Interfaces - A Deeper Dive

Question 1

Describe a ‘loopback’ interface.

Answer 1

1. A virtual interface that is always up as long as the device and one interface is up
   a. You configure an IP address on it, advertise it via a routing protocol
   b. Some protocols can source traffic from the loopback
   c. If you apply a fiewall filter to your loopback interface, you can protect your control plane
   d. Many protocols derive their ‘router iD’ from the IPv4 loopback address

Question 2

There are 3 ways to set a router ID, which are?

Answer 2

1. Manually
2. Derive it from the lowest numbered IPv4 loopback address
3. Derive it from the lowest numbered IPv$ interface
   a. For some protocols, adding a loopback IP addres can restart your adjacencies

Question 3

‘IRB’ is the acronym for?

Answer 3

1, Integrated routing and bridging
a. A virtual layer 3 interface on a switch that is tied to a vlan. On Cisco, referred to an ‘SVI’
b. They can act as a default gateway for hosts in a particular vlan
c. IRBs are not guaranteed to be up. If all the interfaces in a vlan are down, then the IRB tied to that vlan will be down as well

Question 4

What is the command to bind a vlan to an IRB?

Answer 4

1. config-mode - set vlans (VLAN name) l3-interface irb.(logical unit)

Question 5

The ‘show vlans’ command displays the IRB interfaces, True or False?

Answer 5

1. False

Question 6

What command displays the layer 3 IRB interface attached to a vlan?

Answer 6

1. op-mode - show vlans detail or show vlans (vlan name) detail

Question 7

By default, when you ping a device, the IP address of the outgoing interface is the source IP address, True or False?

Answer 7

1. True
   a. You can this by adding the ‘source’ option to a ping command

Question 8

Describe the ‘Is-Preferred’ and ‘Is-Primary’ when an interface has multiple IP address?

Answer 8

1. Junos takes an IP address from a subnet and makes it the preferred address.
   a. Is-Preferred = the source to devices on the same subnet. By default, it is the lowest IP address in a subnet
   b. Is-Primary=the source to devices on a different subnet. By default, the winner is the lowest numbered IP address

Question 9

When you have multiple IP address assigned to an interface, you can override the default behavior by

Answer 9

1. Use ‘preferred’ or ‘primary’ keywoards when configuring an interface
   a. This is helpful on loopback interfaces which is common to have multiple IP address

Question 10

Best route selecton

Answer 10

1. More specific prefixes always winb against less specific prefixes
2. Route preference is used for two more routes for the same prefix from different protocols

Question 11

If you have two or more routes to a destination, and the preferences and prefixes are the same, how does Junos choose a particular path/route

Answer 11

1. Randomness
   a. This happens on a ‘per-prefix’ basis

Question 12

‘ECMP’ is the acronym for?

Answer 12

1. Equal-Cost Multipath
   a. Load balancing across multiple links can be done in a few ways
   I. Some options offer precise load-balancing
   II. Other options map a traffic flow to a link, at the expense of imprecise load-balancing
   b. To configure ECMP, you create a ‘routing policy’ and apply it to the forwarding table

Question 13

What are two types of load-balancing?

Answer 13

1. per-flow
2. per-packet

Question 14

What are the three steps to create a Link Aggregation Group?

Answer 14

1. Enable the ability to create ‘aggregated Ethernet’ (ae) interfaces
   a. Configure the quantity of logical interfaces that you need
   b. config-mode - set chassis aggregated devices ethernet device-count (n)
   c. Remember that interfaces start from the numerical 0 - ex. device-count 1 creates interface ae0, device-count 2 creates ae0 and ae1
2. Configure the (ae) interface itself
   a. It’s just like configuring any other interface. But with one extra protocol
   b. ex. config-mode - set interfaces ae0 unit 0 family inet address 10.1.2.1/24
   c. Optional, but recommended - enable Link aggregation protocol - ex. set interfaces ae0 aggregated-ether-options lacp active
   d. LACP sends packets to check that all links in the bundle are up
3. Add your links into the bundle
   a. Just one line of configuraition is needed to add an interface into a bundle
   b. ex. set interfaces ge-0/0/0 gigether-options 802.3ad ae0
   c. ex. set interfaces ge-0/0/1 gigether-options 802.3ad ae0
   d. Traffic is load-balanced in a ‘per-flow’ fashion

Question 15

What command could you use to verify the aggregated interfaces?

Answer 15

1. op-mode - show interfaces terse | match ae0