Routing Basics

Before you start the hands-on part of this module, you should load the appropriate configuration and verify that the testbed is up and running by executing the corresponding robot file:

In order to get a better understanding, the lab setup is shown in the picture below.

Routing is the process of determining where to send IP packets destined for IP addresses outside of the local network. A router collects and maintains routing information to enable sending and receiving of such data packets. This process is called forwarding and usually done in purpose-built hardware.

There are a couple of different routing protocols. In this tour we will cover

We have already covered the interface routes in the previous module.

All routing protocols contribute to the routing table which provides the best entries for each IP destination. The most important command to view the routing table is the show route command. There are a lot of useful options to narrow down the output of the show route command:

The routing table contains an information called preference. The preference is used as a tiebreak if the same prefix is learned from different route sources. The lowest numerical route preference wins. As direct interface routes are the most reliable one, the default preference is 0.

If you take a close look at the output of the show route command, you will find the instance name in the first line. A routing instance represents a set of interfaces, routing protocols and corresponding routing tables. You can think of a routing instance as a virtual router within RBFS.

Routing instances are configured using the set instance <instance_name> syntax, followed by the address families (AFI and SAFI) that the instance needs to support, such as for example ipv4 unicast or ipv6 labeled-unicast, respectively for normal ipv4 routing and for the transport of ipv6 traffic via MPLS.

Once an instance is defined, you can assign logical interfaces to it with the syntax set interface <interface-name> unit <unit-number> instance <instance-name>.

With static routing, the router learns routes when an administrator manually configures the static route. The administrator must manually update this static route entry whenever an internetwork topology change requires an update. Static routes are user-defined routes that specify the path that packets take when moving between a source and a destination. These administrator-defined routes allow very precise control over the routing behavior of the IP internetwork.

Static routes are commonly used when you are routing from a network to a stub network. A stub network (sometimes called a leaf node) is a network accessed by a single route. Static routes can also be useful for specifying a "gateway of last resort" to which all packets with an unknown destination address will be sent.

In general, a static route consists of two components:

You can define a nexthop-profile using the syntax set instance <instance-name> static nexthop-profile <profile-name> nexthop <address>.
The address can be either IPv4 or IPv6.
Alternatively, you can specify an egress interface using this syntax:
set instance <instance-name> static nexthop-profile <profile-name> exit-interface <interface>
This is often used for a nexthop-profile pointing to the null0 interface, which is often useful for route aggregation.

If you have completed the exercise, you can check the results by executing