Static Routing

If R2 receives packets with the destination address different than the neighborhed networks and routers, it will drop it, because it the routing table does not contain a route to the specific network.

Default Gateway

• End hosts like PC1 and PC4 can send packets directly to destinations in their connected network

  • To send packets to destinations outside of their local network, they must send the packets to their default gateway.

    • It is a route to 0.0.0.0/0 = all netmask bits set to 0. Includes all addresses from 0.0.0.0 to 255.255.255.255.

  • You can express the default gateway as default router.

• End hosts have no need for any more specific routes.

  • They just need to know: to send packets outside of my local network, I should send them to my default gateway.

Static Routes

• When R1 receives the frame from PC1, it will de-encapsulate it (remove L2 header/trailer) and look at the inside packet.

• It will check the routing table for the most-specific matching route.

  • R1 has no matching routes in its routing table.

    • Which means, it will drop the packet.

• To properly forward the packet, R1 needs a route to the destination network (192.168.4.0/24)

  • Routes are instructions: TO send a packet to destinations in the network 192.168.4.0/24, forward the packet to next hop Y.

    • There are two possible paths packets from PC1 to PC4 can take.

      • 1) PC1 - R1 - R3 - R4 - PC4

      • 2) PC1 - R1 - R2 - R4 - PC4

    • It is possible to configure the routers to:

      • Load-balance between path 1) and 2)

      • Use path 1) as the main patch and path 2) as a backup path

• Each router in the path needs two routes: a route to 192.168.1.0/24 and a route to 192.168.4.0/24

  • This ensures two-way reachability (PC1 can send packets to PC4, PC4 can send packets to PC1)

R1 already has a Connected route to 192.168.1.0/24. R4 already has a Connected route to 192.168.4.0/24.

• The other routes must be manually configured (using Static routes)

Static Route Configuration

• The command for adding a static route is:

R1(config)# ip route ip-address net-mask next-hop

Static Route Configuration with exit-interface

Instead of configuring with a next-hop, you can configure it with an exit-interface.

For example, if R2 wants to send a packet to the 192.168.1.0/24 network, the exit-interface is G0/0, because that is the one connected to the G0/1 of the destination network.

You can set up both the next-hop and exit-interface.

R2(config)# ip route ip-address netmask exit-interface
R2(config)# ip route ip-address netmask exit-interface next-hop

When you configure the static route with the exit-interface option, it will be described as a directly connected route, but it is a static route.

• Static routes in which you specify only the exit-interface rely on a feature called Proxy ARP to function.

• This is usually not a problem, but generally you can stick to next-hop or exit-interface next-hop

Default Route

• A default route is a route to 0.0.0.0/0.

  • Is the least specific lest route possible; it includes every possible destination IP address.

  • If the router does not have any more specific routes that match a packet's destination IP address, the router will forward the packet using the default route.

  • Is often used to direct traffic to the Internet.

• Gateway of last resort is not set

  • Which means that no default reoute has been configured yet.

In order to configure a default router, one has to type the command:

R1(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.2