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Questions tagged [interface]

For questions about a physical or virtual interface on a network device. For instance: Interface configurations, troubleshooting and security.

17
votes
7answers
50k views

Cisco command to show which interfaces an ACL is applied to

For Cisco routers and switches, is there a show command, or something similar, that will display what physical and logical interfaces an ACL is implemented on and what direction it is applied in? I ...
10
votes
4answers
126k views

What can cause “FastEthernet is up, line protocol is down” on a Cisco Switch?

I wonder what can cause the first line output of the command show interfaces to be: FastEthernet is up, line protocol is down. In other words, Is the cable that is connected to the local interface, ...
6
votes
3answers
1k views

How not to route on a Cisco router

So we have a router, with a switch card interface, 8 ports Say I have 4 VLANs (A,B,C,D) on it, with 2 switch ports in each. Each VLAN (i.e. all ports) can communicate with each other, because ...
6
votes
2answers
2k views

SVI as a DHCP client

Can a Cisco switch be configured to have a management interface (or SVI, or VLAN interface) become a DHCP client, therefore obtaining its own IP address on the respective network? This is out of ...
6
votes
2answers
54k views

Difference between CRC and input errors - show interface

The command "show interface fa0/1" prints the following output: What is the difference between "CRC" and "input errors"? CRC errors confirms that a frame error has occured, but what about "input ...
6
votes
4answers
3k views

Why do I get “% Warning: use /31 mask on non point-to-point interface cautiously” when I use a /31 subnet mask?

Why do I get % Warning: use /31 mask on non point-to-point interface cautiously when I use a /31 subnet mask over Ethernet, and why am I not able to ping from R1 to R2 when both the interfaces have ...
6
votes
2answers
1k views

Why is Cisco enforcing X:X:X:X::X for link-local addresses when the standard says FE80:/10?

This document shows that you can use one of these options: interface type/number ipv6 address ipv6-prefix/prefix-length eui-64 ipv6 address ipv6-prefix/prefix-length link-local <------...
6
votes
2answers
6k views

SFP / Fiber Link Issue - Can't connect a Fiber CPE

I've a CPE connected to a 4506 switch. The interface is Up but I don't see traffic CPE MAC address on MAC sddes-table I've already replaced the cable, SFP and port. Now I've the minimal config (...
6
votes
3answers
1k views

why does an interface card in cisco 1921 router not restart after cable disconnect?

I've got a cisco 1921 in a lab setting where we've added an extra interface (EHWIC-1GE-SFP-CU) and as long as a network cable is connected it works fine. However if the network cable is disconnected ...
5
votes
6answers
3k views

Why do I need to specify a subnet mask on interface?

When assigning an IP address to an interface, why do I need to specify a subnet mask? I understand the purpose of subnet masks in a routing table for example (to specify a range of IP addresses as ...
5
votes
2answers
46 views

What's the `line protocol` and `Link`?

When I show a switch interface information, there are line protocol and Link: Switch-A(config-if)# show interface gigabitethernet 0/3 Gi0/3 up, line protocol is up (connected) Bridge Port Type: ...
5
votes
2answers
915 views

How does a router know the MTU of a network?

I've done several exercises where you're given the MTU of a net or subnet, but I wonder how routers know the MTU of the nets they're connected. I've learnt that in OSPF protocol this information is ...
5
votes
2answers
166 views

Why the Cisco Router's loopback interface can not be showed up by default?

By default, why the Cisco Router's loopback interface can not be showed up? Router#show ip interface brief Interface IP-Address OK? Method Status Protocol ...
5
votes
2answers
292 views

Can a broadcast storm do physical damage to computers and networking hardware?

We had a broadcast storm that went undiagnosed for several hours. After the problem switch was unplugged and traffic returned to normal, we had a handful of machines and switches that were broken. On ...
5
votes
4answers
5k views

DLink switch loses configuration on power off!

We just bought a DGS-1210-28P smart switch, it is my first experience with a business class switch, however I found a pretty weird behavior. The switch loses all of its configurations, even management ...
5
votes
1answer
11k views

time based interface shutdown on cisco router

Is there a way to shutdown an interface on a Cisco router based on time periods? I would like to shut the cellular interface when it is not necessary. Can this be done with the event manager and ...
5
votes
4answers
7k views

Multiple IP Addresses on VLAN Interface (SVI)

Let's say that there is a SVI for VLAN 100 and it's been assigned the first usable IP address in subnet 192.168.1.0/24. There is a host on VLAN 100 with an IP address of 192.168.1.130. We need to ...
5
votes
2answers
2k views

Network Gateways vs Interfaces

When looking at the contents of a routing table section on the Wikipedia article on Routing Tables, it contains a table as an example of a routing table with two headings which have an unclear ...
4
votes
3answers
570 views

What is responsible for dropping Layer 2 frames that's not addressed to it?

It's my understanding that if a network interface card (NIC) receives a Layer 1 packet, it will extract the Layer 2 frame from it, checks its destination MAC address, and if it is not addressed to it (...
4
votes
5answers
9k views

How many IP addresses can a device or an interface possess?

Can one IP address be assigned to more then one device or interface? In which situations may this happen? Can one device or interface posses more than one IP address?
4
votes
2answers
120 views

Cisco ASA 5585X Internal-Data0/1 interface errors

I have noticed on Cisco ASA 5585 (SSP-20) interface error counter going up specially overrun but so far we haven't seen any production impact or issue, error rate is low so its not noticable but would ...
4
votes
1answer
146 views

EHWIC-1GE-SFP for Cisco 1941

I was able to configure the interface but only the GE port is working, not the SFP port. Did I miss something?
4
votes
1answer
938 views

Cisco ASA startup-config command ordering for sub-interfaces

In relation to another question on this forum, Moving interface names on a Cisco ASA while maintaining the rest of the configuration in place, I modified the startup-config on my Cisco ASA 5525-X from....
4
votes
1answer
4k views

what are IFL IFD and IFA

I came across terms like IFL IFD and IFA 's recently when studying some articles related to my project, I dont clearly understand what they are and the difference between them. I know they are related ...
4
votes
3answers
6k views

interface down time summary

bar show interface link is there an alternative (pretty) way to display a summary of link last in use. Goal is to get a view of how long a link has been unused. Something along the lines of: show ...
4
votes
1answer
826 views

limit bandwidth on a Cisco switch port,

Our company has a 3/.5 DSL connection. On this connection we have a VPN where the execs are able to access files on the network as well as VOIP. On this connection we also have our exchange email ...
4
votes
1answer
13k views

What is “usec” and “delay” a measurement of?

The term usec appears in the delay field of the show interface command on a router: Router# show interfaces fastethernet 0 Fast Ethernet0 is up, line protocol is up Hardware is DEC21140, address ...
3
votes
2answers
300 views

Do the concepts of IP address and network interface not belong to the same layer?

https://unix.stackexchange.com/a/508065/674 You are confusing the device driver layers and the IP layers of the stack. At the IP layer, 127.0.0.1 is just another IP address and treated the ...
3
votes
1answer
209 views

Which physical interface would be used to send/receive packets intended for a loopback interface?

Let us say, I have configured the managed router with a loopback addresses - Interface IP-Address OK? Method Status Protocol GigabitEthernet0/0 ...
3
votes
2answers
294 views

Inter-vlan routing with unmanaged switch?

I have a Cisco 3650 with inter-vlan routing enabled. All vlans reach each other with no problems. The problem appeared when I connected an unmanaged switch to an interface on the cisco. I set up a ...
3
votes
1answer
505 views

Does traceroute show the IP address of the entry port? or exit port?

Does traceroute show the IP address of the entry port? or exit port? For example, if there is an intermediary router in the path with 198.28.56.18 facing the source (entry port), and 65.86.24.136 ...
3
votes
1answer
174 views

OM3 optical fiber 850 nm compatibility with 10GBASE-LRM

I have Cisco equipement with "XENPAK-10GB-LRM" and Optical Fiber Type OM3 wavelength 850 nm. As the LRM interface works at 1310 nm, will it work with this type of Fiber.
3
votes
1answer
747 views

Router equipment and interface info in reverse DNS

Question about pages 17 and 18, of 49, at http://www.nanog.org/meetings/nanog45/presentations/Sunday/RAS_traceroute_N45.pdf It is mentioned that prefixes and numbers in DNS (when doing traceroute) ...
2
votes
3answers
48 views

Interfaces named with `ge` represent what?

I see a information: Most interfaces are named according to: - Interface media type (ge, so, at, and so forth) ... I know the so represent SONET, at represnet ATM, but how about the ge ? if it ...
2
votes
2answers
1k views

What are the most common scenarios in which the 'loopback interface' on a router is used?

I was studying about CISCO routers and we learned that every router has a LOOPBACK interface. How and when are those interfaces used? We never used it though in our configurations.
2
votes
3answers
592 views

Cisco C3560CX Interface number/name question

The Cisco C3560CX's come in flavors that include 10gbs SFP+ ports (e.g. WS-C3560CX-12PD-S), and also less expensive ones that only do 1gbs SFP (e.g. WS-C3560CX-12TC-S). Documentation shows you can put ...
2
votes
2answers
1k views

Why do routers have only a few interfaces?

Why do routers have only a few interfaces? Since one router interface is one subnet itself, a router given G0/0, G0/1 and G0/2 can only have three subnets. Is it sustainable? How can we go beyond ...
2
votes
1answer
3k views

Find negotiated speed on Cisco router

I am a bit lost. I'm trying to find the physical interface speed that is negotiated on a Gigabit interface on a Cisco 2900 and 3900 series ISR G2. On a switch, I am used to giving sh interfaces ...
2
votes
1answer
39 views

What can an IP address be assigned to?

What can an IP address be assigned to? A network interface, which is what I thought originally. (I seem to hear a network interface can have multiple IP addresses assigned to it, and if it is true, ...
2
votes
2answers
2k views

Aruba OS equivalent for default interface

I need to reset or default an interface on an Aruba switch. This may be the same for HP procurve switches, but I am not sure. On Cisco, the command would be default interface in global config mode. Is ...
2
votes
1answer
210 views

Urgent: how to configure different interface using same ip subnet in asa 5506?

Scenario is in old asa firewall 5505 I have switch port access vlan command in different interface. Just like below: interface Ethernet0/0 ! interface Ethernet0/1 switchport access vlan 2 ! ...
2
votes
2answers
269 views

How can two interface communicate between them with different subnet

R1 and R2 are connected with serial cable interface but both of them having different subnet and i know they will not communicate so what are other options two communicate between them without the ...
2
votes
2answers
828 views

Why use two NICs, one external facing and the other internal facing? [closed]

As the question implies, why use two NICs, one external facing and the other internal facing when one would suffice for both functions using NAT.
2
votes
2answers
459 views

Ping loopback without interface IP address

I have two routers attached directly and both of them has loopback address. The interface has no IP address, I have just connected R1 g0/0 to R5 g0/0. If I put IP address on the g0/0 interface of both ...
2
votes
2answers
290 views

Manually set combo interface for RJ45 Cisco 3560-CG Series

I have a Cisco Catalyst 3560-CG that has two combo ports with SFP and RJ45 each. Is it possible to manually set the combo port to use the RJ45 only? If so, how is it done? I am troubleshooting a ...
2
votes
1answer
16k views

How to know the interface utilization on a Cisco switch?

I would like to confirm. Is the above below information in bold sufficient to know the utilization of my interface in percentage? Switch#sho int fa0/8 FastEthernet0/8 is up, line protocol is up (...
2
votes
1answer
3k views

Routing on firewall vs L3 switches

So we have an environment with several VLANs connected to a FortiNet firewall using multiple interfaces trunked back to our L3 Dell PowerConnect 6248P switches. Currently the FortiNet is setup to do ...
2
votes
1answer
47 views

Is a network interface supposed to have no more than one MAC address?

Is a network interface supposed to have no more than one MAC address? Since a network interface is free to have more than one IP addresses, is IP address a way of multiplexing MAC address, if a ...
2
votes
2answers
42 views

Junos[MX]: How to show VPLS instance associated with given subinterface

Suppose I have a subinterface with Vlan-vpls encapsulation, like this one: admin@router> show configuration interfaces ge-2/1/1 flexible-vlan-tagging; encapsulation flexible-ethernet-services; ...
2
votes
1answer
36 views

Wireless Controller 5520 Creating a dynamic interface

I'm hoping to determine the least intrusive way to configuring my WLC. I am working through the GUI interface and have created a new dynamic interface [Controller-> Interfaces] When I went to ...