ROUTING AND SWITCHING
Static routing
FreeSkills.com
09.10.2004
Rating: -4.65- (out of 5)
Static routes are ideal when using something called a stub network.
This is where there is only one way in and out of the network.
The disadvantage of using static routes is that if the connection to
the next hop goes down the router will continue to route traffic there.
It will never be aware of the fact that the path is no longer valid.
Also, once you have more than a handful of static routes they can
become time consuming administrative task to change and update.
FIG 5.2 Traffic Will Leave an Interface Even if the Link is Down
Configuring a Static Route
In order to configure a static route the router has to be in global
configuration mode.
ip route network prefix mask {address |
interface} [distance]
network - the destination network
mask - is the subnet mask for that network
address - IP address of the next hop router
interface - or the interface the traffic is to leave by
distance - (optional) the administrative distance of the route
There are other parameters but these have been removed as they are not
relevant to the CCNA exam.
example:
ip route 10.0.0.0 255.0.0.0 131.108.3.4 110
10.0.0.0 is the destination network. 255.0.0.0 is the subnet mask for
that network and 131.108.3.4 is the next hop for the router to use. The
110 is the administrative distance which we will look at later on.
Alternatively, we could have specified the interface the traffic is to
leave by.
ip route 192.168.1.0 255.255.255.0 serial 0
This tells the router that to get to network 192.168.1.0, leave by
interface Serial 0.
FIG 5.3 A Static Route to Get to Bs Network
When you give the router a static route to use it is vital that the
router knows how to get to that next hop. Normally the next hop would
be part of a directly connected network. Routers automatically are
aware of directly connected networks.
One important fact to remember is that the router on the other side
(destination) must have a route back to the source. If it is not aware
of the source network there will never by a response. Just like if you
don't put a return address on an envelope.
FIG 5.4 The Destination Router Must Know How to Get Back to the
Source
Default Routes
A default route is a special variety of static route. Normally, when a
router is looking for a network that is not in its routing table. It
will simply drop the packet. Remember that routers never ever send
broadcasts by default. You can change this behaviour but it is not
advisable.
On the network below, the stub network has only one way for the traffic
to go, to reach several different networks. To configure several static
routes would be a long winded way of achieving what could be done with
one command.
Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.2
The 0s indicate any network and any subnet mask. i.e. any traffic for
anywhere go via the next hop 192.168.1.2.
You could have specified an exit interface as well.
Router(config)# ip route 0.0.0.0 0.0.0.0 s0
FIG 5.5 All Traffic Will Leave the Stub Router By Serial 0
You could even have some static routes configured and then a default
route at the end. A practical use may be that you work in a small
office and any unknown traffic will be passed to a larger router at
your head office that can make all the routing decisions for you.
One last point, all of my network diagrams are there to illustrate a
point so please do not take them as examples of how to design your own
network.
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