6.8 KiB
id, aliases, tags
| id | aliases | tags | |||
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| 1777544578-NSAK |
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Static Routing
Network diagram
Default Gateway
End hists like PC1 and PC4 can send packets directly to destinations 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 their local network, they must send the packets to their default gateway.
iface eth0 inet static
address 192.168.1.10/24
gateway 192.168.1.1
iface eth0 inet static
address 192.168.4.10/24
gateway 192.168.4.4
The default gateway configuration is also called a default route.
-
It is a route to 0.0.0.0/0 = all netmask bits set to 0. Includes all addresses 0.0.0.0 -> 255.255.255.255
The default route is the least specific route possible, because it includes all [[IP addresses]]. 0.0.0.0 = 4,294,967,296 IP addresses A /32 route (ie. Local route) is the most specific route possible, because it specifies only one IP address 192.168.1.1/32 = 1 IP address
End hosts usually 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
- Src. IP: 192.168.1.10
- Dst. IP: 192.168.4.10
- Dst. MAC = R1 G0/2 MAC
- Src. MAC = PC1 eth0 MAC
- to learn R1 G0/2's MAC address, PC1 will first send an ARP request to 192.168.1.1
-
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 irs routing table.
- 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 network 192.168.4.0/24, forward the packet to next hop Y
-
There are two possible path packets from PC1 to PC4 can take:
- PC1 -> R1 -> R3 -> R4 -> PC4
- PC1 -> R1 -> R2 -> R4 -> PC4
- (it will be learn later)
Static Route configuration
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).
routers don't need routes to all networks in the path to the destination. - R1 doesn't need a route to 192.168.34.0/24. - R4 doesn't need a route to 192.168.13.0/24. -
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 Chart
| Router | Destination | Next-Hop |
|---|---|---|
| R1 | 192.168.1.0/24 | Connected |
| R1 | 192.168.4.0/24 | 192.168.13.3 |
| R3 | 192.168.1.0/24 | 192.168.13.1 |
| R3 | 192.168.4.0/24 | 192.168.34.4 |
| R4 | 192.168.1.0/24 | 192.168.34.3 |
| R4 | 192.168.4.0/24 | Connected |
R1 Configuration
For configuring Static route the command is:
R1(config)# ip route ip-address netmask next-hop
so for R1:
R1(config)# ip route 192.168.4.0 255.255.255.0 192.168.13.3
To check the routes
R1(config)# do show ip route
Codes: L - local, C - connected, S - static ....
//partial return
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.1.0/24 is directly connected, GigabitEthernet0/2
L 192.168.1.1/32 is directly connected, GigabitEthernet0/2
S 192.168.4.0/24 [1/0] via 192.168.13.3
The [1/0] displayed in static routes means:
[adminstrative Distance/Metric]
We will cover these concepts later in the course.
so for R3:
R3(config)# ip route 192.168.1.0 255.255.255.0 192.168.13.1
// to send packets to 192.168.1.0/ 24 send packets to R1
R3(config)# ip route 192.168.4.0 255.255.255.0 192.168.34.4
// to send packets to 192.168.4.0/ 24 send packets to R4
so for R4:
R4(config)# ip route 192.168.1.0 255.255.255.0 192.168.34.3
Test connection
Now to see if PC1 and PC4 can communicate
PC1:$ ping 192.168.4.10
5 packets transmitted, 5 packets received, 0% packet loss
If the ping is successful, that means there is two-way reachability. PC1 can reach PC4, and PC4 can reach PC1
Static Route Configuration with exit-interface
instead of configuring a next hop we can configure an exit interface instead of the ip address we can specify the interface
We will do this for R2:
R2(config)# ip route ip-address netmask exit-interface
R2(config)# ip route 192.168.1.0 255.255.255.0 g0/0
but we can do both exit-interface and next-hop
R2(config)# ip route ip-address netmask exit-interface next-hop
R2(config)# ip route 192.168.4.0 255.255.255.0 g0/1 192.168.24.4
so it returns
R2(config)# do show ip route
// exit interface
S 192.168.1.0/24 is directly connected, GigabitEthernet0/0
// exit interface and next-hop
S 192.168.4.0/24 [1/0] via 192.168.24.4, GigabitEthernet0/1
- 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
- Neither is 'better' than the other: use which you prefer.
Default route
-
A default route is a route to 0.0.0.0/0
- 0.0.0.0/0 is the least specific route possible; it 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.
-
A default route is often used to direct traffic to the Internet.
-
Example for a internal corporate network.:
- More specific routes are used for destinations in the internal corporate network.
- Traffic to destinations outside of the internal network is sent to the internet.
configure default route
R1# show ip route
Gateway of last resort is not set
No default route has been configured yet To configure a default route use this command
R1(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.2
//check
R1(config)# do show ip route
* - candidate default
S* 0.0.0.0/0 [1/0] via 203.0.113.2
Review
- Connected and Local routes Routing Fundamentals
- Intro to Static Routes
- Static Route configuration
R2(config)# ip route ip-address netmask next-hop
R2(config)# ip route ip-address netmask exit-interface
R2(config)# ip route ip-address netmask exit-interface next-hop
- Default Routes