In due process of rectifying my Juniper certifications, I’ve come to review the following JunOS concepts:
- Preferred / Primary addresses
- Default addresses (nothing to do with default route!)
- Primary Interface
Having read Juniper’s explanation on the topic, this time, I found myself asking a few more questions which to me, had no clear answer.
I think that looking at each concept separately, it does make sense. However, things got a little bit blurry in my initial understanding on, how the default interface get its IP address. And since we are talking about interfaces and IP addresses, how do all these concepts actually come together?
In my oppinion, I think Juniper’s article falls short explaining this.
The Primary Interface
Whenever the router has to send locally sourced traffic out, it will have to determine which interface to use. There are occasions when the outgoing interface cannot be matched to one particular interface:
- Sending a directed broadcast ping to 255.255.255.255 or;
- some multicast scenarios
The Primary & Preferred Addresses
As you know, in order to enable IP communication, an interface needs an IP address. Regardless the number of IPs assigned to one single interface, JunOS will always pick a primary and a preferred interface; they could, of course, match on the same IP.
- Primary Address will be used for locally sourced multicast and broadcast traffic;
- Preferred Address will be used for locally sourced, unicast traffic
The Default Address
Lastly, we also have the concept of default address. This is the address used by default to source traffic out un-numbered interfaces. Note that it is also possible to explicitly specify the address to use when configuring the un-numbered interface.