Types of Interfaces
Permanent Interfaces
The permanent interfaces on a Juniper Networks platform perform two vital roles:
- Management -- interface fxp0, does not provide forwarding capabilities for transit data packets
- Operation -- interface fxp1 connects the Routing Engine to the Packet Forwarding Engine. This communications link is how routing protocol packets reach the Routing Engine to update the routing table. The forwarding table updates reach the Packet Forwarding Engine across this interface as well
Transient Interfaces
These interfaces are physically located on a Physical Interface Card (PIC) and can be inserted and removed from the router at any time. This property gives them their transient nature.
Interface Naming
Router’s:
Interfaces == located ==> PIC (Physical Interface Card) == located ==> FPC (Flexible PIC Concentrator)
Interface Naming Structure
media_type-fpc / pic / port.unitmedia_type A two-character designator that uniquely identifies the type of physical interface
fpc The physical slot in the chassis where the interface is located
pic The slot on the FPC that contains the interface
port The location on the PIC where the interface port is located
unit The logical portion of the interface that contains properties, such as an IP address
Media Types
ae Aggregated Ethernet interface
as Aggregated SONET/SDH interface
at Asynchronous Transfer Mode (ATM) interface
ds DS0 interface (including Multichannelized DS-3 interfaces)
e1 E1 interface (including Channelized STM-1 to E1 interfaces)
e3 E3 interface
es Encryption interface
fe Fast Ethernet interface
fxp Management and Internal Ethernet interfaces
ge Gigabit Ethernet interface
gr Generic Route Encapsulation tunnel interface
ip IP-over-IP encapsulation tunnel interface
lo Loopback interface
so SONET/SDH interface
t1 T1 interface (including Channelized DS-3)
t3 T3 interface (including Channelized OC-12 interfaces)
Interface Naming Examples
Suppose a router has two OC-3 PICs in slots 0 and 1 on an FPC in slot 1. Each of the PICs contains two ports. The names of these interfaces are:
so-1/0/0.0
so-1/0/1.0
so-1/1/0.0
so-1/1/1.0
A channelized OC-12 PIC contains 12 logical DS-3 channels. When installed in PIC slot 2 on FPC slot 2, the channels are represented as:
t3-2/2/0:0
t3-2/2/0:1
t3-2/2/0:2
t3-2/2/0:3
t3-2/2/0:4
t3-2/2/0:5
t3-2/2/0:6
t3-2/2/0:7
t3-2/2/0:8
t3-2/2/0:9
t3-2/2/0:10
t3-2/2/0:11
Interface Properties
An interface’s logical properties represent the Layer 3 routing and Layer 2 transmission parameters needed to operate the interface in a network.
Physical Properties
Description -- a user-defined text description is available for all interfaces. This is often used to describe the interface’s purpose.
Diagnostic characteristics -- circuit-testing capabilities, such as loopback settings or Bit Error Rate Test (BERT) tests, are user-configured on a per-physical interface basis.
Encapsulation -- options for encapsulation types vary for different media types.
Frame check sequence (FCS) -- this field is used for error-checking received packets. You can change the default value from a 16-bit field to a 32-bit mode.
Interface clock source -- point-to-point interfaces require a clocking source for synchronization purposes. Options here include internal (the default) or external.
Interface MTU size -- the maximum transmission unit (MTU) of the physical interface can be changed. Each interface has a different default value; the possible range is 256 to 9192 bytes.
Keepalives -- a keepalive is a physical-layer mechanism that is used to determine whether the interface is operating correctly. With the exception of ATM interfaces, each interface uses keepalives by default. You can disable this function.
Payload scrambling -- scrambling is a mechanism used for long-haul communications to assist in an error-free transmission. Most interfaces in the JUNOS software use a default value of payload-scrambler, but you can disable this function as well.
Logical Properties
Each and every interface within the JUNOS software requires at least one logical interface, called a unit. This is where all addressing and protocol information is configured. Some physical encapsulations allow only a single logical unit. PPP and Cisco-HDLC fall into this category. Logical interfaces, such as the loopback, and non-VLAN Ethernet also provide for only one logical unit. In both situations, the logical interface is assigned a unit value of 0.
Multiple logical interface units are often used in ATM, Frame Relay, and VLAN tagged Ethernet networks. In these cases, each logical unit is assigned a Virtual Circuit Identifier (VCI), Data-Link Connection Identifier (DLCI), or Virtual Local Area Network (VLAN) number, respectively. This system allows you to map multiple logical interfaces onto a single physical interface. The JUNOS software views each logical interface as a separate entity.
Common logical interface properties include a protocol family, logical Layer 3 addressing, MTU, and virtual circuit (Layer 2) addressing information.
Protocol Families
Each logical interface in the JUNOS software has the ability to support one or more protocol families. These families enable the logical interface to accept and process data packets for the router. Without their configuration, the interface drops any unknown transmissions.
Currently four possible protocol families are available for your use:
- inet The inet protocol family supports IP version 4 (IPv4) packets.
- inet6 To allow support for IP version 6 (IPv6) data packets, each interface can be configured with the inet6 protocol family.
- iso The Intermediate System to Intermediate System (IS-IS) routing protocol uses a data link encapsulation defined by the International Standards Organization (ISO). The iso protocol family allows the processing of these packet types.
- mpls The mpls protocol family provides support for processing packets encoded with a Multiprotocol Label Switching (MPLS) label. This label information allows the router to forward the data packet. (We discuss MPLS in greater detail in Chapter 11.)
Protocol Addresses
A protocol address is a logical Layer 3 value used to route user packets in a network. For example, an IPv4 address of 192.168.1.1 /24 is a protocol address. The inet family provides the capability to assign multiple addresses to each logical unit, with each address equally represented on the interface. In this situation, you encounter the concepts of the primary address and the preferred address.
A single primary address is assigned to each interface. By default, it is the lowest numerical IP address configured. For example, 10.10.10.1 /24 is a lower value than 172.16.1.1 /24. The primary address is used as the source address of a packet when the destination address is not local to a configured subnet.
Let’s look at an example. Cabernet has both 10.10.10.1 /24 and 172.16.1.1 /24 configured on its fe-0/0/0.0 interface. You use the ping command to form an IPv4 packet with a destination address of 192.168.100.10.
Unlike the primary address, a logical unit may have multiple preferred addresses at the same
time. The preferred address is used when an interface has two addresses configured within the same subnet. The default selection of the preferred address is similar to the primary address in that the lowest numerical prefix is selected. The use of the preferred address is also similar in that it assists the interface in selecting the source IP address of a packet.
We’ve added the 172.16.1.100 /24 address to Cabernet’s fe-0/0/0.0 interface. This time, we issue the ping command to the destination of 172.16.1.200. The outgoing subnet is known to the interface, so the primary address is not automatically used. The local address within the subnet is used instead, but in our case we have two addresses configured in the subnet. The preferred address of 172.16.1.1 is used in this case as the source IP address.
Protocol MTU
An MTU value can be configured for each logical unit in the router. The difference between the protocol MTU and the interface MTU discussed in the “Physical Properties” section earlier in this chapter is quite important. The interface MTU is the largest size packet able to be sent on the physical media. This value includes all Layer 2 overhead information, such as the destination MAC address on Ethernet, or the labels in an MPLS environment. The Cyclic Redundancy Check (CRC) information is not included in this value, however. Each encapsulation type has a payload field where higher-layer information is stored. This payload field is the size of the protocol MTU. This is the largest amount of logical protocol data, including the protocol header, able to be sent on a particular interface.
Virtual Circuit Addressing
ATM VPI and VCI. An Asynchronous Transfer Mode (ATM) network uses the concept of a virtual path and a virtual circuit to connect two devices. The path is represented by a virtual path identifier (VPI), which can be thought of as a logical conduit between the devices. Each VPI in a network may contain multiple logical circuits represented by a virtual circuit identifier (VCI), which is the actual connection between the devices.
Frame Relay DLCI
In a manner similar to ATM, a Frame Relay network uses data link connection identifiers (DLCIs) to address packets at Layer 2. The DLCI value is the logical circuit between the two devices, which is also locally significant.
Ethernet VLAN Tags
For broadcast-capable media, such as Fast Ethernet and Gigabit Ethernet, the JUNOS software supports a subset of the IEEE 802.1Q standard for channelizing an interface into multiple logical interfaces. These channels are referred to as virtual local area networks (VLANs). A VLAN allows many hosts to connect to an Ethernet switch while maintaining separate logical subnets and broadcast domains. Each Ethernet interface on a Juniper Networks router can support up to 1024 VLANs. Gigabit and some Fast Ethernet interfaces use values in the range of 0 to 4094, while the rest of the Ethernet interfaces use values between 0 and 1023. The operation of a VLAN is similar to the Layer 2 operation of ATM and Frame Relay. Two routers share a VLAN value, allowing data packets to be processed by the correct logical interface.
Disabling or Deactivating an Interface
Using the disable command at the [edit interfaces interface-name] hierarchy level allows the router to use the interface configuration. Operationally, the interface is viewed as down, or administratively disabled.
The deactivate command places an inactive tag next to the configuration in the router. As the commit command is issued, the JUNOS software completely ignores the configuration. Operationally, the interface has no configuration—as if you had never entered any commands at all.
Example deactivate:
user@Cabernet> show interfaces so-2/0/0 terse
Interface Admin Link Proto Local Remote
so-2/0/0 up up
so-2/0/0.0 up up inet 10.0.2.1/30
[edit interfaces]
user@Cabernet# deactivate so-2/0/0
[edit interfaces]
user@Cabernet# show
inactive: so-2/0/0 {
description "Connection to Riesling";
sonet-options {
fcs 32;
}
unit 0 {
family inet {
address 10.0.2.1/30;
}
}
}
[edit interfaces]
user@Cabernet# activate so-2/0/0
Example disable:
[edit interfaces]
user@Cabernet# set fxp0 disable
[edit interfaces]
user@Cabernet# show fxp0
description "This is the Ethernet management interface";
disable;
unit 0 {
family inet {
address 172.16.1.1/24;
}
}
user@Cabernet> show interfaces fxp0 terse
Interface Admin Link Proto Local Remote
fxp0 down up
fxp0.0 down down inet 172.16.1.1/24
[edit interfaces]
user@Cabernet# delete fxp0 disable
[edit interfaces]
user@Cabernet# show fxp0
description "This is the Ethernet management interface";
unit 0 {
family inet {
address 172.16.1.1/24;
}
}
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