates every 30 seconds, next
due in 25 seconds Invalid after 180 seconds,
hold down 180, flushed after 240 Outgoing
update filter list for all interfaces is not set
Incoming update filter list for all interfaces
is not set Redistributing rip Default
version control send version 1, receive any
version Interface Send Recv
Key-chain Ethernet0 1 1 2
Serial0 1 1 2 Routing for
Networks 75.0.0.0 Routing Information
Sources Gateway Distance Last
Update 75.2.0.1 120
000003 Distance (default is 120)
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IP Routing can be turned off With the no ip
routing command
Routersh protocols Global values Internet
Protocol routing is enabled Ethernet0 is up, line
protocol is up Internet address is
75.3.0.1/24 Serial0 is up, line protocol is up
Internet address is 75.2.0.2/24 Serial1 is
administratively down, line protocol is down
Shutdown command present
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State of interface

• Serial1 is up, line protocol is up
• Everything great
• Serial1 is up, line protocol is down
• Data Link Layer problem
• Clock rate doesnt match keepalives,
  encapsulation incorrect
• Serial1 is down, line protocol is down
• Physical Layer problem
• Clock rate not set, cable not connected
• Serial1 is administratively down, line protocol
  is down
• Shutdown command used