what is a shared media in networking?

p1xels · October 2014

Hi all,
When we issue the command "show spanning-tree vlan id" ,then we receive a output similar to the following-

spanning-tree enabled protocol ieee

( output omitted)

int role stat cost pri.nbr type

f0/1 atln blk 19 128.11 p2p

What does this p2p under type field above means?

Does that indicate that it is a shared media?

Hope someone can please explain this to me.Will be obliged greaty.

Thanks in advance

With Best Regards

Dieg0M · October 2014

p2p will be a full duplex media between 2 bridges/switches and shr will usually be a link operating as half-duplex

p1xels · October 2014

Thank you Diego.
Then you are saying that 'p2p' is different from 'shr' media , i.e., p2p is not a shared media??

With Regards

Dieg0M · October 2014

As far as I know that is correct and p2p (point to point) is different then shr (shared). Both types identify links that have bridges/switches interconnected but the media setting is different.

Starwars · October 2014

The term ptp is used to describe a link between 2 devices. When there are only 2 devices, the encapsulation doesn't require a src and dst address as once you send a frame on the wire you are sure it will reach the destination device. This doesn't mean every protocol encapsulation runninging over a ptp link doesn't use a src dst address, its just not required. As you start getting deeper into protocols running over ptp links you see how the signalling protocols used are more simplistic due to the destination device being deterministic.
A shared media, means that more than 2 devices are connected over a media, in this case a src dst address are mandatory and usually protocols run some kind of signalling to negotiate a master device on the shared medium.

Starwars · October 2014

As Dieg0M has pointed out, in your scenario, you are looking at spanning-tree, the ptp refers to a link that is full-duplex. Full duplex allows simultaneous bidirectional communication between devices, this is not possible on a shared medium. When spanning-tree see's a port configured as full-duplex it automatically knows the port is a ptp link.
Half duplex is used on shared media, i.e. when a link is connected to a bridge or switch, the bridge or switch interconnect multiple l2 devices. Without a bridge or switch you could only connected 2 devices i.e. ptp. When spanning-tree detects a port working in half duplex it assumes the port is shared i.e. connected to a bridge or switch.

theodoxa · October 2014

Rapid Spanning-Tree has different port types - Point-to-Point, Shared, and Edge. It acts differently based on the port type.

Point-to-Point: A Full Duplex Link between 2 switches. The two switches will negotiate directly with each other to determine the Designated Port.

Shared: A Half-Duplex Link. The switches will use the slower 802.1D (BPDUs + 30-50 seconds to reach Forwarding) methods, because there might be multiple switches connected to the same segment.

Edge: A port connected to a single end user device, such as a PC. An Edge port acts identically to a port with PortFast Enabled. It skips directly to the Forwarding state.

NOTE: A couple decades ago there was something called ThinNet/ThickNet. It was Ethernet using Coaxial cable. Each device was simply connected to a single cable using an adapter. The media was literally shared by every device. Any traffic sent by one device was received by all other devices. As we moved towards UTP and star topologies, switches were still quite expensive, so several hubs would be aggregated to a single switch to provide more ports while saving money. Hubs simply replicated the received signal on all other cables attached to it, so the media was still effectively shared (any traffic sent by one device was received by every other device connected to the hub).

what is a shared media in networking?

Comments