11. Routing Concepts
1.1 Introduction
A router
connects one network to another network. The router is responsible for the
delivery of packets across different networks.
The router uses
its routing table to determine the best path to use to forward a packet. It is
the responsibility of the routers to deliver those packets in a timely manner.
1.2 Characteristics of a Network
Speed, Cost, Security, Availability, Scalability,
Reliability.
1.3 Router is a computer
RAM: IP routing table, Ethernet ARP table) and buffers for packet
processing
ROM(firmware): storage for bootup, basic diagnostic software, and a
limited IOS.
NVRAM: Startup-config
Flash: IOS and other system-related files.
Ports: LAN(local) or WAN, AUX, CONSOLE, USB.
The primary functions of a router are
to:
·
Determine the best path to send
packets (routing table)
·
Forward packets toward their
destination
When the router
receives a packet, it examines the destination address of the packet and uses
the routing table to search for the best path to that network. The routing
table also includes the interface to be used to forward packets for each known
network. When a match is found, the router encapsulates the packet into the
data link frame of the outgoing or exit interface, and the packet is forwarded
toward its destination.
The Data link
encapsulation depends on the type of interface on the router and the type of
medium to which it connects. Ethernet, PPP, Frame Relay, DSL, cable, and
wireless (802.11, Bluetooth).
1.5 Packet Forwarding Mechanisms
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Slow
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packet-triggered / Fast
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change-triggered / Very fast
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-
Process switching: older packet
forwarding mechanism. Packet->control plane (CPU matches the destination
with routing table) and then determines the exit interface and forwards the
packet.
Process for every packets
slow and rarely implemented.
-
Fast Switching: fast-switching cache to
store next-hop information. Flow information stored.
-
Cisco Express Forwarding(CEF): most
recent and preferred packet forwarding. CEF builds a Forwarding Information
Base (FIB), and an ADJACENCY TABLE. The FIB contains pre-computed reverse
lookups, next hop information for routes including the interface and Layer 2
information.
2.
Connect devices
2.1 Connect to a
network
Network devices
and end users typically connect to a network using a wired Ethernet or wireless
connection
2.2 Defaut Gateway
To enable
network access, devices must be configured:
-
IP address: identifies a unique host on
a local network.
-
Subnet mask: Identifies with which
network subnet the host can communicate.
-
Default gateway: dentifies the router to
send a packet to when the destination is not on the same local network subnet.
The default
gateway is the destination that routes traffic from the local network to
devices on remote networks
Note: A router is also usually configured with its own default
gateway. This is sometimes known as the Gateway of Last Resort. (could be
also IP address of another router!)
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2.3 Document Network Addressing
When designing
the documentation is important with this minimum details: Devices name, IP
address and subnet mask, Default gateway.
Topology diagram -
Provides a visual reference that indicates the physical connectivity and
logical Layer 3 addressing. VISIO
DHCP services
can be provided by a Cisco Catalyst switch or a Cisco ISR.
2.4 Devices LEDs
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2.5 Console Access
infrastructure
devices are commonly accessed remotely using Secure Shell (SSH) or HyperText
Transfer Protocol Secure (HTTPS). Console access is really only required when
initially configuring a device, or if remote access fails.
Console access
requires:
-
Console cable - RJ-45-to-DB-9
console cable
-
Terminal emulation software -
Tera Term, PuTTY, HyperTerminal
The Cisco ISR G2
supports a USB serial console connection. To establish connectivity, a USB
Type-A to USB Type-B (mini-B USB)
ßà
(Driver is required download at cisco.com)
2.6 Enable IP on a Switch
Configuring VLAN
1 with IP address and mask also a ip gateway is needed.
2.7 Configure Basic Router Settings
Cisco routers
and Cisco switches have many similarities.
When configuring
a Cisco switch or router, the following basic tasks should be performed
first:
-
Name the device - Distinguishes it from
other routers.
-
Secure management access - Secures
privileged EXEC, user EXEC, and Telnet access, and encrypts passwords to their
highest level.
-
Configure a banner - Provides legal
notification of unauthorized access.
2.8 Configure an IPv4 Router Interface
R1(config)# interface GigabitEthernet0/1
R1(config-if)# description xxxxxx
R1(config-if)#ip address 192.168.1.1
255.255.255.0
R1(config-if)#no shutdown
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In case of interface serial DCE: (IN DTE= ERROR)
R1(config-if)# Clock rate 64000
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2.9 Configure an IPv6 Router Interface
R1(config)# interface GigabitEthernet0/1
R1(config-if)# description xxxxxx
R1(config-if)#ipv6 address x:x:x:x:x:x:x:x/YY
R1(config-if)#no shutdown
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In case of interface serial DCE: (IN DTE= ERROR)
R1(config-if)# Clock rate 64000
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Note: An interface can
generate its own IPv6 link-local address without having a global unicast
address by using the ipv6 enable interface configuration command.
- IPv6
interfaces will typically have more than one IPv6 address
- IPv6 also
supports the ability for an interface to have multiple IPv6 global unicast
addresses from the same subnet.
Create a global
unicast or link-local IPv6 address:
- ipv6 address ipv6-address / prefix-length - Creates a global unicast IPv6
address as specified.
- ipv6 address ipv6-address / prefix-length eui-64
- Configures a global unicast IPv6 address with an interface identifier (ID) in
the low-order 64 bits of the IPv6 address using the EUI-64 process.
- ipv6 address ipv6-address / prefix-length link-local
- Configures a static link-local address on the interface that is used instead
of the link-local address that is automatically configured when the global
unicast IPv6 address is assigned to the interface or enabled using the ipv6
enable interface command. Recall, the ipv6 enable interface command is used to
automatically create an IPv6 link-local address whether or not an IPv6 global
unicast address has been assigned.
3. Configure an
IPv4 Loopback Interface
The loopback
interface is a logical interface internal to the router. It is not assigned to
a physical port and can therefore never be connected to any other device. It is
considered a software interface that is automatically placed in an UP state, as
long as the router is functioning.
(OSPF) routing
process. The router use loopback for identification instead IP address assigned
in port that may goes down.
Configure:
R1 (config)#
interface loopback number ß unique number. Router accept
various loopback.
R1 (config-if)#
ip address ip-address subnet-mask
R1 (config-if)#
exit
4
Verify Connectivity of
Directly Connected Networks
4.1
Verify IPv6 Interface Settings
R1#show ipv6
interface brief
R1#show ipv6
interface gigabitethernet
R1#show ipv6
route
4.2
Filter Show Command Output
Command, enter a
pipe (|) character after the show (section, include, exclude, begin)
4.3
Command History Feature
To recall
commands: UP arrow or Crt+p
To recall Most recent
commands: Down arrow or Ctrl+N
R1#show history
R1#terminal
history size <0-256> Size of
history buffer
5. Routing
decisions
Switching
Packets Between Networks
5.1 Router Switching
Function
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Router:
Primary function = forwarding packets
Accomplished by using a switching
function, which is the process used by a router to accept a packet on one
interface and forward it out of another interface.
Note: In this context, the term “switching” literally means moving
packets from source to destination and should not be confused with the
function of a Layer 2 switch.
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5.2 Send a
Packet
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AND operation is performed to check if the destination is in the same
network.
[ IPv4
address + subnet mask ]
PC1 have ARP cache for the MAC address
If not:
ARP request to acquire the address
IPv4: ARP process
IPv6: ICMPv6 process
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5.3 Forward to
the Next Hop
The following
processes take place when Router receives the Ethernet frame from PC:
1)Examines the destination MAC address, which matches
the MAC address of the receiving interface. copies the frame into its buffer.
2)Identifies the Ethernet Type field as: 0x800 (Ethernet
frame contains an IPv4 packet)
3)Ethernet frame contains an IPv4 packet
4)The destination IPv4 address of the packet does not match any of
the directly connected networks, the router consults its routing table to route
this packet. With a network , next-hops and exit interface the router
encapsulate the ipv4 packet in a new Ethernet frame with the destination MAC
address of the IPv4 address of the next-hop router.
For the exit of
this packet a new ARP request is
performed for the next-hops address.
5.4 Packet Routing
Same process of
the previous example but the exit in this case is a Serial interface he doesn’t
need to
resolve the
next-hop IPv4 address with a destination MAC address.
Example in point-to-point
(P2P) serial connection Frame addressing:
Address:
0x8F (source address)
Control:
0x00 (destination address)
When the
interface is a point-to-point (P2P) serial connection, the router encapsulates
the IPv4 packet into the proper data link frame format used by the exit
interface (HDLC, PPP, etc.). Because there are no MAC addresses on serial
interfaces, R2 sets the data link destination address to an equivalent of a
broadcast.
5.5 Reach the
Destination
The following
processes take place when the frame arrives
1)copies the
data link PPP frame into its buffer.
2) de-encapsulates
the data link PPP frame
3) Searches the
routing table for the destination IPv4 address of the packet. The routing table
has a route to a directly connected network on R3. This means that the packet
can be sent directly to the destination device and does not need to be sent to
another router.
6. Path
Determination
6.1 Routing Decisions
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The routing table search results in one
of three path determinations:
· Directly connected
network : packet is a host address on the same
network as the interface of the router.
· Remote network: Remote networks can only be reached by forwarding packets to
another router.
· No route determined: A Gateway of Last Resort is set when a default route is configured
on a router.
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6.2 Best Path
Determining the
best path involves the evaluation of multiple paths.
value metric= quantitative
value used to measure the distance to a given network
The best path to
a network is the path with the lowest metric.
Dynamic routing
protocols typically use their own rules and metrics to build and update routing
tables
The routing
algorithm generates a value, or a metric, for each path through the network.
Metrics can be
based on either a single characteristic or several characteristics of a path.
Some routing
protocols can base route selection on multiple metrics, combining them into a
single metric.
Dynamic
protocols and the metrics they use:
·
Routing Information Protocol
(RIP) - Hop count
·
Open Shortest Path First (OSPF)
- Cisco’s cost based on cumulative bandwidth from source to destination
·
Enhanced Interior Gateway Routing
Protocol (EIGRP) - Bandwidth, delay, load, reliability
6.3 Load
Balancing
In routing table 2 equally cost
metric path the reach the same
destination = equal cost load balancing
FOR STATIC ROUTING AND DYNAMIC ROUTING
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Router forwards
packets using the multiple exit interfaces listed in the routing table.
Load balancing
can increase the effectiveness and performance of the network.
Note: Only EIGRP supports
unequal cost load balancing.
6.4 Administrative
Distance
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Router with different protocols for
routing.
This table helps for the decision.
Example: We have a routing table filled
and the router is configured with static, RIP and EIGRP the decision chosen
will be static
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7. Routing operations
7.1 the Routing
Table (Stored in RAM)
Routing table of
a router stores information about:
·
Directly connected routes -
These routes come from the active router interfaces. Routers add a directly
connected route when an interface is configured with an IP address and is
activated.
·
Remote routes - These are
remote networks connected to other routers. Routes to these networks can either
be statically configured or dynamically configured using dynamic routing
protocols.
7.2 Routing
Table Sources
The show ip route display IPv4 routing
table of a router.
AND provides
additional route information, including how the route was learned, how long the
route has been in the table, and which specific interface to use to get to a
predefined destination.
Entries in the
routing table can be added as:
-
Local Route interfaces - Added when an
interface is configured and active. This entry is only displayed in IOS 15 or
newer for IPv4 routes and all IOS releases for IPv6 routes.
-
Directly connected interfaces - Added to
the routing table when an interface is configured and active.
-
Static routes - Added when a route is
manually configured and the exit interface is active.
-
Dynamic routing protocol - Added when
routing protocols that dynamically learn about the network, such as EIGRP or
OSPF, are implemented and networks are identified.
CCNA CODE:
L - Identifies the address assigned to a
router’s interface. This allows the router to efficiently determine when it
receives a packet for the interface instead of being forwarded.
C - Identifies a directly
connected network.
S - Identifies a static
route created to reach a specific network.
D - Identifies a
dynamically learned network from another router using EIGRP.
O - Identifies a
dynamically learned network from another router using the OSPF routing
protocol.
7.3 Remote
Network Routing Entries
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1)Route
source:
2)Destination
network:
3)Administrative
distance (AD):
4)Metric:
4)Next-hop:
5)Route
timestamp:
6)Outgoing
interface:
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8. Directly
Connected Routes
8.1 Directly
Connected Interfaces
A new deployed
router without a configuration doesn’t have routing table.
Before an
interface is considered UP/UP most have:
-
Be assigned a valid IPv4 or
IPv6 address
-
Be activated with the no
shutdown command
-
Receive a carrier signal from
another device (router, switch, host, etc.)
8.2 Directly
Connected Routing table Entries
An active,
properly configured, directly connected interface actually creates two routing
table entries.
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Route
source - how the route was learned. Directly
connected interfaces have two route source codes. ‘C’ identifies a directly
connected network. ’L’ identifies the IPv4 address assigned to the router’s
interface.
Destination
network - remote network.
Outgoing
interface - exit interface
NOTE: Prior IOS 15 (L) Not displayed.
IPV6: show ipv2 route 2001:db8:acad:1::/64
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9. Static
Routes (CODE: “S”)
After directly
connected interfaces are configured and added to the routing table, then static
or dynamic routing can be implemented. Static routes are manually configured.
Two common types of static routes in the
routing table:
-
Static route to a specific
network
-
Default static route
IPv4:
R1(config)#ip route network mask {next-hop-ip | exit-intf}
R1(config)#ip route 0.0.0.0 0.0.0.0 {exit-intf | next-hop-ip} Default candidate “*”
IPv6:
R1(config)#ipv6 route x:x:x:x:x:x:x:x /mask {ipv6-address
| interface-type interface-number}
R1(config)#ipv6 route ::/0 {ipv6-address | interface-type
interface-number} ßDefault 0.0.0.0 0.0.0.0(ipv4)
10. Dynamic Routing Protocols
10.1 Dynamic Routing
Dynamic routing protocols perform several
activities, including network discovery and maintaining routing tables. These
networks, and the best path to each, are added to the routing table of the
router, and identified as a network learned by a specific dynamic routing
protocol.
10.2 IPv4 Routing
Protocols
Cisco ISR routers supported ipv4 protocols:
-
EIGRP - Enhanced Interior
Gateway Routing Protocol (CCNA)
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OSPF - Open Shortest Path First
(CCNA)
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IS-IS - Intermediate
System-to-Intermediate System
-
RIP - Routing Information
Protocol
To find with one is supported by our
router:
R1(config)# router ?
The entry beginning with ‘D*EX’ identifies
that the source of this entry was EIGRP (‘D’). The route is a candidate to be a
default route (‘*’), and the route is an external route (‘*EX’) forwarded by
EIGRP.
10.3 IPv6 Routing
Protocols
-
RIPng (RIP next generation)
-
OSPFv3
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EIGRP for IPv6
To enable IPv6 routers to forward traffic
R1(config)# ipv6 unicast-routing
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