J.T. Jansson*
* Oulu University of Applied Sciences, School of Engineering, Oulu, Finland
Abstract because there is more than one path to the destination and because the switch or the hub forwards the data to The availability must be considered when allports. [6] building a network. Business runs all day, every day and even in off hours and so reliable network The solution to looping is to use some protocols availability is required all the time. This is referred like spanning tree protocol (STP) that block some ports to five nines (99.999) uptime where the small on the switch and open others so that only one path is percentage of downtime is accounted for unforeseen existed to the destination. The concept of looping and incidents, or ‘scheduled maintenance’. Fast STP are applied only to devices connected through a convergence around link or component failures is a LAN and not to routers. [6] must with growing technologies such as Voice over Device failure occurs when a network device fails IP (VoIP) and Video over IP. In this paper the and thus is unable to forward the packets. The device purpose and implementation of network redundancy of IP traffic in the mobile network is discussed. failure can have a big impact on the network if it occurs in the core layer of the network which connects the Keywords: network redundancy, IP traffic, link failure, whole network together. Therefore the solution is network device failure, STP, VRRP suggested to be applied at the core layer where two network devices (2N) are connected to the network to 1. Introduction forward the data. One network device is the primary and 2. the other is the secondary. If primary device fails, the The mobile network must be operable during the secondary becomes available. [6] [3] whole day regardless of failures and thus some The above technique is configured on the device. techniques must be taken to make the network available Some protocols like Virtual Router Redundancy all the time. The possible failures that might occur Protocol (VRRP) are developed to accomplish this during the network operation are categorized into two function. When implementing it, one must connect the types: link failure and network device failures. Some device at the core which must be redundant to the methods how to overcome these failures are studied. proper ports of other devices and connect similar device One of the ways to increase availability is to provide to the rest of the network in the same manner the redundancy for critical components when building primary device is connected. If the primary fails, this networks. This usually involves duplicating routers, secondary device takes it’s role. [6] [3] switches and links to ensure continuity of service across failures. Routing protocols are used to keep the network running despite the network problems. [3] 4. Spanning tree protocol (STP) Spanning tree protocol (STP) is Data Link Layer 3. Possible network failures and methods to protocol. STP implements the 802.1D IEEE algorithm overcome them by exchanging BPDU messages with other switches to detect loops, and then removes the loop by shutting The solution in link failures is to provide multiple down selected bridge interfaces. This algorithm links between devices such that when a link is down, guarantees that there is one and only one active path other link takes its role. This can lead to problems between two network devices. [1] because when there are more than one link connecting the devices the data will find multiple links and the switch will forward the data to multiple links and the receiving end will receive the data more than one time. Also looping can occur because the data will be forwarded across the links forever. This will occur 4.1 STP algorithm learning states of STP. The default is 15 seconds, which means that out of the box we spend 15 seconds in listening and 15 seconds in learning. [2]
3.3 STP states The different states of STP are as follows: Blocking — In the blocking state the port is essentially shut down. The switch discards frames received on the interface. It will receive BPDUs from the DP on the segment but will not pass them along to other switches. A switch will go through the blocking state when it is first initialized (boots up) and it will place ports that could cause L2 loops into blocking when necessary. The blocking state is typically only seen during indirect link Picture 1 L2 switching diagram failures.[2] There are four steps in STP algorithm: Listening — In listening state the port is starting to transition into doing something. In First there is root election. In this case Cat2 is this state, the switch will actually process the the root bridge because we have manually BPDUs it receives on the port although we are given it the lowest priority. By receiving still discarding frames at this point. Note that superior BPDUs from other switches they all per the RFC Listening and Learning MUST be eventually agree on who is the root bridge. the same amount of time. [2] The root bridge is the bridge with the lowest BID. A BID is a priority appended to a MAC Learning — In the learning state the port address (See picture 1). [2] continues it’s transition by learning MAC addresses on the port, continuing to receive Each Non-Root bridge elects a root-port (RP) and process BPDUs, and transmitting BPDUs which is the port on that switch with the on to neighboring switches.[2] lowest cost path to the root bridge. In the event of a tie, they will go with lowest sending BID, Forwarding — In the forwarding state the port and finally lowest port-priority. [2] is up and running. At this point the port actually forwards frames and continues to On each segment, a designated port (DP) is monitor BPDUs. [2] elected. The DP is the port on that particular segment with the lowest cost path to the root Disabled — This isn’t really a state of STP. bridge. The DP has the responsibility of This means STP is essentially turned off. [2] sending BPDUs on to the segment. [2] 3.4 Example of Direct and Indirect link failure At the end of all this, if a port is not a RP or a In picture 1 can be that Fa0/23 on Cat3 goes DP, it is put into the blocking state. [2] into the blocking state to prevent an L2 loop from 3.2 STP timers occurring. In addition the link between Cat1 and Cat2 is shut down. This will be an example of an indirect link Hello Timer – This is how often the root failure from the perspective of Cat3 and a direct link bridge will send out BPDUs. These BPDUs failure from the perspective of Cat1. [2] get relayed down the spanning-tree to all the other switches. The default is 2 seconds. [2] Cat1: Max Age Timer – This is how often a bridge Cat1 lost it’s root-port and has no idea who the will actually save the BPDU information it root bridge is. Therefore, Cat1 advertises itself receives from other switches. Think of it as as the root bridge out fa0/21 towards Cat3 sort of a hold timer. The default is 20 seconds, immediately. [2] and it helps prevent against loops in the event After max-age expires over on Cat3, Cat3 of indirect link failures. [2] transitions Fa0/23 into listening mode which Forward-Delay — This determines how long a means it now forwards BPDUs from the path switch will spend in each of the listening and Cat2 –> Cat4 –> Cat3 over to Cat1. Cat1 realizes it is not the real root bridge and backup for both of these VRIDs. If rD or rF submits to Cat2 being the real root. [2] fails, rE will become the master for that VRID. Cat3: In fact, both rD and rF could fail at the same time; the fact that a VRRP router is a master When the Cat1/Cat2 link goes down Cat3 starts for one VRID does not preclude it from being receiving BPDUs from Cat1 who is now master for another. [4] claiming to be the root bridge. Cat3 will ignore Router rG is the WAN gateway for the these claims completely until the max-age Backbone LAN. All of the routers attached to timer expires. [2] the backbone are sharing routing information Cat3 transitions Fa0/23 from blocking into with the routers on the WAN using a dynamic routing protocol such as OSPF. VRRP is not listening after max-age expires. It learns it’s involved in this, although Router rC will new root-port is via Fa0/23 and awaits to move advertise that the path to the Client LAN it into learning and finally forwarding. [2] subnet is via the VIP of VRID 3. [4] 15 seconds after going into listening Cat3`s Router rH is the master of VRID 10 and Fa0/23 goes into learning. [2] backup for VRID 11. Router rJ is the master 15 seconds after going into learning Cat3`s for VRID 11 and the backup for VRID 10. This is a VRRP load-sharing configuration and it Fa0/23 goes into forwarding. [2] illustrates that multiple VRIDs can exist on a The total convergence time for the whole single router interface. [4] network here is 50 seconds. [2] VRRP can be used as part of a network design that 5. Virtual Router Redundancy Protocol (VRRP) provides almost total routing redundancy for all systems Virtual Router Redundancy Protocol (VRRP) is a in the network. [4] non-proprietary redundancy protocol described in RFC 3768 designed to increase the availability of the default gateway servicing hosts on the same subnet. VRRP introduces the concept of a “virtual router” that is addressed by IP clients requiring gateway service. The actual routing service is provided by physical routers running the VRRP protocol. An example of this is shown in Picture 2. Two or more physical routers are then configured to stand for the virtual router, with only one doing the actual routing at any given time. If the current physical router that is routing the data on behalf of the virtual router fails, an arrangement is made for another physical router to automatically replace it. In a VRRP configuration, one router is elected as the virtual router master, with the other routers acting as backups in case the virtual router master fails. [3] [4]
4.1 VRRP concepts
VRRP concepts are showed in Picture 2. Router rA is the master of virtual router VRID 1, and the backup for VRID 3. At this time, it handles the routing of packets addressed to the VIP for VRID1, and is ready to take on the routing role for VRID 3. [4] Router rB is the master of virtual router VRID Picture 2 VRRP concepts 3, and the backup for VRID 1. At this time, it handles the routing of packets addressed to the VIP for VRID3, and is ready to take on the routing role for VRID 1. [4] 4.2 Simple environment with VRRP Router rC does not have VRRP function, but uses the VIP for VRID 3 to reach the ClientLAN subnet. [4] Router rD is the master of VRID 2. Router rF is the master of VRID 5. Router rE is the over IP (VoIP) and Video over IP, fast convergence around link or component failures is a must. Because things break and unforeseen events do take place, there is the need for creating an architecture that is ‘highly available’.
8. References
[1 ] Cisco, spanning tree protocol introduction http://www.cisco.com/en/US/tech/tk389/tk621/tsd_tech nology_support_protocol_home.html Date of data acquisition 1 May 2011
Picture 3 Routing with VRRP [2 ] Joe Astorino, By configurating VRRP into environment, the Spanning-Tree Direct VS Indirect Link Failures redundancy can be provided for outgoing traffic. Available on the Internet In picture 3, Virtual IP (VIP) is configured as the http://blog.ipexpert.com/2010/03/22/spanning-tree- default gateway to linux guests (sA, sB, sC and sD). direct-vs-indirect-link-failures/ Now VRRP will provide a continuous router service Date of data acquisition 1 May 2011 across the two routers. [4] [3 ] 6. Results and Discussion Cisco IOS and NX-OS Software, Configurating VRRP One of the ways to increase availability is to Available on the Internet provide redundancy for critical components when http://www.cisco.com/en/US/docs/ios/ipapp/configurati building networks. This usually involves duplicating on/guide/ipapp_vrrp.html links, switches and routers to ensure continuity of Date of data acquisition 11 May 2011 service across failures. [4] [4 ] The 802.1D Spanning Tree Protocol (STP) standard Linux on IBM zSeries and S/390, was designed at a time when the recovery of Virtual Router Redundancy Protocol on connectivity after an outage within a minute or so was VM Guest LANs considered adequate performance. Rapid Spanning Tree Available on the Internet Protocol (RSTP; IEEE 802.1w) can be seen as an http://www.redbooks.ibm.com/redpapers/pdfs/redp3657 evolution of the 802.1D standard. [5] .pdf Dynamic routing protocols are used to keep the Date of data acquisition 12 May 2011 network running, routing traffic around network problems. It can be difficult to provide this level of [5] redundancy at the endpoints of the network. Due to Cisco, Understanding Rapid Spanning Tree Protocol prohibitive cost and duplication of horizontal cabling it (802.1w) is impractical to provide multiple network connections Available on the Internet for end-stations. Running dynamic routing protocols on http :// www . cisco . com / en / US / tech / tk 389/ tk 621/ technolo end-stations, to allow them to take advantage of gies _ white _ paper 09186 a 0080094 cfa . shtml # conclusion multiple network paths and/or multiple gateways, is not Date of data acquisition 14 May 2011 feasible due to the network overhead and resulting complexity of the routing environment. VRRP gives [6] network designers a way to provide reliable, redundant WindowsNetworking.com,The importance of network gateway service for IP end-stations. [4] redundancy Available on the Internet 7. Conclusions http://www.windowsnetworking.com/articles_tutorials/I mportance-Network-Redundancy.html Today’s businesses require reliable network Date of data acquisition 1 May 2011 connectivity. Switched networks must fulfill stringent robustness, resiliency, and high-availability requirements. With growing technologies such as Voice