Spanning Tree – Root Bridge
Every spanning tree switched network or broadcast domain has a switch designated as the root bridge. The root bridge serves as the reference point for all spanning-tree enabled switches. The spanning tree algorithm determines which redundant paths to block. It is selected as an election process.
The figure below illustrates the bridge ID (BID) fields. BID is a unique identity when the switch is the part of a network. It is a 64 bits field which is divided into three parts. The first part is 4 bits Bridge Priority field, the second field is 12 bits extended system ID while the third part is the 48 bits MAC address of the switch. The Bridge Priority is configurable while the MAC address is unique among all switches and the sum of these two ensure a unique Bridge ID.
Root Bridge election
All switches in the broadcast domain join the election process. When switches complete their booting process, it begins sending out BPDU frames every two seconds containing the switch BID and the root ID. The adjacent switches receive the BPDU frames and read the root ID information from the BPDU frames. If the root ID of BPDU sending switch is lower than the root ID on the receiving switch, then the receiving switch updates its root ID, identifying the adjacent switch as the root bridge.
It doesn’t need to be the adjacent switch. It could be any other switch in the broadcast domain. The switch then sends out new BPDU frames with the lower root ID to the other adjacent switches. Finally, the switch with the lowest BID ends up being selected as the root bridge for the spanning-tree instance.
Now look the election process at another angle. Before configuring the bridge priority manually all the switches have a default priority, therefore, it is a tie based on priority. Now, the switch with the lowest Mac address will become a root bridge. In the figure below switch-3 is elected as Root Bridge because the bridge priority ID is default which is 32769, So the switches elect root bridge based on MAC address. The switch-3 is the lowest MAC address, so all the ports of the root bridge become in the forwarding state i.e. designated port.
Spanning Tree – Port Roles
IEEE 802.1D Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol (RSTP) uses the Spanning Tree Algorithm (STA) to decide which switch ports on a network have to be put in blocking state to preventing loops. The Spanning Tree Algorithm designates one switch in the network as the root bridge. The root bridge is considering […]
Spanning Tree Protocol
In the previous article, I have explained the redundancy. It increases the network availability by protecting the network from a single point of failure, such as a fault in a network cable or fault in a switch. When engineers introduce the physical redundancy in design, loops and duplicate frames happen. I have written in the […]
Network Redundancy
Reliability is a quality of computer network that consistently performs according to its specifications. It has long been considered one of three related attributes that must be considered when planning network. Redundancy is one of the key factors to maintain network reliability. Multiple physical connections between network devices provide redundant network paths. The network can […]
NAT for IPv6
The NAT and private IPv4 addresses have slowed down the depletion of IPv4 addresses but NAT has some disadvantages. The one major benefits providing by NAT is the security. It hides the private IPv4 network from the public Internet which provides a perceived level of security by denying computers on the public Internet from accessing […]
Port Forwarding Configuration – Cisco Router
The port forwarding configuration on Cisco routers is similar to the configuration of static NAT. It is a static NAT translation with a specific TCP or UDP port number. The figure above shows an example port forwarding configuration using Cisco IOS commands on router R2, 192.168.11.100is the inside local IPv4 address of the webserver. The […]
Introduction to Port Forwarding
Port forwarding is also known as port mapping and tunnelling. It is the method of forwarding traffic destined to a specific network port from one network node to another. The external user can access a specific port on a private IPv4 address inside a LAN from the outside, through a NAT-enabled router. In other words, […]
Port Address Translation (PAT) Configuration
Port Address Translation (PAT) is also known as NAT overload. Port Address Translation preserve addresses in the inside global address pool. Port Address Translation (PAT) allows the router to use one inside global address for many inside local addresses simultaneously. We can use a single public IPv4 address for hundreds, even thousands of internal private […]
Dynamic NAT Configuration
Dynamic NAT maps inside local addresses to inside global addresses automatically. The inside global addresses are usually public IPv4 addresses. Dynamic NAT uses a pool of public IPv4 address or a group public IPv4 addresses for translation. It also requires the configuration of the inside and outside interfaces participating in NAT like Static NAT. The […]
Networkustad
- Spanning Tree – Root Bridge October 18, 2019 Shahab Ali
- Spanning Tree – Port Roles October 17, 2019 Shahab Ali
- Spanning Tree Protocol October 17, 2019 Shahab Ali
- Network Redundancy October 16, 2019 Shahab Ali
- NAT for IPv6 October 15, 2019 Shahab Ali
- Port Forwarding Configuration – Cisco Router October 14, 2019 Shahab Ali
- Introduction to Port Forwarding October 13, 2019 Shahab Ali
- Port Address Translation (PAT) Configuration October 13, 2019 Shahab Ali
- Dynamic NAT Configuration October 12, 2019 Shahab Ali
- Static NAT Configuration October 11, 2019 Shahab Ali
