Dealing with Nonconfigurable Interfaces

Problem

You see interfaces in the show interfaces command output that you didn't configure, and you want to know what to do with them.

Solution

The JUNOS software internally generates a number of interfaces that you cannot configure:

	aviva@router1> show interfaces terse
	Interface Admin Link Proto Local Remote
	dsc up up
	fxp0 up up
	fxp0.0 up up inet 192.168.71.246/21
	fxp1 up up
	fxp1.0 up up inet 10.0.0.4/8
	 tnp 4
	gre up
	ipip up
	lo0 up up
	lo0.0 up up inet 127.0.0.1 --> 0/0.
	lo0.16385 up up inet
	 inet6 fe80::2a0:a5ff:fe12:3ed5
	lsi up up
	mtun up up
	pimd up up
	pime up up
	tap up up

 

Description

Toward the end of the show interfaces output, you see a number of interfaces that don't correspond to any of the physical interfaces you have configured on the router. Most of these special interfaces are internal interfaces that are created and used by the JUNOS software to route traffic within the router. The fxp0 (the out-of-band management) and loopback (lo0) interfaces, which are configurable, provide connection to the Routing Engine.

The fxp1 interface is an internal Ethernet interface that connects the Packet Forwarding Engine (all the chassis hardware components) to the Routing Engine, which handles all routing protocol operations. The output in this recipe shows that fxp1 has an internal IPv4 address of 10.0.0.5/8 and a Trivial Network Protocol ( TNP) address of 5. JUNOS software uses the proprietary TNP internally to communicate between the forwarding and Routing Engines.

As a side note, the fxp0 and fxp1 interfaces take their names from the FreeBSD fxp Ethernet device driver.

On J-series routers, the Packet Forwarding Engine and Routing Engine functionality is on the same chip, so there is no fxp1 interface or its equivalent. Also, because the fe-0/0/0 serves the function of the fxp0 interface, it isn't listed at the end of the show interfaces terse command output with the nonconfigurable interfaces:

	aviva@RouterA> show interfaces terse
	Interface Admin Link Proto Local Remote
	fe-0/0/0 up up
	fe-0/0/0.0 up up inet 10.0.16.1/24
	 172.19.121.142/24
	gr-0/0/0 up up
	ip-0/0/0 up up
	ls-0/0/0 up up
	lt-0/0/0 up up
	mt-0/0/0 up up
	pd-0/0/0 up up
	pe-0/0/0 up up
	sp-0/0/0 up up
	sp-0/0/0.16383 up up inet
	fe-0/0/1 up up
	fe-0/0/1.0 up up inet 10.0.15.2/24
	se-0/0/2 up down
	se-0/0/3 up up
	se-0/0/3.0 up up inet 10.0.21.1/24
	dsc up up
	gre up up
	ipip up up
	lo0 up up
	lo0.0 up up inet 192.168.42.1 --> 0/0
	lo0.16385 up up inet 10.0.0.1 --> 0/0
	 10.0.0.16 --> 0/0
	lsi up up
	mtun up up
	 
pimd up up
	 
pime up up
	pp0 up up
	tap up up

dsc is a virtual interface that is used to discard packets, which you might want to do if the router is experiencing a denial-of-service (DoS) attack. You can configure one discard interface. If you associate an output filter with the interface, you can log or count DoS traffic before discarding it to determine the source of the attack.

The remaining interfaces are created by the kernel and are used internally by the JUNOS software. Some have corresponding configurable interfaces for setting up tunnels on Tunnel Services PICs. You set up unicast tunnels with the gr- or ip-physical interfaces, in place of the nonconfigurable gre and ipip interfaces, to use generic route encapsulation or IP-IP encapsulation, respectively. PIM tunnels, used by PIM sparse mode, have two interfaces, pe- and pd-, for encapsulating and deencapsulating PIM Register messages. When the router has a Tunnel Services PIC installed, the software automatically configures one multicast tunnel interface, mt- (corresponding to mtun), for each virtual private network (VPN) you configure.

The lo0.16385 interface is created by the Internet Routing process ( irsd) and is used for internal routing. The tap interface is used to copy discarded packets. lsi is a label-switched interface that is used by MPLS label-switched paths (LSPs).