JFL
  • Member for 6 years, 6 months
  • Last seen this week
  • Auvergne, France
What is "oversubscription" in Networking?
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41 votes

Suppose you have a core switch that connect to several access switches (leaf and spine topology). If your access switches have each 48 1Gbs ports, you can potentially aggregate 48Gbs of traffic to be ...

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What is the difference between 0.0.0.0 and a loopback IP address?
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40 votes

The statement: The IP address 0.0.0.0 [...] means ‘‘this network’’ or ‘‘this host.’’ is misleading. It is not a "or" but "This host on this network." From RFC1122: { 0, 0 } ...

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Why can't devices on different VLANs, but on the same subnet, communicate?
29 votes

The whole point of Virtual LAN, is to create separate Layer 2 LANs on a single physical device. It is like building an armored and sonic-proof wall in a room to create 2 rooms. The people in each ...

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Network latencies between opposite ends of the Earth
19 votes

Google in particular uses distributed datacenters around the globe. They announce the same IP network at various places and due to the way routing protocols work, you reach the nearest one. This is ...

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What happened to IPv5?
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18 votes

The first four bits in the IP header are used to store the Protocol version (4 or 6). The version 5 was assigned to another protocol, the Internet Steam Protocol and so was not available for the ...

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5 nines, how is it built from 4 nines links
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15 votes

4 nines = 99.99 %. That means the probability that a link fail is 0.01 % or 0.0001 in terms of probability (scale 0 - 1). Assuming independence, The probability that both link fails is 0.0001 x 0....

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What purpose is the Uplink port in layman's terms and do I need it?
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13 votes

An uplink port is a port on which Transmit and Receive are reversed. They are used to connect together 2 switches with a standard straight-trough cable. (otherwise it would require a cross cable where ...

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How come SFP modules are so incompatible?
12 votes

In reality they are very compatible. I almost never use the official vendor SFPs and never had any issue (but I always use two well known third-party SFP vendors). There's two main points for vendors ...

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Encapsulation: can a higher protocol encapsulate a lower protocol?
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12 votes

Yes, encapsulation hide the details of what is encapsulated and doesn't really care about the payload nature. VxLAN is a sensible example of this, with layer2 (VLAN) being encapsulated in layer 4 (UDP)...

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How to Disable telnet on Cisco switch totaly?
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12 votes

If you actually configured line vty 04 Then you have something like: line vty 0 3 transport input telnet ssh line vty 4 transport input ssh This is because you configured only the fifth VTY ...

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What are the negative impacts of using non RFC1918 addresses on a private network?
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11 votes

If those devices will actually never access the Internet, then the devices themselves will be fine, however there's still some potential issues: what about other system that must communicate with ...

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Why is the ICMP protocol considered to run at layer 3 on the Wikipedia page?
11 votes

The original RFC for ICMP, RFC777 state that: ICMP, uses the basic support of IP as if it were a higher level protocol, however, ICMP is actually an integral part of IP, and must be implemented by ...

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Do I need to re-authenticate every time I switch a access point?
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10 votes

If your access points are independent, then yes, each time you connect to an access point, you need to authenticate. This happens automatically in the background but incurs some delay and the ...

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How to find the client who downloads a big file?
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10 votes

The easy way to do it could be: On your WLC home page you should have something like: You can reset the stat through the gear icon on the top right corner, selecting "Clear Dashboard Data" Then ...

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How will be the ARP Request (Unicast/Broadcast) if Subnet Mask is Unknown
10 votes

A device NEVER knows the subnet mask of another device. A device knows its IP address and its own subnet mask and based on this information knows in which network it resides. When host A sends a ...

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Difference between default route, partial and full routing table for a BGP session
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10 votes

A default route is the route that will be used if there's no other route that matches the destination in the router's forwarding table. A full routing table is a table which contain all the routes the ...

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What's use of Port Isolation vs traditional VLANs?
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10 votes

Port isolation -also called private VLAN (thanks @Stuggi)- is a very useful feature for switches that connect end users. In a typical network you will have many end-users computers grouped together ...

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Is rerouting possible in UDP?
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10 votes

This is not the hosts that decide which route a packet will follow, each router in the path make it's own decision. (Actually, the originating host could use the IP strict source option to force the ...

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Duplex Mismatch Detection
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9 votes

Packet Tracer is a simulator aimed at CCNA level courses. It doesn't accurately simulate real devices but only a subset of their functions. (and still those simulations aren't perfect). I would not ...

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iperf: send specific volume?
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9 votes

The -l option is for the buffer and doesn't influence the amount of data transferred. You have to specify the desired amount of data with the client-only option -n in KByte or MByte. So for 10GB, ...

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Why traceroute is still Used?
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9 votes

Traceroute doesn't give inexact information, it performs tests in a specific way and give some results based on the response it receives (or do not receive). You have to know how to interpret the ...

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Learning of MAC addresses with cut-through switching?
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9 votes

Cut-trough switching is not a standard. Actually it violates the 802.1D standard in some (minor) aspects. So each vendor implement it on its own way and there's not a single answer to this question. ...

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What range is this in IPV4
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9 votes

31.13.68.0/24 is a network in CIDR notation. It encompasses all IP addresses between 31.13.68.0 and 31.13.68.255. You can get more explanation on CIDR easily by searching on the Internet, here is the ...

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Fiber between two sites - no signal lights
8 votes

From the J9150D datasheet you linked, the max transmission distance over an OM1 fiber is 33m. Thus you need other transceivers, or to change the fiber.

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2960X Management VLAN
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8 votes

You can disregard the dedicated management interface and simply assign an IP address to the VLAN 500 interface on the switch and use this IP for management purpose, with appropriate access lists. FE0 ...

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Two computers connected by straight through Ethernet cable, communicating with APIPA addresses
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8 votes

Nowodays, many ethernet interfaces are 'auto MDI/MDIX'. They detect the type of device connected and if needed reverse internally Transmit and Receive. This allow connection of 2 devices of the same ...

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How much RAM you actually need to keep whole global BGP routing table?
Accepted answer
7 votes

Here is the info given but a production router that does IPv4 BGP only, as of 2021 October 5: 1547576 RIB nodes, using 142 MiB of memory 1687609 BGP routes, using 103 MiB of memory 8 Static routes, ...

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How do I know the switch is layer 2 or layer 3?
7 votes

In addition to @Zac67 answer, in the Catalyst product line, layer 3 capabilities are often unlocked by licenses. So a given switch can be layer 3 capable but, depending on the installed license, may ...

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How can twinaxial cables be faster than CAT cables
7 votes

Simply put, twinaxial cables allow only very short distances, under 10 meters. And they are used in networking, for example both 40Gb and 100Gb standards support twinaxial cables... ...up to 7 ...

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Is IP an abstraction
7 votes

Is your home postal address an abstraction? When someone send you a paper mail, they put your address on the envelope, then put the envelope in the mailbox (the first "router"). Then this mail goes ...

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