Transcript Introduction to Static Routing

Routing and Routing Protocols
Introduction to Static Routing
Routing Decisions
• Routing is the process that a router
uses to forward packets toward a
packet’s destination.
• Routing decisions are based on the
destination IP address of a packet.
Routing Methods
• Routers must learn the direction to remote
networks in order to forward packets.
• Two ways to learn this information:
 Dynamically
• Information is learned from other routers
– Often through RIP, OSPF, or EIGRP routing
protocols
 Statically
• Configured manually
– Requires the network administrator to add and
delete static routes when topology changes
– In large networks it requires a tremendous
amount of administrative time
– On small, or unchanging networks, it requires
very little maintenance
Static Route Operation
• Static route operations can be
divided into 3 stages:
 A network administrator manually
configures the static route on the
router
 The router installs the route in its
routing table
 Packets are routed using the
static route
Configuring Static Routes
Send traffic through an interface:
Destination
Network
Subnet Mask
Local Router’s
Outgoing
Interface
Send traffic to the next router’s address:
Destination
Network
Subnet Mask
Next Hop IP
Address
(address of
next router)
Configuring the Outbound
Interface
Configuring the Next-Hop
Address
Routes and Interfaces
• If a router cannot reach the outgoing interface
that is being used in a route, the route will not
be installed in the routing table.
• This means if that interface is down, the route
will not be placed in the routing table.
Administrative Distance
• The administrative distance is a
number that measures the
trustworthiness of the source of the
route information.
 The lower the administrative distance,
the more trustworthy the source.
• If a path has the lowest
administrative distance, it is
installed in the routing table.
Default Administrative
Distances
Multiple Routes to the Same
Destination
• When two or more routes point to
the same destination, the
administrative distance is used by
the router to determine which route
is entered into the routing table.
 All routes are remembered, but only
the best route makes it into the
routing table.
Static Routes as Backups
• Often static routes are used for
backup purposes, such as when
the dynamically learned route fails.
Backup Route Example
• For Router0, the
preferred path to
the 192.168.0.0
network is
through the
switch.
• If RIP is used to
exchange routes
between the two
routers, this route
will be marked as
the best.
No Routing Protocol
192.168.0.0/24
192.168.2.1
192.168.1.1
192.168.2.2
192.168.1.2
Router0 Routing Table
No Routing Protocol
192.168.0.0/24
192.168.2.1
192.168.1.1
192.168.2.2
192.168.1.2
Backup Route Creation
No Routing Protocol
192.168.0.0/24
192.168.2.1
• To configure a
static route for the
56 kbps backup
serial line:
192.168.1.1
192.168.2.2
192.168.1.2
Router0 New Routing Table
No Routing Protocol
192.168.0.0/24
192.168.2.1
• Now, the static
route shows
up in the
routing table,
instead of the
better route
learned
through RIP.
192.168.1.1
192.168.2.2
192.168.1.2
Fixing the Routing Table
Problem
No Routing Protocol
192.168.0.0/24
192.168.2.1
•
•
RIP’s default
administrative
distance is 120.
Creating the
route with an
AD higher than
120 will ensure
the RIP route is
placed in the
routing table.
192.168.1.1
192.168.2.2
192.168.1.2
The Backup Route In Action
No Routing Protocol
192.168.0.0/24
192.168.2.1
• When the
RIP-learned
route is
unavailable,
the static
backup route
is placed in
the routing
table.
192.168.1.1
192.168.2.2
192.168.1.2
The Backup Route In Action
No Routing Protocol
192.168.0.0/24
192.168.2.1
• Once the
better route is
available, the
RIP route will
be re-entered
into the
routing table,
automatically.
192.168.1.1
192.168.2.2
192.168.1.2
Configuring Default Route
Forwarding
• Default routes are used to route packets to
destinations that do not match any of the
other routes in the routing table.
• A default route is a special static route that
uses the quad-zero format:
ip route 0.0.0.0 0.0.0.0 [next-hopaddress | outgoing interface]
• If the packet does not match a specific route
in the routing table, it will be routed to the
0.0.0.0 network.
 Any IP address will always yield the network
address 0.0.0.0 when ANDed with the mask
0.0.0.0.
Default Routing Example
• In this example,
no routing
protocol is used.
• Instead, a default
route will be used
to allow Router0
to direct traffic to
the ISP router.
No Routing Protocol
192.168.0.1
192.168.0.2
10.0.0.1
10.0.0.0/24
Router0 Routing Table Prior to
Default Route
No Routing Protocol
192.168.0.1
192.168.0.2
• Router0 can
see the two
directly
connected
networks and
cannot route
traffic to the
Internet.
10.0.0.1
10.0.0.0/24
Adding a Default Route
No Routing Protocol
192.168.0.1
192.168.0.2
• Entering the
quad-zero
route on
Router0 allows
the router to
send all remote
traffic to the
next-hop.
10.0.0.1
10.0.0.0/24
Viewing the Default Route
No Routing Protocol
192.168.0.1
192.168.0.2
• Entering the
quad-zero
route on
Router0 allows
the router to
send all remote
traffic to the
ISP.
10.0.0.1
10.0.0.0/24
Default Route Note
• The routers on the
Internet must have a
route in their routing
tables that points to
Router0 (or a default
route that
accomplishes the
same).
• Otherwise, Router0
will be able to route
traffic to servers on
the Internet, but the
Internet routers
would not be able to
route the responses
back to Router0.
No Routing Protocol
192.168.0.1
192.168.0.2
10.0.0.1
10.0.0.0/24