12 votes

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

I wonder what can cause that the first line output of the command "show interfaces" will be: "fastEthernet is up, line protocol is down". Cisco ethernet interfaces are normally down / down if they ...
12 votes
Accepted

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

Your problem is that you are addressing in two separate networks: 216.21.5.0/31 and 216.21.5.2/31. The addresses you are using are in the same /30 network, but two separate /31 networks. You need to ...
  • 94.6k
11 votes
Accepted

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? If you actually stopped to think this through, clearly this can happen. All you need to ...
  • 26.8k
10 votes

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

Super simple answer... It tells the computer what range of addresses it can talk to before sending the traffic to the gateway to decide where it goes next. The traffic in that range never goes to ...
  • 918
10 votes
Accepted

How not to route on a Cisco router

While Ron and Ron's answers actually enable routing but prevent traffic, their suggestions still pack both host groups into the same routing instance. There's another way to think about this: a) ...
8 votes
Accepted

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

It depends on routing table of router2 and other devices in your network (if you have any between router1 and router2). Router1 does not influence on it, it can be do something only when packet ...
8 votes
Accepted

Switching interface VLAN ID Mid-Production

I was wondering if there was a way to switch out the VLAN ID from VLAN1 to VLAN10 without impacting the connectivity of the end users? No. First, you must remove the addressing from VLAN 1 before ...
  • 94.6k
7 votes
Accepted

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

The document to which you link claims a specific IOS version. It may be incorrect; that would not be unusual in Cisco documentation. In reality, having tested this on several IOS 15.X versions, you ...
  • 94.6k
7 votes
Accepted

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

The router uses the IP address and mask configured on an interface to determine what network is attached to that interface. That network and mask then become part of the routing table as a directly ...
  • 94.6k
7 votes

Why do routers have only a few interfaces?

You forget that routers can have virtual interfaces. For example, you can create GigbitEthernet0/0.10. You can have many, many different VLANs, each with its own network, on virtual interfaces of a ...
  • 94.6k
7 votes
Accepted

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

The details of this are highly dependent on the hardware, but your description is correct: for many years now it is normally the network interface card which discards the frames which don't match the ...
  • 15.9k
6 votes
Accepted

Difference between CRC and input errors - show interface

The "input errors" counter will increment whenever the interface receives a frame with any sort of input error, which includes CRC among other types of errors. Each frame is only counted once, no ...
  • 26.8k
6 votes
Accepted

EHWIC-1GE-SFP for Cisco 1941

You can change to the SFP interface. Under the interface configuration, configure: media-type sfp To change back to the copper interface: media-type rj45
  • 94.6k
6 votes
Accepted

What's the relationship of a Router with its loopback interface in logic?

Loopback interfaces are virtual interfaces that are considered directly connected interfaces. Your Loopback0 interface is defined with a /32 network, which, by definition, is a local route. How you ...
  • 94.6k
6 votes
Accepted

What can an IP address be assigned to?

IP addresses are assigned to interfaces, either physical or virtual. For example, for a physical interface, such as a NIC, or a virtual interface, such as a loopback or SVI. Some people think IP ...
  • 94.6k
6 votes
Accepted

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

Tim, you are trying very hard to map abstract concepts to real life hardware and software. They don't map exactly, so you always will have things that don't fit. Software is modularized for ease of ...
  • 63.5k
6 votes

Serial interfaces of router are used nowadays?

As Ricky Beam has already written, even though Serial connections might not be common where you live, that doesn't mean they don't exist. Over 90% of the connections we have in Europe use either ...
  • 1,700
5 votes
Accepted

Routing on firewall vs L3 switches

Am I missing some obvious crucial piece to getting this working as I had expected? You musn't confuse the latency of traffic destined to the switch with the latency through the switch. When you ping ...
5 votes

Difference between CRC and input errors - show interface

See this page, table 4-6: http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1904.html Description of input errors from that page: Includes runts, giants, no buffer, CRC, frame,...
  • 151
5 votes
Accepted

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

It is the address on the interface which responds back to with ICMP timeout. Which, by default, would be the interface closest to the source.
  • 94.6k
5 votes

How to know the interface utilization on a Cisco switch?

What you are looking at is the bit rate over the last five minutes. You can change the time period, but not below 30 seconds with the load-interval command. I'm not sure what you mean by "utilization....
  • 94.6k
5 votes

What is the difference between WAN and LAN ports?

A LAN is typically something completely within your own premises (your organization's campus, or building, or office suite, or your home), so it's something you build and own yourself, all the way ...
5 votes
Accepted

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

I don't think this is possible. A broadcast storm from a bridge loop will effectively bring down much/all of your network (depending on bandwidth) but doing some permanent damage to the hardware is ...
  • 72.8k
5 votes

Why it is neccesary to assign IP adresses to interfaces?

You're right: you can have unnumbered point-to-point interfaces, for the reason you gave. Have a read of this explanation of unnumbered interfaces work in Cisco routers. We normally do number ...
  • 15.9k
5 votes
Accepted

Interfaces named with `ge` represent what?

I guess you're asking about JunOS, the Operating System of Juniper for their hardware. ge means Gigabit Ethernet. This a list of ethernet interfaces and names used in JunOS: ae— Aggregated Ethernet ...
  • 4,437
5 votes

How not to route on a Cisco router

You can simply create access lists to restrict traffic, and place those access lists on the VLAN interfaces. For example an ACL to prevent traffic from 10.11.12.0/24 to 10.11.13.0/24 can be created ...
  • 94.6k
5 votes

i cant configure the ip adress and subnetmask ,i only have one router "1941" and 2 pc's

You misspelled "address". Using the tab-completion feature on most devices usually helps with spelling errors :)
  • 95
5 votes
Accepted

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

The ASA 5506 doesn't have the switchport commands like the 5505. In routed mode, each interface is on a separate subnet, like a router.
  • 63.5k

Only top scored, non community-wiki answers of a minimum length are eligible