2.2.1 Creating a Layer 3 Interface Follow these steps to create a Layer 3 interface. You can create a VLAN interface, a loopback interface, a routed port or a port-channel interface according to your needs. Oct 06, 2005 When the port is listed as 2/1-4, spanning tree is treating ports 2/1, 2/2, 2/3 and 2/4 as one port. It is commonly a loopback address (127.0.0.x). T Series,M Series,MX Series. Checklist for Using Loopback Testing for Fast Ethernet and Gigabit Ethernet Interfaces, Diagnose a Suspected Hardware Problem with a Fast Ethernet or Gigabit Ethernet Interface, Create a Loopback, Verify That the Fast Ethernet or Gigabit Ethernet Interface Is Up, Configure a Static Address Resolution Protocol Table Entry, Clear Fast Ethernet or Gigabit Ethernet.
A switching loop or bridge loop occurs in computer networks when there is more than one Layer 2 (OSI model) path between two endpoints (e.g. multiple connections between two network switches or two ports on the same switch connected to each other). The loop creates broadcast storms as broadcasts and multicasts are forwarded by switches out every port, the switch or switches will repeatedly rebroadcast the broadcast messages flooding the network.[1] Since the Layer 2 header does not support a time to live (TTL) value, if a frame is sent into a looped topology, it can loop forever.
A physical topology that contains switching or bridge loops is attractive for redundancy reasons, yet a switched network must not have loops. The solution is to allow physical loops, but create a loop-free logical topology using the shortest path bridging (SPB) protocol or the older spanning tree protocols (STP) on the network switches.
Broadcasts[edit]
In the case of broadcast packets (broadcast radiation) over a switching loop, the situation may develop into a broadcast storm.
In a very simple example, a switch with three ports A, B, and C has a normal node connected to port A while ports B and C are connected to each other in a loop. All ports have the same link speed and run in full duplex mode. Now, when a broadcast frame enters the switch through port A, this frame is forwarded to all ports but the source port, i.e. ports B and C. Both frames exiting ports B and C traverse the loop in opposite directions and reenter the switch through their counterpart port. The frame received on port B is then forwarded to ports A and C, the frame received on port C to ports A and B. So, the node on port A receives two copies of its own broadcast frame while the other two copies produced by the loop continue to cycle. Likewise, each broadcast frame entering the system continues to cycle through the loop in both directions, rebroadcasting back to the network in each loop, and broadcasts accumulate. Eventually, the accumulated broadcasts exhaust the egress capacity of the links, the switch begins dropping frames, and communication across the switch becomes unreliable or even impossible.
MAC database instability[edit]
Switching loops can cause misleading entries in a switch's media access control (MAC) database and can cause endless unicast frames to be broadcast throughout the network. A loop can make a switch receive the same broadcast frames on two different ports, and alternatingly associate the sending MAC address with the one or the other port. It may then incorrectly direct traffic for that MAC address to the wrong port, effectively causing this traffic to be lost, and even causing other switches to incorrectly associate the sender's address with a wrong port as well.
Multiple frame transmissions[edit]
In a redundant switched network it is possible for an end device to receive the same frame multiple times.
Misinterpretations[edit]
It is not true that within a switching loop packets will circulate the network until their time to live (TTL) value expires, as no TTL concept exists at Layer 2. In practice, the packet will circulate until it is dropped, e.g. due to resource exhaustion.
References[edit]
- ^https://accedian.com/enterprises/blog/identify-fix-network-switching-loop/
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Switching_loop&oldid=930886635'
Loopback
Loopback is a commonly used term in telecommunications. It refers to the process of transmitting electronic signals or digital data streams and returning to their sending point without any intentional processing or modification. Therefore, by comparing transmitting signals with the receiving signals, the loopback test is used to debug physical connection problems. But what a loopback test means for fiber optic network and how to make use of it will be the issues that we will explore in this post.
Why Need Fiber Loopback Test?
To conduct a fiber loopback test, the communication devices will be involved, like the transceivers and the switch. As you know, the transceiver is the basic component of fiber optic communication network equipment. We can take the transceiver as a case. Conventionally, a transceiver has a transmitting port and a receiving port, in that way, the loopback test can be applied to test the ports to diagnose whether the transceiver is working well and the configuration of the switch is right. For its unique working mode, the test is a convenient way to maintain transceivers. In the next part, we will deliver how to do the fiber loopback test on the transceiver.
How to Conduct Loopback Test?
In this part, we will introduce two types of tests to troubleshoot transceiver and switch port: single-port test and dual-port test.
To perform tests, things you need to prepare are listed below:
- Transceivers (2pcs), such as 10G SFP+ SR transceiver.
- Simplex fiber cable (1 pc).
- Switch (1 pc), like Cisco switch.
- Duplex fiber cable (1 pc).
- Two loopback cables (optional), like LC or SC loopback cable. To know more about loopback cable, you can move to the article: What Is Loopback Cable And How to Use It?
Figure 1: Loopback Cable
Single-port Loopback Test
Figure 2: Single-port Loopback Test
1. Connect your transceiver with one simplex fiber cable or loopback cable, such as LC fiber cable or LC loopback cable. At this step, you can examine whether the port and transceiver parameters are normal.
2. Check the software version of the switch.
3. Review the interfaces status to confirm the working status of all ports on the switch.
Figure 3: Display the Working Status of All Ports
4. Check the working status of the port you are connecting, such as the port 50 in the following figure.
Loopback 2
Figure 4: Working Status of Interface 50
5. Go over the DDM information to review whether the transceiver works in normal status.
Figure 5: DDM Information of Port 50
Figure 6: Dual-port Loopback Test
1. Connect two transceivers with one duplex fiber cable or two loopback cables. At this step, you can examine whether the port and transceiver data rate are matching as well as the link is normal or not.
2. Check the interfaces status to confirm the working status of all ports on the switch.
Figure 7: Ports Working Status Display
3. Check the working status of the two ports you are connecting, such as the ports 50 and 52 in the following figure.
Figure 8: Working Status of Interfaces 50 and 52
Loopback 2 1 3 Switch Wiring
4. Go over the DDM information to review whether the transceiver works in normal status.
Loopback 2 1 3 Switch Wiring
Figure 9: DDM Information of Ports 50 and 52 Pastebot 2 1 2.
Network Switch Loopback
Summary
To troubleshoot the circuit connectivity as well as the transceiver and the switch port, loopback test is a cost-effective way. In this post, we have an overview of loopback and make a demonstration of how to conduct the loopback test on a switch to debug the transceiver and the switch port.