Routing and Routing Protocols

1
Routing and Routing Protocols

• Introduction to Static Routing

2
Routing Decisions

• Routing is the process that a router uses to
  forward packets toward a packets destination.
• Routing decisions are based on the destination IP
  address of a packet.

3
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

4
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

5
Configuring Static Routes

• Send traffic through an interface
• Send traffic to the next routers address

6
Configuring the Outbound Interface
7
Configuring the Next-Hop Address
8
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.

9
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.

10
Default Administrative Distances
11
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.

12
Static Routes as Backups

• Often static routes are used for backup purposes,
  such as when the dynamically learned route fails.

13
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
192.168.2.2
192.168.1.2
192.168.1.1
14
Router0 Routing Table
No Routing Protocol
192.168.0.0/24
192.168.2.1
192.168.2.2
192.168.1.2
192.168.1.1
15
Backup Route Creation
No Routing Protocol
192.168.0.0/24
192.168.2.1
192.168.2.2

• To configure a static route for the 56 kbps
  backup serial line

192.168.1.2
192.168.1.1
16
Router0 New Routing Table
No Routing Protocol
192.168.0.0/24
192.168.2.1
192.168.2.2

• Now, the static route shows up in the routing
  table, instead of the better route learned
  through RIP.

192.168.1.2
192.168.1.1
17
Fixing the Routing Table Problem
No Routing Protocol
192.168.0.0/24
192.168.2.1
192.168.2.2

• RIPs 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.

192.168.1.2
192.168.1.1
18
The Backup Route In Action
No Routing Protocol
192.168.0.0/24
192.168.2.1
192.168.2.2

• When the RIP-learned route is unavailable, the
  static backup route is placed in the routing
  table.

192.168.1.2
192.168.1.1
19
The Backup Route In Action
No Routing Protocol
192.168.0.0/24
192.168.2.1
192.168.2.2

• Once the better route is available, the RIP route
  will be re-entered into the routing table,
  automatically.

192.168.1.2
192.168.1.1
20
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-hop-address
  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.

21
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
22
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
23
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
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
25
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