Virtual Router Redundancy Protocol (VRRP) is very similar to HSRP and can create a virtual gateway but the difference that it is not the Cisco Proprietary Protocol.
It is a standard protocol defined by the IETF in RFC 3768. If you learn and understood HSRP then it is very easy for you to understand the Virtual Router Redundancy Protocol (VRRP).
The Virtual Router Redundancy Protocol (VRRP) is an election protocol that dynamically assigns responsibility for one or more virtual routers to the VRRP routers on a LAN and elects the role of the router.
Virtual Router Redundancy Protocol (VRRP) allows several routers on a multi-access link to utilize the same virtual IP address. In a VRRP configuration, one router is elected as the virtual router master and other routers acting as backups in case the virtual router master fails.
As in the previous two articles, Router2 and Router3 are neighbouring to the LAN segment. We will use the same topology for the Virtual Router Redundancy Protocol (VRRP) configuration. Router2 will be the master router and Router3 role is the backup router.
In the above topology, we are using two Cisco routers for VRRP. We have been tasked to configure VRRP. The IP addresses, routing and other pre-requisite have done. This is important to know that each device participating in the VRRP process also needs to be in the same broadcast domain and also requires a unique IP address on the interface or VLAN where you are configuring VRRP. Let’s go-ahead to the configuration and add the VRRP configuration.
When we type vrrp in the command prompt we are stating to the router that we are configuring vrrp on this interface, the number next to it is the VRRP group we are configuring, the range of the group is from 1 to 254.
The groups required to be matched on all equipment participating in the VRRP election process, and the last part of the configuration we are stating what the floating IP address is. We have added the group 1 to configuration.
We have set the priority of both routers. Router2 is configured with higher value, so this will be a master router. If Router2 fails, the Router3 will become master. The preemption is enabled, so whenever Router2 recover from failure, it will again become a master router.
Gateway Load Balancing Protocol (GLBP) is the Cisco-proprietary FHRP. It also allows load sharing between a group of redundant routers. GLBP was specially designed to overcome the limitations of HSRP and VRRP.
In HSRP and VRRP, only the active/master router can forward traffic while all the standby/backup routers are idle but Gateway Load Balancing Protocol overcome this problem, all routers in the GLBP group can be efficiently utilized and forward traffic, so in addition to redundancy we also get load balancing. We can configure GLBP in the Cisco IOS router using three types of an algorithm: round-robin, weighted, or host-dependent.
The routers participating in the Gateway Load Balancing Protocol group are called Active Virtual Forwarders (AVF). Like HSRP, one router still must control the Gateway Load Balancing Protocol operation, and this router is called the Active Virtual Gateway (AVG).
An Active Virtual Gateway (AVG) is elected when GLBP is activated on the network. It decides the next AVF that will be used to forward the packet based on the selected algorithm. A router with the AVG role by default also acts as AVF at the same time.
The Gateway Load Balancing Protocol (GLBP) configuration on the Cisco router is easy and similar to the configuration of the Hot Standby Router Protocol (HSRP) and Virtual Router Redundancy Protocol (VRRP). We will use the same topology for the GLBP configuration that we used for the HSRP configuration.
Each gateway will be configured in a GLBP group with the same group number, and at least one gateway in the GLBP group must be configured with the virtual IP address to be used by the group. We can use the “glbp group ip” command to enable GLBP from the global configuration mode. The configuration on both Router2 and Router3 is as under:
We are using GLBP group number 1 and virtual IP 192.168.10.1, and this virtual IP address should be configured on each router. We also configured the priority for both routers to elect as AVG; the priority of Router2 is 110, and the priority of Router1 is 100. The default priority value of GLBP is 100. The router with the highest priority number will be elected as the AVG.
If the AVG fails, the next router with the highest priority value will be granted the role of AVG. Preempt is enabled on both routers, which means that they can get back the AVG role from the current AVG if they have higher priority.
We can verify GLBP configuration, using the command show glbp brief from any GLBP members. To verify GLBP configuration, we can simply issue a command show glbp brief from any of the GLBP members:
GLBP Round-Robin Algorithm
Round-robin is the default load-balancing algorithm with the Gateway Load Balancing Protocol configuration. With this algorithm, AVG replies to each ARP request for the GLBP virtual IP with each AVF’s virtual MAC address, so all AVFs are used equally.
GLBP Weighted Algorithm
The weighted algorithm allocates traffic to each AVF based on its weight value. An AVF with a bigger weight value will redirect more traffic to it.
The weighting range is from 1 to 254, and the default weight value is 100. The router with a higher weight value will be assigned more traffic to the router with a lower weight value. The configuration command for the above example scenario is as follows:
Router2:
Router2>enable
Router2#config terminal
Router2(config)# interface gigabitEthernet 0/1
Router2(config-if)#glbp 1 load-balancing weighted
Router2(config-if)#glbp 1 weight 150
Router2(config)#do wr
Router3:
Router3>enable
Router3#config terminal
Router3(config)# interface gigabitEthernet 0/1
Router3(config-if)#glbp 1 load-balancing weighted
Router3(config-if)#glbp 1 weight 200
Router3(config)#do wr
GLBP Host-Dependent
This algorithm uses the host’s MAC address to determine which virtual MAC address to direct the host to use. If the number of active virtual forwarders (AVF) does not change, this algorithm usually guarantees the same virtual MAC address that was previously assigned.
In some situations, this comes up as the best option, for example, in the case of stateful NAT. The configuration of the Gateway Load Balancing Protocol host-dependent for the above topology is as follows:
GLBP allows automatic selection and simultaneous use of multiple available gateways and automatic failover between those gateways. Various routers share the load of frames that, from a client viewpoint, are sent to a single default gateway address.
We can fully use network resources without the administrative burden of configuring multiple groups and managing multiple default gateway configurations. Gateway Load Balancing Protocol has the following characteristics:
GLBP allows full use of network resources on all devices without the administrative burden of creating multiple groups.
GLBP provides a single virtual IP address with several virtual MAC addresses.
GLBP routes traffic to a single gateway distributed across multiple routers.
In the event of failure and change, it provides automatic rerouting.
HSRP is a Cisco-proprietary First Hop Redundancy Protocol (FHRP) planned to allow for transparent failover of a first-hop IPv4 device. It allows configuring two or more routers as standby routers and only a single router as an active router at a time which ensure high network availability by providing first-hop routing redundancy for IPv4 hosts on networks configured with an IPv4 default gateway address.
All the routers in a single HSRP group share a single MAC address and IP address, which acts a default gateway to the local network. It selects an active router as well as a standby router in the group.
The active router responsible for forwarding traffic to the network and sends the hello packet to the standby router. If it fails and the Standby router doesn’t receive a hello packet for three specific times, it takes up all the responsibilities of the active router and forwards the traffic.
The HSRP configuration can be complicated at times, so this article will cover the basic points step by step. Now, look at the topology in the figure below.
Basic HSRP Configuration
The above topology contains two HSRP routers (Router2 and Router3) both can act as the default gateway for the network 192.168.10.0/24 network. At a particular time, only one of these routers will be active with a virtual IP address of 192.168.10.1.
So, virtual IP 192.168.10.1 will be configured as the default gateway for all devices on the network 192.168.10.0/24. So let’s configure the Router1 and Router2 as HRSP router. The following configuration is enough for achieving basic HSRP configuration, the configuration on Router2 is as follow:
Standby 1, 1 is the HSRP group number. This group number can be any number between 0 and 255 in HSRP version 1 and the only requirement is that you must use the same number across devices in the same HSRP group. So now we can verify the HSRP configuration using the “show standby” command.
The two figures below illustrate the output of the show standby command on Router2 and Router3. You can see the virtual IP address 192.168.10.1 on both router HSRP configuration, which is the default gateway for all the hosts in the network 192.168.10.0/24. You can also see the virtual MAC address for the virtual router.
You can see that Router3 is in the Active state while Router2 is in the standby state. Currently, we have not configured the priority, and both routers are configured with default priority 100.
So, with the same priority, the router with the highest IP address will be elected as the active router. However, we can control the configuration of the active router by configuring the priority that will be also discussed in this article.
Now the entire host will be configured with the default gateway address 192.168.10.1 and the Router3 which is currently active will receive traffic destined for the address 192.168.10.1. We have not advertised the network 192.168.10.0/24 in any routing protocol because Routes are not replicated between HSRP routers.
This means that Router1 and Router2 must individually know how to reach the 172.31.10.1 network used in our example. We have configured the default route on both routers to reach the 172.16.10.0/24 network, suppose it is the internet server, or it is your ISP. So, now if router3 is failed for any reason Router2 will start act as the active router.
HSRP Priority Configuration
HSRP the priority value can control the Active Router election. We can change the default priority value which is 100. For example, what if we wanted Router2 to be the Active router instead of Router3? To force a particular router to be the active router in an HSRP group you will need to use the priority command.
The higher priority determines the active router. If both routers are set to the same priority, the first router to come up will be the active router otherwise router with the highest IP address will be active. Let’s configure the Router priority higher then Router3, so Router2 will ever active when it is power up.
Router2>enable
Router2#config terminal
Router2(config)# interface gigabitEthernet 0/1
Router2(config-if)#standby 1 priority 110
Router2(config-if)#exit
Router2(config)#do wr
We have changed and increased the priority value of the Router2 but, it will remain in the standby mode because preemption is disabled.
HSRP Preempt
If Rouer3 fails, Router2 will become active as we have seen. But, if Router3 restores and returns to service, Router2 will continue to stay active. There are times when you may always want Router3 to be in an active state in the HSRP group.
In our case, we want Router2 as an active router ever for which we increase the priority of the HRSP on the Router2. But it not changes into active mode.
Cisco provides a method for users to control this by using the preempt command. Preempt to ensure a router to be active after recovering from a failure.
So we want Router2 to take over the active function when it recovers from a failure, we can change our configuration on Router2 to include the standby 1 preempt command.
Router2>enable
Router2#config terminal
Router2(config)# interface gigabitEthernet 0/1
Router2(config-if)#standby 1 preempt
Router2(config-if)#
%HSRP-6-STATECHANGE: GigabitEthernet0/1 Grp 1 state Standby -> Active
Router2(config-if)#exit
Router2(config)#do wr
Advanced HSRP Configuration – Load Balancing
Load Balancing is an important aspect of any network. In the above topology, one router will ever standby. This is not a good practice.
To solve this problem, we can configure HSRP load balancing between routers. The load balancing is not possible in the single HSRP group, but for multiple HSRP groups, we can extend the load and have each HSRP group be active on different routers.
For our example, let’s add Laptop0 and Laptop1 to another group (Group-2). Router2 will be active for Group1 while Router3 will be active for Group2. This means that R2 will be standby for Group2 while Router3 will be standby for Group1. The HRSP load balancing configuration is following for both routers.
The full configuration on Router2 is as follows:
Router2>enable
Router2#config terminal
Router2(config)# interface gigabitEthernet 0/1
Router2(config-if)# ip address 192.168.10.2 255.255.255.0
Router2(config-if)# standby 1 ip 192.168.10.1
Router2(config-if)#standby 1 priority 110
Router2(config-if)#standby 1 preempt
Router2(config-if)#standby 1 name Group1
Router2(config-if)#standby 2 ip 192.168.1.250
Router2(config-if)#standby 2 name Group2
Router2(config-if)#exit
Router2(config-)#exit
Router2#wr
The full configuration on Router3 is as follows:
Router3>enable
Router3#config terminal
Router3(config)# interface gigabitEthernet 0/1
Router3(config-if)# ip address 192.168.10.3 255.255.255.0
Router3(config-if)# standby 1 ip 192.168.10.1
Router3(config-if)#standby 1 name Group1
Router3(config-if)#standby 2 ip 192.168.1.250
Router3(config-if)#standby 2 priority 110
Router3(config-if)#standby 2 preempt
Router3(config-if)#standby 2 name Group2
Router3(config-if)#exit
Router3(config-)#exit
Router3#wr
This configuration allows and puts both routers in working mode. We have also configured the HSRP group name to help better describe each HSRP group. This is helpful in the case of many HSRP groups.
To test this configuration, we can use the traceroute command to 172.16.10.1 from both Groups. Remember that Laptop0 and Laptop1 are in Group2 and the default gateway for these hosts is 192.168.10.250 and the other two hosts on Group1 use the previous Default Gateway 192.168.10.1.
When the active router fails, the First Hop Redundancy Protocol (FHRP) changes the standby router to an active router. If the active router fails, the standby router stops seeing Hello messages from the forwarding router.
Then, the standby router assumes the role of the forwarding router because it assumes both the IP and MAC addresses of the virtual router, and the host devices see no disturbance in service. This process is possible due to the First Hop Redundancy Protocols (FHRPs). The difference FHRPs are the following:-
Host Standby Router Protocol (HSRP)
It is a Cisco-proprietary First-Hop Redundancy Protocol (FHRP) planned to allow transparent failover of a first-hop IPv4 device. It allows configuring two or more routers as standby routers and only a single router as an active router at a time. This ensures high availability by providing first-hop routing redundancy for IPv4 hosts on networks configured with an IPv4 default gateway address.
All the routers in a single HSRP group share a single MAC address and IP address, which acts as a default gateway to the local network. It selects an active router and a standby router in the group.
The active roster forwards traffic to the network and sends the hello packet to the standby router. If it fails and the Standby router doesn’t receive a hello packet for three specific times, it takes up all the responsibilities of the active router and forwards the traffic.
HSRP for IPv6
This is also Cisco-proprietary FHRP, which is the same as HSRP. The difference is that HSRP works in an IPv4 environment, and HSRP for IPv6 works in IPv6 environments. An HSRP IPv6 group uses a virtual MAC address resulting from the HSRP group number and a virtual IPv6 link-local address.
Periodic router advertisements (RA) messages would be sent to the HSRP virtual IPv6 link-local address when the HSRP group is active. When the group becomes inactive for any reason, the Router Advertisement (RAs) messages stop after a final RA is sent when the group would be leaving the active state.
Virtual Router Redundancy Protocol Version 2 (VRRPv2)
Virtual Router Redundancy Protocol (VRRP) is an election protocol that provides redundancy to routers within a Local Area Network.
It is a non-proprietary protocol that allows several routers on a multi-access link to use the same virtual IPv4 address. VRRP was designed to remove a single point of failure in a static default-route environment by dynamically assigning virtual IP routers to participating hosts.
In a VRRPv2 configuration, one router is elected as the virtual router master, and other routers act as backups if the master fails.
VRRPv3 supports IPv6 addresses, including IPv4 addresses, for dual-stack networks configured with VRRP or VRRP-E. It is compliant with RFC 5798. It provides a faster switchover to backup devices than can be achieved using standard IPv6 neighbor discovery mechanisms.
With VRRPv3, a backup router can become a master router only in seconds with less overhead traffic and no relations with the hosts. It works in multi-vendor environments and is more scalable than VRRPv2.
Gateway Load Balancing Protocol (GLBP)
GLBP is the Cisco-proprietary FHRP. It is the abbreviation of the Gateway Load Balancing Protocol, which protects data traffic from a failed router or circuit, like HSRP and VRRP. GLBP also allows load sharing between a group of redundant routers.
GBLP is specially designed to overcome the limitations of HSRP and VRRP. Gateway Load Balancing Protocol’s additional feature is loading share between the gateways. In HSRP and VRRP, the standby routers are configured for redundancy and act as standby only, becoming active only when the active router fails.
GLBP for IPv6
It is also a Cisco-proprietary FHRP that provides the same function as GLBP but in an IPv6 environment. It provides automatic router backup for IPv6 hosts configured with a single default gateway on a LAN. Multiple first-hop routers on the local area network join to offer a single virtual first-hop IPv6 router while sharing the IPv6 packet forwarding load.
ICMP Router Discovery Protocol (IRDP)
RFC 1256 specifies it as a legacy FHRP. The ICMP Router Discovery Protocol (IRDP) allows IPv4 hosts to locate routers that provide IPv4 connectivity to nonlocal IP networks.
It uses Internet Control Message Protocol (ICMP) router advertisements and router solicitation messages to allow a host to discover the addresses of operational routers on the subnet.
FAQs
Q1: What is a First Hop Redundancy Protocol (FHRP)?
A1: FHRP is a computer networking protocol designed to ensure the availability of a default gateway in a network. It allows multiple routers to provide backup for a single IP address, ensuring continuous network availability even if one router fails.
Q2: Why is FHRP important in networking?
A2: FHRP is crucial because it provides redundancy and high availability for the default gateway, essential for maintaining network connectivity and minimizing downtime in case of router failures.
Q3: What are the common types of FHRP?
A3: The most common types of FHRP include:
Hot Standby Router Protocol (HSRP)
Virtual Router Redundancy Protocol (VRRP)
Gateway Load Balancing Protocol (GLBP)
Q4: How does HSRP work?
A4: HSRP works by creating a virtual router with a virtual IP address and MAC address. Multiple routers participate in the HSRP group, one acting as the active router and another as the standby router. If the active router fails, the standby router takes over the virtual IP address, ensuring continuous network service.
Q5: What is the difference between HSRP and VRRP?
A5: While both HSRP and VRRP provide redundancy for the default gateway, their implementation and standards differ. HSRP is a Cisco proprietary protocol, whereas VRRP is an open standard protocol defined by the IETF. Additionally, VRRP allows multiple routers to share the load, whereas HSRP typically has one active and one standby router.
Q6: Can FHRP protocols be used together?
A6: FHRP protocols can be used together in a network to enhance redundancy and load balancing. For example, HSRP can be used with GLBP to provide redundancy and load balancing.
Q7: What are the benefits of using GLBP?
A7: GLBP provides load balancing in addition to redundancy. It allows multiple routers to share the network traffic load, improving overall network performance and resource utilization.
Q8: How do I configure FHRP on my network?
A8: Configuring FHRP involves setting up the protocol on your routers, assigning virtual IP addresses, and defining priority levels for the routers. The specific configuration steps vary depending on the protocol (HSRP, VRRP, or GLBP) and the network equipment used.
Q9: What are some common issues with FHRP, and how can they be resolved?
A9: Common issues with FHRP include misconfigurations, network topology changes, and hardware failures. These can be resolved by verifying configuration settings, ensuring proper network design, and regularly monitoring and maintaining network equipment.
Q10: Where can I find more information about FHRP?
A10: More information about FHRP can be found in networking textbooks, online resources, and vendor documentation from companies like Cisco. Additionally, the original article on NetworkUstad provides a detailed overview of FHRP.
A virtual router is a method to prevent a single point of failure at the default gateway. To implement virtual router redundancy, several routers are configured to work jointly as a single router to the hosts on the LAN. The routers share an IP address and a MAC address and act as a single virtual router.
The IP address of the virtual router is used as the default gateway for the local network on a particular IP segment. When hosts on the local network sending data to the internet using the default gateway, the sending host resolve the MAC address of the default gateway using ARP.
The ARP returns the MAC address of the virtual router and the data that are sent to the MAC address of the virtual router can then be physically processed by the currently forwarding (active) router within the virtual router group.
The redundancy protocol is used to recognize two or more routers as the devices that are responsible for processing data frames for the MAC or IP address of a single virtual router.
The Host devices on the local network send traffic to the address of the virtual router but the physical router process and forwards this traffic is transparent to the host devices.
The redundancy protocol decides which router should take the active role in forwarding traffic. The protocol also decides when the forwarding role must be taken over by a standby router. The switch from one forwarding router to another is transparent to the end devices.
The end devices don’t know about the change of the physical router. Thus the network dynamically recovers from the failure using router redundancy. The device acting as the default gateway in the router redundancy protocol known as the first hop redundancy.
Hosts at the access layer of a hierarchical network take advantage of the alternate default gateways. If a gateway (router interface) fails, the hosts configured with that gateway are cut off from outside networks. A method is needed to provide alternate default gateways in switched networks where two or more routers are connected to the same VLANs.
The function of the router and multilayer switch is not different at the distribution layer. So, the multilayer switch mainly acts as the default gateway for each VLAN in a switched network. Each client receives only one gateway in a switched network. There is no method to configure a secondary gateway, even if a second path is available to carry packets off the local segment. Look at the figure below:-
In the figure, Switch4 is the default gateway for routing packets from PC0. It is a layer 3 switch. If Switch4 becomes unavailable, the spanning-tree protocols can dynamically converge the network.
Switch3 will now route packets from outside networks that would have gone through switch2. But, traffic from the inside network associated with switch4, including traffic from hosts configured with switch4 as their default gateway, is still sent to switch3 and dropped.
End devices are usually configured with a single gateway; the gateway address does not change when the network topology changes. If that gateway is unreachable for any reason, the host of this gateway cannot send packets except to the local network segment.
Still, if a redundant path exists that could serve as a default gateway for that segment, there is no dynamic way for these devices to connect to a new gateway.
In most networks, the best Spanning-Tree Protocol topology is determined as part of the network blueprint. The Spanning-Tree Protocol is implemented by configuring Spanning-Tree Protocol priority and cost values. Several things could go wrong.
You can expect something, but your switches can give you something different. Situations also occur when the Spanning-Tree Protocol was not measured in the network planning and implementation or was measured or implemented before the network’s growth and change.
In these situations, it is important to analyze the actual Spanning-Tree Protocol topology in the operational network to troubleshoot the Spanning-Tree Protocol. The steps for analyzing a spanning tree are the following:-
In the first step, find the layer 2 topology. If it was prepared previously, consult the network documentation for the topology.
Use the “show cdp neighbors” command to help find the layer 2 topology.
When Layer 2 topology is discovered, use spanning-tree protocol knowledge to resolve the predictable Layer 2 path.
It is also important to know the root bridge. Use the“show spanning-tree vlan <vlan_id >” command to resolve which switch is the root bridge.
Use the show spanning-tree vlan <vlan_id > command on all switches to find the port state and confirm your expected Layer 2 path.
Expected Topology vs Actual Topology
Comparing the network’s actual state against its expected state and spotting the differences can help troubleshoot the problem. A network administrator can examine the switches, resolve the actual topology, and recognize the superior spanning-tree topology.
Overview of Spanning Tree Status
The overview of the spanning tree plays an important role in troubleshooting. For an overview, we can use the “show spanning-tree” command without specifying any additional options to provide a quick overview of the status of STP for all VLANs.
We can limit the command’s output by specifying a particular VLAN. The command syntax for specifying a VLAN is “show spanning-tree vlan vlan_id.” The command output will display information about the role and status of each port on the switch.
The port role and state can be Root, Designated, alternate, etc. The command’s output also provides information about the bridge ID of the local switch, including the bridge ID of the root bridge.
Spanning Tree Failure Consequences
Two types of failure can occur with STP. In the first problem, the STP may block the wrong port planned in the forwarding state. This problem might be caused by lost traffic that would normally pass through this switch, but other networks remain unaffected.
The second type of failure is much more troublemaking, as shown in the Figure below. It happens when the Spanning Tree Protocol wrongly moves one or more ports into the forwarding state.
Master the art of troubleshooting Spanning Tree Protocol (STP) for a seamless network experience.
Recall that an Ethernet frame header does not contain a TTL field, so any frame that enters a bridging loop remains continuous, forwarding from switch to switch indefinitely.
The frames that have their destination address recorded in the MAC address table of the switches are simply forwarded to the port associated with the MAC address and do not enter a loop. But, any frame flooded by a switch enters the loop. The flooded traffic may include broadcasts, multicasts, and unicasts with a globally unknown destination MAC address.
What is the sign of STP failure? The load on all links starts increasing as more frames enter the loop. The frames also affect other links in the switched network because they flood them. Only the corresponding VLAN is affected if the failure is on a single VLAN. Switches and trunks that are not related to this VLAN operate normally.
The spanning-tree failure can create bridging loops. In this case, traffic is increasing exponentially, and the switches will flood the broadcasts out multiple ports. This creates copies of the frames each time the switches forward them.
When traffic like OSPF or EIGRP hello packets enters the loops, the devices running these protocols quickly become overloaded. Their CPUs quickly reach 100 percent utilization.
The network switches to change the MAC address table frequently. If a loop exists, a switch may see a frame with a particular source MAC address received on one port and another with the same source MAC address on a different port.
So, the switch will update the MAC address table twice for the same MAC address. Due to the high load and maximum CPU utilization, these devices become unreachable, making troubleshooting difficult.
Repairing a Spanning Tree Problem
The first method of resolving the problem is to remove redundant links in the switched network. The redundant link can be removed both physically and through configuration.
When the loops are removed and broken, traffic and CPU loads should quickly return to normal levels, and device connectivity should also be restored.
This restores the network troubleshooting, but this is not the end of the troubleshooting process. Because all redundant paths have been removed from the network, it needs to restore the redundant links.
If the problem of the spanning tree failure has not been fixed, there is a chance that a new broadcast storm will be triggered again during the restoration of the redundant links. So, before restoring the redundant links, find out and correct the original fault.
FAQs
What is the Spanning Tree Protocol (STP)?
Spanning Tree Protocol (STP) is designed to prevent network loops by creating a loop-free logical topology in Ethernet networks.
What are common issues with STP?
Common issues with STP include incorrect root port selection, spanning tree disabled, blocked VLANs, and topology changes causing network instability.
How can I troubleshoot STP issues?
To troubleshoot STP issues, verify physical connections, check configuration settings, use diagnostic commands like show spanning-tree, and ensure proper STP features are enabled.
What diagnostic commands are useful for STP troubleshooting?
Useful diagnostic commands include show spanning-tree, show spanning-tree vlan, and show interface to view the state of STP and root port elections.
How can I prevent STP issues?
Prevent STP issues by enabling features like Unidirectional Link Detection (UDLD), Loop Guard, PortFast, and BPDU Guard and by minimizing the number of blocked ports.
I hope this meets your needs. Let me know if there’s anything else you’d like to discuss!
The default configuration of the spanning-tree protocol (STP) on a Catalyst 2960 Series switch is PVST+. A Catalyst 2960 series switches support PVST+, Rapid PVST+, and MST, but only one version can be active for all VLANs. Rapid PVST+ is the Cisco execution of RSTP supporting RSTP on a per-VLAN basis.
We can control the configuration of the VLAN spanning-tree instance using the Rapid PVST+ commands. A spanning-tree instance is created while an interface is assigned to a VLAN.
When all the interfaces are removed from the VLAN or moved to any other VLAN, the instance is removed. The STP switches and port parameters can be configured before a spanning-tree instance is created. These configured parameters are applied when a spanning tree instance is created.
Spanning-Tree Mode Configuration
The command for configuring the Rapid PVST+ on a Cisco switch is “spanning-tree mode rapid-pvst” The command can be used in global configuration mode. We can use the same command to change the spanning-tree mode, i.e., PVST, MST, etc.
We can The show spanning-tree vlan <vlan_number> command to show the spanning tree configuration for a specific VLAN. We can also use the show running-config command to verify the Rapid PVST+ configuration.
The point-to-point link-type parameter for Rapid PVST+ is not necessary because it is abnormal to have a shared link-type. The difference between configuring PVST+ and Rapid PVST+ is the spanning-tree mode rapid-pvst command. The command for the link-type parameter is “spanning-tree link-type point-to-point”
FAQs
What is Rapid Per-VLAN Spanning Tree (Rapid PVST)?
Rapid PVST is an updated Spanning Tree Protocol (STP) implementation that provides a separate spanning tree for each VLAN, ensuring loop-free network topology and faster convergence.
How does Rapid PVST improve network performance?
Rapid PVST improves network performance by quickly adapting to topology changes, minimizing downtime, and ensuring efficient traffic flow across multiple VLANs.
What are the key components of Rapid PVST?
The key components include determining the root bridge, electing root ports, and assigning port roles such as designated, non-designated, alternate, and backup ports.
How do you configure Rapid PVST on a network switch?
Configuration involves setting the root bridge ID, configuring port priorities, and adjusting the path cost method to ensure optimal spanning tree topology for each VLAN.
I hope this meets your requirements! If there’s anything else you need, just let me know.
Network load balancing is the technique to manage traffic across a network without the use of complex routing protocols such as BGP.
Network Load Balancing divide load across multiple links, which use the network resources more efficiently and avoid network overload. Load balancing may be accomplished through software or hardware.
Now, look at the topology in the Figure below. There are three switches with 802.1Q trunks connecting them. Two VLANs are configured in the network, VLAN 100 and 200.
The purpose is to configure Switch-1 as the primary root bridge for VLAN 100 and the secondary root bridge for VLAN 200 and Switch-2 as the primary root bridge for VLAN 200 and secondary root bridge for VLAN 100.
Port F0/0 on Switch-3 is the forwarding port for VLAN 100 and blocking the port for VLAN 200. Port F0/1 on Switch-3 is the forwarding port for VLAN 200 and the blocking port for VLAN 100.
We have also planned the secondary root bridge for both VLANs in case of fault of the primary VLAN. A secondary root bridge becomes the root bridge for a VLAN if the primary root bridge going fails.
If other bridges in the VLAN keep their default STP priority, this switch configured as a secondary root bridge becomes the primary root bridge if the primary root bridge fails. In the topology, Switch-1 is the primary bridge for VLAN 100 and the secondary root bridge for VLAN 200. So, the configurations of the switch-1 are as under:-
After configuring the switch-1, configure the switch-2. The switch-2 is the primary root bridge of the VLAN-200 and secondary root bridge for VLAN-100. The configurations of the switch-2 are is under.
We can also configure the root bridge using the bridge priority specifying the root bridge is to set the spanning-tree priority on each switch to the lowest value so that the switch is selected as the primary bridge for its associated VLAN.
We can set switch priority for any spanning-tree instance. A lower value increases the chances that the switch is selected. The range is 0 to 61,440 in increments of 4,096. The configuration of root bridge priority we have done in the method of bridge priority ID configuration.
Now examine the topology, the Switch 1 is configured as the primary root bridge for the VLAN-100 and secondary root bridge for VLAN-200, and Switch-2 is configured as the primary root bridge for VLAN-200 and secondary root bridge for VLAN 100.
We can verify the configuration using show spanning-tree activecommand which displays spanning tree configuration details for the active interfaces only. Another command we can use the show spanning-treecommand.
The switch port directly connected to an end station cannot create bridging loops. So, the port can rapidly enter the forwarding state by skipping the listening and learning state. Portfast is a Cisco proprietary solution that allows the switch port to skip the listening and learning state and immediately enter the forwarding state. It is working in PVST+ environments.
We can use PortFast on access ports to allow the immediate connection of the attached devices to the network rather than waiting for IEEE 802.1D STP to converge on each VLAN. Access ports are usually connected to a single workstation or a server. A switch-enabled port fast cannot generate a topology change and notification for an interface with Port Fast enabled.
In a Port Fast configuration, BPDUs should never be received because receiving BPDUs indicates that another bridge or switch is connected to the port. Receiving BPDUs causes a spanning tree loop. The BPDU guard is a feature called BPDU guard. BPDU guard is Cisco proprietary, and it puts the port in an error-disabled state on receipt of a BPDU.
Port Fast is functional for Dynamic Host Configuration Protocol (DHCP). If Port Fast is not enabled on the switch, the host can send a DHCP request before the port is in the forwarding state, which prevents the host from receiving an IP address and related information from the DHCP server. If PortFast is enabled, the port state changes immediately to the forwarding state, and the host can get a usable IP address.
We can enable Port Fast on a switch port using the command spanning-tree portfast in interface configuration mode. If we want to configure Port Fast for all router interfaces, we can use the command spanning-tree portfast default in global configuration mode.
The command enables Port Fast on all non-trunking interfaces. The figure below illustrates the Port Fast configuration on the switch-3 interface fa0/2.
To configure BPDU guard on an access port, use thespanning-tree bpduguard enablecommand in interface configuration mode. If we want to configure the BPDU guard on all interface use the command spanning-tree portfast bpduguard defaultin global configuration mode. The command enables BPDU guard on all PortFast-enabled ports.
We can verify the PortFast and BPDU guard enabled for the switch port using the show running-config command. By default, PortFast and BPDU guard are disabled on all interfaces.
