Static routing
Static routing
Layer3 overview
Note: The colored boxes represent a LAN whereas 4 routers connected together represents a WAN (Wide Area Network - one that extends over a large geographical area).
PC1 wants to send a packet to PC4.
The packet has source (PC1) and destination (PC4) IP address.
PC1 asks itself if the destination IP address is in the same network.
NO, becuase both are in different networks (Class C address). So PC1 can not forward the packet directly to PC4.
Since the destination is in a different network, a different LAN, routing logic tells PC1 to send the packet to the 'default gateway'.
The default gateway is the device to which the host will forward data that is destined for another network.
To reach other networks, the end host, PC1, will send the packet to its network's router, R1, the default gateway.
After receiving the packet, R1 will compare the packet's destination IP address to the routing table.
Each router keeps a routing table, which stores a list of destinations and how to reach the destination.
Assuming R1 has an entry in its routing table for the 192.168.4.0/24 network, the entry will look like : 192.168.4.0/24 via 192.168.12.2, Gi0/0 ('destination' via 'nexthop', 'interface'). Consider we configured R1 to send it via R2.
Now the packet has reached R2 and it follows the same process to forward to the packet to R4.
In R4, the entry in the routing table will look like : 192.168.4.0/24 is directly connected, Gi0/2.
In-depth process on how a packet travels with Layer3 and Layer2 steps - Life of a Packet
Packet journey
Routing table
show ip route
Done in Privileged exec mode.
PC1#show ip route -> displays the routing table.
Below the codes, 2 routes are displayed (ignore the first line without code - it is a classful address).
C and L to the left of the 2 routes display the type of route.
Connected route (C) - the network address the interface is connected to. Local route (L) - the actual IP address on the interface (with a /32 mask - used to identify a specific address).
If you configure an IP address on an interface and enable the interface, the connected and local routes of that interface are added AUTOMATICALLY.
But the host (PC1) does not have a route to the destination, which is not a problem. An end host like PC does not need to know the route to every destination. All it needs is a default gateway to which it can send any traffic destined for a location outside the local network.
On a Cisco router, the default gateway is called 'gateway of last resort', which is not set yet.
Default route
A default route is a route that matches ALL possible destinations.
To configure the 'gateway of last resort' on a Cisco router, you must configure a default route.
It is used only if a more specific route match isn't found in the routing table.
When a router looks up a route to a destination in its routing table, it looks for the most specific match and chooses that route. ^mostspeificroute
The default route is the LEAST specific possible route.
It uses an IP address of 0.0.0.0 with a mask of 0.0.0.0
To set the default route, configure a route to 0.0.0.0/0 (the /0 means that there is no network portion of the address and all the octets are 'not fixed'. covers the range of 0.0.0.0 through 255.255.255.255 - all possible addresses).
Configuring a static route
A static route is a route you manually configure yourself.
Done in Global-config mode
ip route destination-address mask next-hop
To set PC1's default route to R1 -> PC1(config)#ip route 0.0.0.0 0.0.0.0 192.168.1.254.
Now check the routing table (PC1(config)#do show ip route) -> 'Gateway of last resort is 192.168.1.254 to network 0.0.0.0'.
The default route also appears with the other routes at the bottom with a code 'S*' at the left (S - static, * - candidate default).
Since PC1 now has a default gateway configured, which is R1, it sends the packet there.
Configuring R1
R1#show ip route
It shows the routing table with the connections for 3 networks and its connected and local routes.
But the destination network of the packet is not in the routing table.
Switches flood frames with unknown destinations (destinations not in the MAC address table). Routers drop packets with unknown destinations.
Unless you configure a route on R1, the ping (packet which is sent) will not succeed.
ip route destination-address mask exit-interface
Instead of specifying the IP address as the next hop, you can just specify the interface out of which R1 should send the packet -> R1(config)#ip route 192.168.4.0 255.255.255.0 g0/0
We don't need a route to the exact destination. Since the destination address '...4.1' is included in the '...4.0/24' range, it matches.
'192.168.4.0/24 is directly connected, Gi0/0' is added to the routing table with the code 'S'.
This static route is not added as the 'gateway of last resort', because it is for a specific route, not for 0.0.0.0 which matches all networks.
Now the packet reaches R2.
Configuring R2
R2(config)#ip route 192.168.4.0 255.255.255.0 192.168.24.4
You can specify the interface the packet gets out or the next-hop IP address (R4 here).
'192.168.4.0/24 [1/0] via 192.168.24.4' is added to the routing table.
Then R2 looks for the closest match for the 'next-hop address' (R4's address) we specified.
R4 is directly connected to R2. So it will be shown in the routing table of R2.
So R2 will forward the packet out of the interface which is connected to R4.
Routing table on R4
Since R4's g0/2 interface has an IP address of 192.168.4.254 configured, the 192.168.4.0/24 network has automatically been added to the routing table as a connected route.
So R4 sends the packet out of its g0/2 interface and SW4 forwards it to PC4.
Two-way reachability
Now use the command 'ping 192.168.4.1' on PC1 to send a ping to PC4 -> the ping fails.
The packet from PC1 was able to reach PC4, as R1, R2, R4 have routes to the destination network (192.168.4.0/24).
However, to send a reply back, PC4 sends the packet with the destination address as '192.168.1.1'. R4 and R2 have no route to reach the destination network.
Configuring
Configure the default route on PC4.
Configure on R4 the static route to 192.168.1.0/24 network with the next-hop address as R2's IP address.
Configure on R2 the static route to 192.168.1.0/24 network with the next-hop address as R1's IP address.
Note
When a router looks up a destination address in its routing table, it looks for the most specific matching route.
Most specific = longest prefix length (/32 > /24 > /16 > /8 > /0)
Example 1
A ping is issued on R1 -> ping 192.168.4.1
Before finding the MOST SPECIFIC route, find the matching routes to the destination address from the routing table.
192.168.4.1 is an exact match, so it is also the most specific match.
192.168.4.0/24 also matches. Although the last octet is different, it isn't covered by the mask, so it doesn't matter.
192.0.0.0/8 also matches. /8 means only the first octet is covered by the mask, the other 3 can vary and they will still match.
But 192.168.4.1 is the MOST SPECIFIC MATCH because it has the exact address and it also has the longest prefix length.
Example 2
Here the most specific match that the router will choose is 192.168.4.0/24, because it has the longer prefix length.
Example 3
Quiz
The IP address configured on a router interface will appear in the routing table as what kind of route?
Local (since they use a /32 mask, it specifies the exact address configured on the interface).
The command to manually configure a static route
ip route destination-address mask next-hop|exit interface.
Lab
Filter
R1#show running-config | include ip route -> shows only the iproute part of the running config
Delete the configuration statement
R1#show running-config | include ip route -> shows the configuration statement -> copy it.
Goto global config mode. Paste the statement and type 'no' at the start (deletes the configuration statement).
If you do not delete the wrong route when adding a correct one, both will be kept in the routing table and the router will load balance over the 2 routes -> sending some packets through both the interfaces. Sometimes load balancing is a good thing. But when a route is completely wrong, you should remove it.
However when configuring IP address, when you enter a new one, it will overwrite the old one, unlike IP routes.
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