Rapid Spanning Tree Capsule

RSTP defined new 4 port roles:- 1. Root Port (Old One) 2. Designated Port (Old One) 3. Alternate Port (New One) 4. Backup Port (New One) Alternate Port:- The alternate port is the port blocked by superior BPDU from a different bridge. Backup Port:- The backup port is the port blocked by the BPDU sent from the same switch.
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Bidirectional Forwarding Detection

Business critical application demands convergence very fast but on shared IP infrastructure, it’s difficult to meet the requirement. Even with redundant network, in case of failure of device or link how fast the convergence can happen? May be in seconds or minutes. BFD(Bi Directional Forwarding Detection) is a protocol which is used to achieve the convergence of links in sub seconds. The fastest convergence could be achieved by IGP also which is almost equal to one minute not less than that but by using BFD it could be in sub seconds like we get in SDH/SONET. BFD is a protocol which uses hello packets to exchange the information of neighbor with irrespective of the protocol used (It can work with OSPF, EIGRP, BGP, LDP, ISIS. PWE3 etc.). The Internet Draft for BFD does not specify a specific encapsulation type for BFD control packets; rather, it recommends the use of an encapsulation “appropriate to the medium and the network”. BFD payload control packets will be encapsulated in UDP packets, using destination port 3784 and a source port in the range of 49152 to 65535.
The primary mode is known as Asynchronous mode. In this mode, the systems periodically send BFD Control packets to one another, and if a number of those packets in a row are not received by the other system, the session is declared to be down.
The second mode is known as Demand mode. In this mode, it is assumed that a system has an independent way of verifying that it has connectivity to the other system. Once a BFD session is established, such a system may ask the other system to stop sending BFD Control packets, except when the system feels the need to verify connectivity explicitly, in which case a short sequence of BFD Control packets is exchanged. Demand mode may operate independently in each direction, or simultaneously.
An adjunct to both modes is the Echo function. When the Echo function is active, a stream of BFD Echo packet is transmitted in such a way as to have the other system loop them back through its forwarding path. If a number of packets of the echoed data stream are not received, the session is declared to be down. The Echo function may be used with either Asynchronous or Demand mode. Since the Echo function is handling the task of detection, the rate of periodic transmission of Control packets may be reduced (in the case of Asynchronous mode) or eliminated completely (in the case of Demand mode).

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E-Line Carrier Ethernet Services

E-Line Carrier Ethernet Service is divided into two parts:-
1. Ethernet Private Line aka EPL:- Ethernet Private Line is the replacement of legacy dedicated TDM private line for point to point connections consequences save cap-ex as well as op-ex. It is dedicated port and ethernet Virtual connection per UNI.

Figure 1

E-Line - Ethernet Private Line

2. Ethernet Virtual Private Line aka EVPL:- Ethernet Virtual Private Line is the replacement of legacy frame relay and atm circuits. Multiple services can be multiplexed in single UNI.

Figure 2

E-Line - Ethernet Virtual Private Line

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Inter Vlan Routing Without Layer 3 Device

It is always said that if single vlan wants to communicate with another vlan, inter vlan routing is required. But in the depicted scenario, vlan 10 is able to communicate with vlan 20 without any layer 3 device. The logic behind the working is very simple and it is totally based on tagged and untagged concept. Access ports always tagged the incoming traffic with vlan configured on port and make untagged the outgoing traffic. The same is happening in the given scenario, when the traffic is being forwarded by PC-1 and Switch A access port receives the traffic, mark that traffic with vlan 10 and look forwards for the vlan 10 ports. After that Switch A another port which is part of vlan 10 receives the traffic and forward it towards Switch B. During forwarding Switch A port untagged that vlan and forwards the traffic without any vlan tag. Switch B port receives the incoming traffic and tagged it with vlan 20 and forwards the traffic towards PC-2. This way end to end communication takes place without the use of router or any layer 3 device. So we can say inter vlan routing without any layer 3 device.

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Symbolic Analysis of Relay and Switching Circuits

Zero and One also fascinates me and could not able to understand why Digital Circuits are always linked with Mathematics. Claude Shannon father of Information Theory, was the one who gave us that concept. Fortunately during googling, I got the thesis published by Claude Shannon in 1940 which tells all about the building block of using mathematics in digital circuits. Click here to access his thesis of “A symbolic Analysis Of Relay and Switching Circuits".
A Symbolic Analysis of Relay and Switching Circuits, Claude Elwood Shannon proved that Boolean algebra and binary arithmetic could be used to simplify the arrangement of the electromechanical relays then used in telephone routing switches, then turned the concept upside down and also proved that it should be possible to use arrangements of relays to solve Boolean algebra problems.

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What is Inter Packet Gap or Inter Frame Gap?

Let's assume if a Gigi port which could transfer raw data up to 125,000,000 bytes per second with minimum frame size of 64 bytes. Device must need some time to process one packet after another. The difference of time to process from one packet to another is known as inter-frame or inter-packet gap. The minimum inter frame gap period is 96 bits or 12 bytes which amounts to 96 nano seconds + 7 byte of preamble and 1 byte of delimiter consequence 84 bytes.

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Switching Interview Questions For CCNA and CCNP Engineers

After IGP, BGP and MPLS interview question, Now I am adding switching basic questions. Please read the below questions:-

1. Difference between hub, bridge and switch?
2. What is mac address and why it is required?
3. In layer 2 domain do we need ip address for communication?
4. What is arp and why it is required?
5. What is Spanning Tree Protocol aka STP?
6. What is the difference between STP, MSTP, PVST and RSTP?
7. Can we use the two same paths for same vlan?
8. What is the difference between broadcast and collision domain?
9. Define type of lan traffic.
10. What is destination address of broadcast frame?
11. Can we connect a switch to switch with straight cable?
12. Define functions of switch.
13. What is arp timeout?
14. What is aging process?
15. What is BPDU?
16. What is path cost?
17. Define selection criteria of STP root bridge.
18. How to non bridge decide which port will elect as root port?
19. If a nonroot bridge has two redundant ports with the same root path cost, how does the bridge choose which port will be the root port?
20. Port states of spanning tree protocol.
21. If the users face delay during initial login, what you will suggest to implement?
22. Why spanning tree BPDU filter is used?
23. Can I use BPDU filter on trunk ports?
24. What is port security?
25. I want to learn only a single mac from the port, what need to be configured?
26. Can we use spanning port-fast on trunk ports?
27. If management ip address is changed, will user’s traffic will be dropped?
28. Difference between trunk and access port?
29. What is UDLD and why it is required?
30. What is interface vlan on switch?
31. How to perform inter vlan routing without layer 3 device?
32. How to stop superior bpdu participating in switching domain?
33. How Vlan In Local Switching Domain is selected?
34. How to provide redundancy to MPLSVPN customer?
35. What are deep buffers in switching?
36. How to build Low Latency Data Center Switching Fabric?


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Network Outage: %ETHCNTR-3-LOOP_BACK_DETECTED

The network is performing well, unfortunately we received a call that the few of the directly connected to each other are not working and the links attached to the switches were also not working. After logging on to the switches, we saw a weird error message "%ETHCNTR-3-LOOP_BACK_DETECTED: Keepalive packet loop-back detected on FastEthernet0/1". Fa0/1 was uplink to another switch and the port state was showing error disabled. The switch port goes in error disable mode if it detects any error situation on the port. The port is shut down and need to enable manually or automatically if configured. By default, all the cisco switches and routers send loopback frames (ethertype 0x9000) with SA=DA=It’s own mac-address. If the Keepalive packet is received on the same port it was sent from, then a loop exists which even cannot be detected by spanning tree protocol. This is likely the same as of split horizon technique in layer three devices. The problem most likely arises because of unmanaged switches or hubs connected to the switches.
To overcome the problem, we need to add “no keepalive” command under interface which prevents the port from being disabled. This issue is documented in Cisco bug ID CSCea46385.

Disable "no errdisable detect cause loopback" command is another way to handle the problem. By doing this when the switch detects the error disable message of loopback, it will not take any action.

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One Port Of Switch Is Up and Another Port Of Another Switch Is Down

One port is up and another port is down, the issue seems to be weird one but it happens when the two switches are connected over fiber. This happens because one side of the switch is not receiving the signal. In the given diagram, see the connectivity of the ports.

The failure of one fiber because of any reason make that port down. The port which is receiving the signal is showing up and the port which is not receiving the signal is showing down. This type of problem leads to the spanning tree loop in the metro ethernet or ring topology. To overcome this problem, UDLD (Uni Direction Link Detection) is required. This protocol forces the both end ports to make down.
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Choking Bandwidth Wan HSRP

In previous post of HSRP, a problem has highlighted of reverse and forwarding traffic when customer use the HSRP in LAN. This post is highlighting the problems when HSRP runs in PE-CE wan. Below is the diagram shown for reference purpose.


Customer R6 is connected with two PE routers for high availability. R3 is the active router with priority 255 and R2 is the standby router with priority 200. R1 ie receiving the lan routes from R2 as well as from R3 but selecting R2 as the best path because of smallest router-id assuming the cost of the both links in cloud is same. So the forwarding from R6 is happening via R3 router and reverse traffic is coming via R2.
Which Problems Could Come
1.The different reverse and forwarding could cause problems ofin customers applications if applications are jitter or latency senstive.
2. If the primary and backup path has different bandwidth allocation, In that case backup could be obstructed at anytime.


How to overcome the problem
1.Change the IGP path cost which may bring other difficulties.
2.Change the router id.

Apart from these remedies I am seeking another solution which could solve the current problem. If anyone is having anyidea , please share.

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HSRP For Lan - MPLS VPN

Customers look for the redundancy of lan and wan because of more uptime . In those scenarios , two routers deply at customer premises and two wan links from different PE are required. In this topology, I am assuming the customer is using the same service provider with redundant pops. Hot Standy Routing Protocol aka HSRP with single group is deployed for lan and static routing is being used between PE and CE. R5 is having the higher priority and working as active router and R4 is working as standby router.
Click Here to Download Full Article.

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Jumbo Frame Support

A good document of cisco which describes how to set the mtu and jumbo frame size on cisco switches. This is required mainly in service provider network when MPLS is implemented or customer which are coming on service provider network need more MTU sizes.

Click Here To Download


regards
shivlu jain
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Selection Of Vlan In Local Switching Domain

Till now I have not bring out anything on switching design. In this post I will let you know how to select the vlans in switching domain especially if spanning tree is working in that domain. Have you ever thought during switching design which vlan number should be used for which type of vlans. Let say if we are having one vlan for data, one for voice and one for vlan. For this I select vlan 10 for data, vlan 20 for video and vlan 30 for voice. It’s so simple and easy. But what will happen if STP is running in that domain and the design is like Switch1-Switch2-Switch3-Switch1. One port will be in blocking mode and if the links fails another will be in forwarding mode. This is the way of spanning tree works. A question comes in mind what vlans have to do with this scenario. Definitely vlans plays a major role during the convergence from one link to another; during the convergence always small number vlans get preference over the bigger number vlans. As I mentioned earlier about the three vlans in that always data vlans will flow without any impact because it is shortest in number and voice vlan number is largest one and get to come in picture last. So think about the scenario where voice is always prioritized first during the convergence it will the least one. So if little change is made in the numbers than the voice vlan traffic is always preferred without any delay. So during lan switching design always take the suggest shortest number for critical applications.


regards
shivlu jain
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How to stop superior bpdu participating in switching domain

If you are working in a service provider network then might be facing a problem of links flapping whic are attached towards the cpe. A very common issue which is always being ignored many times. But during the log analysis we always a weired error of receiving bpdu which smaller cost and most of the time we hardly diagnose the probelm. So eventually one of my collegaue added a command callled spanning-tree bpdufilter enable. After adding the command the link flapping stopped. After that we analysed what happened after adding the command, why the link flaping stopped suddenly and as soon as we removed we get stucked into the same. Actually from clinet end superior bpdu are receiving by the switch port and due to which port may be going in block state. After adding the command whenevve the switch receives the bpdu from clinet end it sliently drops it and issue resolved smartly.So we finalize make a best practise of adding the command to every client facing interface.

regards
shivlu jain
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