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<Network redundancy of IP traffic of mobile network> J.T. Jansson* * Oulu University of Applied Sciences, School of Engineering, Oulu, Finland [email protected] Abstract The availability must be considered when building a network. Business runs all day, every day and even in off hours and so reliable network availability is required all the time. This is referred to five nines (99.999) uptime where the small percentage of downtime is accounted for unforeseen incidents, or ‘scheduled maintenance’. Fast convergence around link or component failures is a must with growing technologies such as Voice over IP (VoIP) and Video over IP. In this paper the purpose and implementation of network redundancy of IP traffic in the mobile network is discussed. Keywords: network redundancy, IP traffic, link failure, network device failure, STP, VRRP 1. Introduction The mobile network must be operable during the whole day regardless of failures and thus some techniques must be taken to make the network available all the time. The possible failures that might occur during the network operation are categorized into two types: link failure and network device failures. Some methods how to overcome these failures are studied. One of the ways to increase availability is to provide redundancy for critical components when building networks. This usually involves duplicating routers, switches and links to ensure continuity of service across failures. Routing protocols are used to keep the network running despite the network problems. [3] 2. Possible network failures and methods to overcome them The solution in link failures is to provide multiple links between devices such that when a link is down, other link takes its role. This can lead to problems 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 because there is more than one path to the destination and because the switch or the hub forwards the data to allports. [6] The solution to looping is to use some protocols like spanning tree protocol (STP) that block some ports on the switch and open others so that only one path is existed to the destination. The concept of looping and STP are applied only to devices connected through a LAN and not to routers. [6] Device failure occurs when a network device fails and thus is unable to forward the packets. The device failure can have a big impact on the network if it occurs in the core layer of the network which connects the whole network together. Therefore the solution is suggested to be applied at the core layer where two network devices (2N) are connected to the network to forward the data. One network device is the primary and the other is the secondary. If primary device fails, the secondary becomes available. [6] [3] The above technique is configured on the device. Some protocols like Virtual Router Redundancy Protocol (VRRP) are developed to accomplish this function. When implementing it, one must connect the device at the core which must be redundant to the proper ports of other devices and connect similar device to the rest of the network in the same manner the primary device is connected. If the primary fails, this secondary device takes it’s role. [6] [3] 3. Spanning tree protocol (STP) Spanning tree protocol (STP) is Data Link Layer protocol. STP implements the 802.1D IEEE algorithm by exchanging BPDU messages with other switches to detect loops, and then removes the loop by shutting down selected bridge interfaces. This algorithm guarantees that there is one and only one active path between two network devices. [1] 3.1 STP algorithm Forward-Delay — This determines how long a switch will spend in each of the listening and 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: Picture 1 L2 switching diagram There are four steps in STP algorithm: First there is root election. In this case Cat2 is the root bridge because we have manually given it the lowest priority. By receiving superior BPDUs from other switches they all eventually agree on who is the root bridge. The root bridge is the bridge with the lowest BID. A BID is a priority appended to a MAC address (See picture 1). [2] Each Non-Root bridge elects a root-port (RP) which is the port on that switch with the lowest cost path to the root bridge. In the event of a tie, they will go with lowest sending BID, and finally lowest port-priority. [2] On each segment, a designated port (DP) is elected. The DP is the port on that particular segment with the lowest cost path to the root bridge. The DP has the responsibility of sending BPDUs on to the segment. [2] At the end of all this, if a port is not a RP or a DP, it is put into the blocking state. [2] 3.2 STP timers Hello Timer – This is how often the root bridge will send out BPDUs. These BPDUs get relayed down the spanning-tree to all the other switches. The default is 2 seconds. [2] Max Age Timer – This is how often a bridge will actually save the BPDU information it receives from other switches. Think of it as sort of a hold timer. The default is 20 seconds, and it helps prevent against loops in the event of indirect link failures. [2] 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 failures.[2] Listening — In listening state the port is starting to transition into doing something. In this state, the switch will actually process the BPDUs it receives on the port although we are still discarding frames at this point. Note that per the RFC Listening and Learning MUST be the same amount of time. [2] Learning — In the learning state the port continues it’s transition by learning MAC addresses on the port, continuing to receive and process BPDUs, and transmitting BPDUs on to neighboring switches.[2] Forwarding — In the forwarding state the port is up and running. At this point the port actually forwards frames and continues to monitor BPDUs. [2] Disabled — This isn’t really a state of STP. This means STP is essentially turned off. [2] 3.4 Example of Direct and Indirect link failure In picture 1 can be that Fa0/23 on Cat3 goes into the blocking state to prevent an L2 loop from occurring. In addition the link between Cat1 and Cat2 is shut down. This will be an example of an indirect link failure from the perspective of Cat3 and a direct link failure from the perspective of Cat1. [2] Cat1: Cat1 lost it’s root-port and has no idea who the root bridge is. Therefore, Cat1 advertises itself as the root bridge out fa0/21 towards Cat3 immediately. [2] After max-age expires over on Cat3, Cat3 transitions Fa0/23 into listening mode which means it now forwards BPDUs from the path Cat2 –> Cat4 –> Cat3 over to Cat1. Cat1 realizes it is not the real root bridge and submits to Cat2 being the real root. [2] Cat3: 4. When the Cat1/Cat2 link goes down Cat3 starts receiving BPDUs from Cat1 who is now claiming to be the root bridge. Cat3 will ignore these claims completely until the max-age timer expires. [2] Cat3 transitions Fa0/23 from blocking into listening after max-age expires. It learns it’s new root-port is via Fa0/23 and awaits to move it into learning and finally forwarding. [2] 15 seconds after going into listening Cat3`s Fa0/23 goes into learning. [2] 15 seconds after going into learning Cat3`s Fa0/23 goes into forwarding. [2] The total convergence time for the whole network here is 50 seconds. [2] Virtual Router Redundancy Protocol (VRRP) Virtual Router Redundancy Protocol (VRRP) is a 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] 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 backup for both of these VRIDs. If rD or rF fails, rE will become the master for that VRID. In fact, both rD and rF could fail at the same time; the fact that a VRRP router is a master for one VRID does not preclude it from being master for another. [4] Router rG is the WAN gateway for the Backbone LAN. All of the routers attached to the backbone are sharing routing information with the routers on the WAN using a dynamic routing protocol such as OSPF. VRRP is not involved in this, although Router rC will advertise that the path to the Client LAN subnet is via the VIP of VRID 3. [4] Router rH is the master of VRID 10 and backup for VRID 11. Router rJ is the master for VRID 11 and the backup for VRID 10. This is a VRRP load-sharing configuration and it illustrates that multiple VRIDs can exist on a single router interface. [4] VRRP can be used as part of a network design that provides almost total routing redundancy for all systems in the network. [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 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] Picture 2 VRRP concepts 4.2 Simple environment with VRRP requirements. With growing technologies such as Voice 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’. 7. 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 By configurating VRRP into environment, the redundancy can be provided for outgoing traffic. In picture 3, Virtual IP (VIP) is configured as the default gateway to linux guests (sA, sB, sC and sD). Now VRRP will provide a continuous router service across the two routers. [4] 5. Results and Discussion One of the ways to increase availability is to provide redundancy for critical components when building networks. This usually involves duplicating links, switches and routers to ensure continuity of service across failures. [4] The 802.1D Spanning Tree Protocol (STP) standard was designed at a time when the recovery of connectivity after an outage within a minute or so was considered adequate performance. Rapid Spanning Tree Protocol (RSTP; IEEE 802.1w) can be seen as an evolution of the 802.1D standard. [5] Dynamic routing protocols are used to keep the network running, routing traffic around network problems. It can be difficult to provide this level of redundancy at the endpoints of the network. Due to prohibitive cost and duplication of horizontal cabling it is impractical to provide multiple network connections for end-stations. Running dynamic routing protocols on end-stations, to allow them to take advantage of multiple network paths and/or multiple gateways, is not feasible due to the network overhead and resulting complexity of the routing environment. VRRP gives network designers a way to provide reliable, redundant gateway service for IP end-stations. [4] 6. Conclusions Today’s businesses require reliable network connectivity. Switched networks must fulfill stringent robustness, resiliency, and high-availability [2 ] Joe Astorino, Spanning-Tree Direct VS Indirect Link Failures Available on the Internet http://blog.ipexpert.com/2010/03/22/spanning-treedirect-vs-indirect-link-failures/ Date of data acquisition 1 May 2011 [3 ] Cisco IOS and NX-OS Software, Configurating VRRP Available on the Internet http://www.cisco.com/en/US/docs/ios/ipapp/configurati on/guide/ipapp_vrrp.html Date of data acquisition 11 May 2011 [4 ] Linux on IBM zSeries and S/390, Virtual Router Redundancy Protocol on VM Guest LANs Available on the Internet http://www.redbooks.ibm.com/redpapers/pdfs/redp3657 .pdf Date of data acquisition 12 May 2011 [5] Cisco, Understanding Rapid Spanning Tree Protocol (802.1w) Available on the Internet http://www.cisco.com/en/US/tech/tk389/tk621/technolo gies_white_paper09186a0080094cfa.shtml#conclusion Date of data acquisition 14 May 2011 [6] WindowsNetworking.com,The importance of network redundancy Available on the Internet http://www.windowsnetworking.com/articles_tutorials/I mportance-Network-Redundancy.html Date of data acquisition 1 May 2011