Chapter_2_Static Routing

Static Routing
Routing Protocols and Concepts – Chapter 2
Objectives
Define the general role a router plays in networks.
Describe the directly connected networks, different router interfaces.
Examine directly connected networks in the routing table and use the CDP protocol.
Describe static routes with exit interfaces.
Describe summary and default route.
Examine how packets get forwarded when using static routes.
Identify how to manage and troubleshoot static routes.
General Role of the Router
Functions of a Router
Best Path Selections
Forwarding packets to destination
Introducing the Topology
3 1800 series routers connected via WAN links
Each router connected to a LAN represented by a switch and a PC
General Role of the Router
Connections of a Router for WAN
A router has a DB-60 port that can support 5 different cabling standards
Connections of a Router for Ethernet
2 types of connectors can be used: Straight through and Cross-over
Straight through used to connect:
Switch-to-Router, Switch-to-PC, Router-to-Server, Hub-to-PC, Hub-to-Server
Cross-over used to connect:
Switch-to-Switch, PC-to-PC, Switch-to-Hub, Hub-to-Hub, Router-to-Router
Interfaces
Examining Router Interfaces
Show IP router command – used to view routing table
Show Interfaces command – used to show status of an interface
Show IP Interface brief command – used to show a portion of the interface information
Show running-config command – used to show configuration file in RAM
Interfaces
Configuring an Ethernet interface
By default all serial and Ethernet interfaces are down
To enable an interface use the No Shutdown command
Interfaces
Verifying Ethernet interface
Show interfaces for fastEthernet 0/0 – command used to show status of fast Ethernet port
Show ip interface brief
Show running-config
Ethernet interfaces participate in ARP
Interfaces
Configuring a Serial interface
Enter interface configuration mode
Enter in the ip address and subnet mask
Enter in the no shutdown command
Example:
R1(config)#interface serial 0/0
R1(config-if)#ip address 172.16.2.1 255.255.255.0
R1(config-if)#no shutdown
Interfaces
Examining Router Interfaces
Physically connecting a WAN Interface.
A WAN Physical Layer connection has sides:
Data Circuit-terminating Equipment (DCE) – This is the service provider. CSU/DSU is a DCE device.
Data Terminal Equipment (DTE) – Typically the router is the DTE device.


Interfaces
Configuring serial links in a lab environment
One side of a serial connection must be considered a DCE.
This requires placing a clocking signal – use the clock rate command.
Example:
R1(config)#interface serial 0/0
R1(config-if)#clockrate 64000
Serial Interfaces require a clock signal to control the timing of the communcations.

Routing Table and CDP Protocol
Purpose of the debug ip routing command
Allows you to view changes that the router performs when adding or removing routes.
Example:
R2#debug ip routing
IP routing debugging is on
Routing Table and CDP Protocol
To configure an Ethernet interface
Example:
R2(config)#interface fastethernet 0/0
R2(config-if)#ip address 172.16.1.1 255.255.255.0
R2(config-if)#no shutdown
Routing Table and CDP Protocol
When a router only has its interfaces configured & no other routing protocols are configured then:
The routing table contains only the directly connected networks
Only devices on the directly connected networks are reachable
Routing Table and CDP Protocol
Routing Table and CDP Protocol
Checking each route in turn
The ping command is used to check end to end connectivity
Routing Table and CDP Protocol
Purpose of CDP
A layer 2 cisco proprietary tool used to gather information about other directly connected Cisco devices.
Concept of neighbors
2 types of neighbors:
Layer 3 neighbors
Layer 2 neighbors
Routing Table and CDP Protocol
CDP show commands
Show cdp neighbors command
Displays the following information:
Neighbor device ID
Local interface
Holdtime value, in seconds
Neighbor device capability code
Neighbor hardware platform
Neighbor remote port ID
Show cdp neighbors detail command
Useful in determining if an IP address configuration error
Routing Table and CDP Protocol
Disabling CDP
To disable CDP globally use the following command
Router(config)#no cdp run
Static Routes with Exit Interfaces
Purpose of a static route
A manually configured route used when routing from a network to a stub network
Static Routes with Exit Interfaces
IP route command
To configure a static route use the following command: ip route
Example:
Router(config)# ip route network-address subnet-mask {ip-address | exit-interface }
Static Routes with Exit Interfaces
Dissecting static route syntax
ip route - Static route command
172.16.1.0 – Destination network address
255.255.255.0 - Subnet mask of destination network
172.16.2.2 - Serial 0/0/0 interface IP address on R2, which is the "next-hop" to this network
Static Routes with Exit Interfaces
Configuring routes to 2 or more remote networks
Use the following commands for R1
R1(config)#ip route 192.168.1.0 255.255.255.0 172.16.2.2
R1(config)#ip route 192.168.2.0 255.255.255.0 172.16.2.2
Static Routes with Exit Interfaces
Zinin’s 3 routing principles
Principle 1: “Every router makes its decision alone, based on the information it has in its own routing table.”
Principle 2: “The fact that one router has certain information in its routing table does not mean that other routers have the same information.”
Principle 3: “Routing information about a path from one network to another does not provide routing information about the reverse, or return path.”
Static Routes with Exit Interfaces
Using Zinin’s 3 routing principles, how would you answer the following?
Would packets from PC1 reach their destination?
Yes, packets destined for 172.16.1.0/24 and 192.168.1.0/24 networks would reach their destination.
Does this mean that any packets from these networks destined for 172.16.3.0/24 network will reach their destination?
No, because neither R2 nor R3 router has a route to the 172.16.3.0/24 network.
Static Routes with Exit Interfaces
Resolving to an Exit Interface
Recursive route lookup - Occurs when the router has to perform multiple lookups in the routing table before forwarding a packet. A static route that forwards all packets to the next-hop IP address goes through the following process (reclusive route lookup).
The router first must match static route’s destination IP address with the Next hop address.
The next hop address is then matched to an exit interface.
Static Routes with Exit Interfaces
Configuring a Static route with an Exit Interface
Static routes configured with an exit interface are more efficient because the routing
The routing table can resolve the exit interface in a single search instead of 2 searches
Example of syntax require to configure a static route with an exit interface
Static Routes with Exit Interfaces
Modifying Static routes
Existing static routes cannot be modified. The old static route must be deleted by placing no in front of the ip route
Example:
no ip route 192.168.2.0 255.255.255.0 172.16.2.2
A new static route must be rewritten in the configuration
Static Routes with Exit Interfaces
Verifying the Static Route Configuration
Use the following commands
Step 1 show running-config
Step 2 verify static route has been entered correctly
Step 3 show ip route
Step 4 verify route was configured in routing table
Step 5 issue ping command to verify packets can reach destination and that Return path is working
Static Routes with Exit Interfaces
Ethernet interfaces and ARP
If a static route is configured on an Ethernet link and packet is sent to the next-hop router then…
The destination MAC address will be the address of the next hop’s Ethernet interface
This is found by the router consulting the ARP table
If an entry isn’t found then an ARP request will be sent out
Summary and Default Route
Summarizing routes reduces the size of the routing table.
Route summarization is the process of combining a number of static routes into a single static route.
Summary and Default Route
Configuring a summary route
Step 1: Delete the current static route
Step 2: Configure the summary static route
Step 3: Verify the new static route
Summary and Default Route
Default Static Route
This is a route that will match all packets. Stub routers that have a number of static routes all exiting the same interface are good candidates for a default route.
Like route summarization this will help reduce the size of the routing table.
Configuring a default static route
Similar to configuring a static route. Except that destination IP address and subnet mask are all zeros.
Example:
Router(config)#ip route 0.0.0.0 0.0.0.0 [exit-interface | ip-address ].
Summary and Default Route
Static routes and subnet masks
The routing table lookup process will use the most specific match when comparing destination IP address and subnet mask
Default static routes and subnet masks
Since the subnet mask used on a default static route is 0.0.0.0 all packets will match
Static Routes and Packet Forwarding
Packet forwarding with static routes (recall Zinin’s 3 routing principles)
Router 1
Packet arrives on R1’s Fastethernet 0/0 interface
R1 does not have a route to the destination network, 192.168.2.0/24
R1 uses the default static route
Static Routes and Packet Forwarding
Packet forwarding with static routes (recall Zinin’s 3 routing principles)
Router 2
The packet arrives on the Serial 0/0/0 interface on R2
R2 has a static route to 192.168.2.0/24 out Serial0/0/1
Static Routes and Packet Forwarding
Packet forwarding with static routes. (recall Zinin’s 3 routing principles)
Router 3
The packet arrives on the Serial0/0/1 interface on R3
R3 has a connected route to 192.168.2.0/24 out Fastethernet 0/1
Static Routes and Packet Forwarding
Troubleshooting a Missing Route
Tools that can be used to isolate routing problems include:
Ping - tests end to end connectivity
Traceroute - used to discover all of the hops (routers) along the path between 2 points
Show IP route - used to display routing table & ascertain forwarding process
Show ip interface brief - used to show status of router interfaces
Show cdp neighbors detail - used to gather configuration information about directly connected neighbors
Static Routes and Packet Forwarding
Solving a Missing Route
Finding a missing or mis-configured route requires methodically using the correct tools
Start with PING - if ping fails then use traceroute to determine where packets are failing to arrive
Issue: show ip route to examine routing table
If there is a problem with a mis-configured static route remove the static route then reconfigure the new static route
Static Routes and Packet Forwarding
Solving a Missing Route
Summary
Routers
Operate at layer 3.
Functions include best path selection & forwarding packets.
Connecting Networks
WANs
Serial cables are connected to router serial ports.
In the lab environment clock rates must be configured for DCE.
LANs
Straight through cables or cross over cables are used to connect to fastethernet port. (The type of cable used depends on what devices are being connected.)
Cisco Discovery Protocol
A layer 2 proprietary protocol.
Used to discover information about directly connected Cisco devices.
Summary
Static Routes
This is a manually configured path that specifies how the router will get to a certain point using a certain path.
Summary static routes
This is several static routes that have been condensed into a single static route.
Default route
It is the route packets use if there is no other possible match for their destination in the routing table.
Forwarding of packets when static route is used
Zinin’s 3 routing principles describe how packets are forwarded
Troubleshooting static routes may require some of the following commands:
Ping
Traceroute
Show IP route
Show ip interface brief
Show cdp neighbors detail