Static Routes with Exit Interfaces

Purpose of a static route

A manually configured route used when routing from a network
to a stub network

 IP route command

 To configure a static route use the following command: ip route
 Example:
-Router(config)# ip route network-address subnet-mask {ipaddress
| exit-interface }

  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

 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

 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."

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.

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

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

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

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

Ethernet interfaces and ARP.

– If a static route is configured on an Ethernet link
-If the 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