Static Routing - 11(2)

(Updated: )
IT

Overview

  • Connected and Local Routes
  • Static Routes
  • Static Route configurations
  • Default routes

Routing Packets: Default Gateway

End hosts like PC1 and PC4 can send packets directly to destinatiosn in their connected network.

  • PC1 is connected to 192.168.1.0/24
  • PC4 is connected to 192.168.4.0/24

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

Linux stores this data in a text file.

  • The default gateway configuration is also called the default route.

    • It is a route to 0.0.0.0/0 = all netmask bits are set to 0
      • Includes all addresses from 0.0.0.0 to 255.255.255255.
    • The default route is the least specific route possible (whereas a /32 route is the most specific route possible.)

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

    • End hosts just need to know that in order to send a packet outside of a local network, it should be send to the default gateway.

  • Say PC1 wants to send a packet to PC4 which resides outside of the LAN.

    • Layer 3:
      • Source IP: 192.168.1.10
      • Destination IP: 192.168.4.10
    • Layer 2: to learn R1 G0/2’s MAC address, PC1 will send an ARP request ot 192.168.1.1
      • Source MAC: R1 G0/2
      • Destination MAC: PC1 eth0 MAC

  • Once R1 receives the packet, it will de-encapsulate the L2 header/trailer, which exposes the L3 header.

  • Then it checks the routing table for the most-specific matching route.
    Assume this is R1’s routing table.

  • There is no matching IP address for “Destination IP: 192.168.4.10”.

  • The packet is dropped.

  • There are two possible paths from R1 to 192.168.4.0/24.

  • Let’s consider the path PC1 -> R1 -> R3 -> R4 -> PC4.

  • Each router, therefore, needs two routes. A route to 192.168.1.10 and 192.168.4.10, ensuring two-way reachability.

  • If we initially configured the router’s interfaces, Cisco routers would have already configured connected routes from R1 to 192.168.1.0/24. And R4 with a connected route of 1982.168.4.0/24.

  • Other routes need to be configured manually.

Static Route Configuration: (config)# ip route ip address netmask next-hop

Static Route Configuration R1

  • The connected route is configured. So only need to one configuration.
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    2
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    R1(config)#
    R1(config)# ip route 192.168.4.0 255.255.255.0 192.168.13.3
    R1(config)# do show ip route

Static Route Configuration R3

  • Two configurations are needed. 
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    2
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    R3(config)# ip route 192.168.1.0 255.255.255.0 192.168.13.1
    R3(config)# ip route 192.168.4.0 255.255.255.0 192.168.34.4
    R3(config)# do show ip route

Static Route Configuration R4

  • The connected route is configured. So only need to one configuration.
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    R4(config)# ip route 192.168.1.0 255.255.255.0 192.168.34.3
    R4(config)# do show ip route

Packet Traveling from PC1 to PC4

Static Route configuration with “exit-interface”

R2 wants to sent a packet to 192.168.1.0/24.
Specifies the exit interface

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R2(config)# ip route 192.168.1.0 255.255.255.0 g0/0
  • Keep in mind that the routing table will state “192.168.1.0/24 is directly connected, GigbitEthernet0/0” even though it’s not”. – just a byproduct of an exit-interface that relies on Proxy ARP.

Static Route configuration with “exit-interface” and next-hop

R2 wants to sent a packet to 192.168.4.0/24 network.
Specifies the exit interface and next-hop

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R2(config)# ip route 192.168.1.0 255.255.255.0 g0/1 192.168.24.4

Default Route

  • A default route is 0.0.0.0/0
    • Least specific route and includes every possible destination IP address.
  • If the router doesn’t have any more specific routes that match a packet’s destination IP address, the router will forward the packet using the default route.
    • So a default route is used to direct traffic to the internet.
    • More specific routes are used for destinations inside internal networks
    • Traffic destinations outside of the internal network is sent to the internet.

Configuring a default route.

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R1(config)# ip route 0.0.0.0 0.0.0.0 203.0.133.2

Questions