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25

Not sure how much you know about switching and spanning tree but basically when starting out all switches claim that they are the root. All switches send BPDUs (Bridge Protocol Data Unit) which contain a priority and the BID (Bridge ID). The BID is 8 bytes long. 6 bytes is used for the MAC address of the bridge. 12 bits is used to indicate the VLAN, this ...


20

No performance gain will exist without everyone using larger packets. The point of jumbo frames is to pack more payload with the same overhead. The NAS's ability to send larger packets is meaningless if the clients don't as well. There will be no "fragmentation" at all. Layer-2 (ethernet) has no means if indicating "fragmentation needed". This is figured ...


20

ARP is used by a host on a LAN to resolve a layer-3 address to a layer-2 address so that a frame can be built for the LAN. A router is just another host on a LAN, and it will need to resolve layer-3 addresses to layer-2 addresses, the same way a PC on a LAN does.


20

Only one device is allowed to transmit at any given time. At any other given time, another device is allowed to transmit. How can you have a conversation at a dinner table if only person can speak at any time?


18

Some LAN protocols, on some media, are half duplex. That means that only one host on a LAN can send a frame at any given time. The classic example of this is the original ethernet, but the modern example is Wi-Fi. The original ethernet ran on coax, and it used CSMA/CD (Carrier Sense Multiple Access with Collision Detection) to detect collisions where two ...


17

Modulation and symbols the number of occurrences of a repeating event per unit time. So what is repeating in the wire per unit time? The voltage patterns on the wire repeat. In extremely simple communication systems, you might cycle the line's DC voltage above or below a threshold, as shown in your ASCII-art... __|‾‾|__|‾‾|__|‾‾|__|‾‾. Suppose your ...


17

Yes, from the packet switching point-of-view, VXLAN is just a matter of sticking some encapsulation on top of an L2 frame: something that other protocols do as well. The real difference it makes is at the control and management layer. VXLAN evolved as a Data Center technology, so the ability to span a WAN is just an additional advantage, not the thing that ...


13

You're asking about STP theory: https://en.wikipedia.org/wiki/Spanning_Tree_Protocol But the basic idea is that each switch has a Bridge ID which is a combination of its priority and its MAC address. By default, all switches use 32768 as their priority so by default the switch with the lowest MAC will act as the Root Bridge. You can manipulate this by ...


13

Mike offered an excellent answer but not exactly to what you were asking. Bandwidth, by definition, is a range of frequencies, measured in Hz. As you've said, the signal __|‾‾|__|‾‾|__|‾‾|__|‾‾ can be broken down (using Fourier) into a bunch of frequencies. Let's say that we've broken it down, and saw that our signal is (mostly) made up of frequencies 1Mhz,...


13

Generally, a giant frame is a frame that is too large for the receiving interface. As a malformed frame it is dropped. A jumbo frame is a frame that is larger than the standard allows (1518 bytes for Ethernet w/o tags, or 1500 bytes L3 payload (= L3 PDU = L2 SDU) plus L2 overhead). It may still be acceptable, depending on the interface configuration. For ...


12

You really, really do not want to disable STP where you connect switches to other switches. That is the entire purpose of STP. If you disable STP, and there is a problem, it will really be too late because your entire network could crash when you notice it, and recovering from a broadcast storm is no fun at all. By the way, portfast doesn't actually disable ...


12

You need to understand the concept of layers. An application will send data to the Transport Layer. The Transport Layer protocol will encapsulate the data inside headers for the Transport Layer protocol, and pass those to the Network Layer. The Network Layer will encapsulate the datagrams it receives inside Network Layer headers, and those are called ...


11

When you create 'interface vlan x' you are creating a brand new logical interface that happens to be automagically associated with the corresponding L2 vlan. When you put an IP address on 'interface vlan x' then you are simply giving that logical interface an IP that is on the same broadcast domain (or subnet or vlan or L2 domain or whatever terminology you ...


11

The forwarding information base (FIB) is the actual information that a routing/switching device uses to choose the interface that a given packet will use for egress. For example, the FIB might be programmed such that a packet bound to a destination in 192.168.1.0/24 should be sent out of physical port ethernet1/2. There may actually be multiple FIB's on a ...


11

Why then does the client need to encrypt the broadcast using the GTK? It doesn't. Since the AP broadcasts, not the client, the client doesn't use the GTK to encrypt the frame. The AP does. Why can't the client just encrypt the broadcast frame using its PTK, and the AP decrypt it, ... Exactly. This is what happens. ... the AP decrypt it, then encrypt ...


10

As soon as I offer a bounty I find the answer, typical! The magic commaned I needed is show mac-address-table bridge-domain 20 ME3600#show mac-address-table bridge-domain 20 Mac Address Table ------------------------------------------- BD Mac Address Type Ports ---- ----------- -------- ----- 20 1111.2222.96e2 ...


10

This would have to be commands provided from an interface on a L3 capable switch. The no switchport command puts the interface in L3 mode (known as "routed port") and makes it operate more like a router interface rather than a switch port. The ip address command assigns an IP address and network mask to the interface. Routed ports can be configured with a ...


10

Adding to jonathanjo's answer: Ethernet has components in both layers 1 (because it can run over different media) and 2 (because the frames are the same on the different media). The Preamble, SoF Delimiter, and Inter-packet Gap are really in layer-1 (waking up the receiver, etc.), while the frame (including the header, payload, and FCS) is in layer-2. The ...


10

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 in a VLAN that communicates with some servers in other networks. Those computers have no need to communicate together, so it's best to block those unwanted ...


10

Layer 3 (mostly IP) generally relies on the underlying layer-2 network (mostly Ethernet or Wi-Fi) for delivery. Just like a layer-2 network uses layer-1 links to actually move the bits. The difference in moving data at layer 1, 2 or 3 is the complexity of the devices. Layer-1 devices (repeaters) just copied bits - simple, yet inefficient and long obsolete. ...


9

So does storm-control unicast only measure the pps rate (or bps rate) of frames that are for destinations not in the CAM tables, or all destinations that aren't a broadcast or multicast address (so just the total number of unicast frames)? I have to confess that I mistakenly trusted the Nexus doc below which said only unknown unicast is rate-limited; ...


9

One key use for per port MAC addresses on switches is for Spanning-tree BPDU's. These are Layer-2 multicasts with source MAC address of the egress switch port. I would have to brush up on other Layer-2 protocols such as TRILL and SPB, but they might also take advantage of a per port MAC. Does that help?


9

In general, forwarding refers to a device sending a datagram to the next device in the path to the destination, switching refers to moving a datagram from one interface to another within a device, and routing refers to the process a layer-3 device uses to decide on what to do with a layer-3 packet. A host sending data to another host through an ethernet ...


9

it seems that routers decapsulate the frame on arrival, and encapsulate the packet in a frame in order to send it. Yes. A router must strip off the layer-2 frame in order to get to the layer-3 packet. The router then routes the packet to the next interface toward the destination, based on the layer-3 destination address. At the next interface, it must ...


9

not quite sure why the book (official Cisco) says that there are 5 collision domains in this example. Isn't it supposed to be 4? No, there are five collision domains. each bridge/switch interface is a separate collision domain, as is each router interface. The bridge in the drawing has two collision domains, and the switch has three; that gives you five ...


8

If Layer-2 has a checksum, shouldn't this validation be enough? Why we are having checksum validation in different OSI Layers? Simply put, different layers of the OSI model have checksums so you can assign blame appropriately. Suppose there is a webserver running on some system (assume TCP port 80, i.e. OSI Layer 4) Suppose there is a software error ...


8

If you have an end-to-end vlan topology, where vlans are not terminated on switches, you can use somethink like this algorithm: On the root switch get the mac address table, use filter output: sh mac add | i 8a06 1 0015.5d02.8a06 dynamic ip,ipx,assigned,other TenGigabitEthernet4/5 Now you know, that mac is behind interface TenGigabitEthernet4/5. ...


8

It's true that you have to know it in advance in order to calculate the CRC. The correct polynomial depends on the application of the CRC. For Ethernet, for example, the CRC-32 polynomial is part of the IEEE 802.3 standard. For you trivia buffs, it's 0x82608EDB.


8

No. The protocol is specified as it is. VxLAN (Virtual Extensible LAN) was developed to overcome this limitation. See RFC7348 for details


8

No, ethernet is connectionless, as is IP, and if you use UDP, the application would need to perform any connection-related handshaking (if needed). Typically, it is the responsibility of the transport or application protocol, e.g. TCP, to establish connections. how does a router know whether its neighbors are down or not? Sometimes (statically configured ...


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