Ccnp route study ospf drbdr elections and how to rig them. It is possible to configure multiaccess networks on something like framerelay, and if a fullmesh is not provided, the default ospf drbdr election process wont work reliably if you do not have a full mesh. Open shortest path first ospf is a dynamic, hierarchical routing protocol designed to support routing in tcpip networks. The router with the next highest priority becomes the new bdr. First discard any routers that are ineligible to become designated routers, such as routers with the ospf priority set to 0 the default is 1. Advanced singlearea ospf configurations drbdr election process. Slave routers only become full neighbours with the dr and bdr, remaining at the 2way state with the other nondrbdr routers. Ospf drbdr another concept that requires some explanation is the election of designated router dr and backup designated router bdr elected on broadcast and nbma networks such as framerelay, atm, x. Configure ospfv2 interface priority to determine the dr and bdr in part 3, you will configure router interface priority to determine the dr bdr election, reset the ospfv2 process, and then verify that the dr and bdr routers have changed ospf interface priority overrides all other settings in determining which routers become.
Verifying drbdr adjacencies to verify the ospfv2 adjacencies, use the show ip ospf neighbor command. It also sets a wait timer with a value equal to the routerdeadinterval. It is the duty of the dr to share routing updates with the rest of the routers in the network. If there is not, then an election is held for drbdr selection after the drbdr have been elected, the other router known as drothersestablish adjacencies with dr and bdr only neighbors are still tracked on multicast address. Red font color or gray highlights indicate text that appears in the instructor copy only topology. Objectives modify the ospf interface priority to influence the designated router dr and backup designated router bdr election. For more information on the dr election process, refer to dr election.
In the event that the dr fails, the bdr can smoothly take over. The role of the dr is critical, therefore a bdr is elected to shadow the dr. First omit any routers that have declared themselves as a dr in their hello. If set to 0, the router is ineligible for the election. Priority is a configurable option at the interface level. If there is a router that has a higher opsf priority that comes up after the election has taken place, it will not become dr or bdr until the dr and bdr fail. It is a role that is valid on per link basis broadcast and nbma. The router id is chosen depending on what is configured on the router.
It also starts a wait timer of the value of the dead interval. Then at the end of twoway state drbdr election is done. Packet tracer determining the dr and bdr cisco networking. Slave routers only become full neighbours with the dr and bdr, remaining at the 2way state with the other non dr bdr routers. During the dr and bdr election process, the dr will be the one with the highest interface priority. Election based on interface priority and routerid priority 0 255 higher better 0 never routerid highest loopback interface ip can be statically set higher better. Packet tracer determining the dr and bdr instructor version instructor note. Then at the end of twoway state dr bdr election is done. But i created as a serial interfaces without using the nbma there is no election process.
In this state, the routers and their dr and bdr establish a. In another lesson im going to teach you drbdrfor now just hold the thought that the drbdr election happens right after the twoway state ok. That is because a dr bdr election does not take place till the existing dr bdr leaves the network. Red font color or gray highlights indicate text that appears in the answer copy only. Jun 11, 2017 by default, all ios ospf routers are assigned a dr priority of 1.
Jun 02, 2011 yes great but you are using the fastethernet interface there elected dr bdr election process dr bdr elected as 224. This also means that route updates are only shared between the routers and the dr and bdr. Existing interfaces on a multiaccess network record the addresses of the dr and the bdr in the interface data structure. We need to clear ospf process for another dr bdr election,which is not good in a production enviroment. If there is not, then an election is held for dr bdr selection after the dr bdr have been elected, the other router known as drothersestablish adjacencies with dr and bdr only neighbors are still tracked on multicast address. Here is a detailed list of what happens in the dr bdr election. The router with the highest priority on the data link. Cisco networking academy mind wide open dr and bdr elections instructor version instructor note. Dr bdr election dr bdr chosen through election process election based on interface priority and routerid priority 0 255 higher better 0 never routerid highest loopback interface ip can be statically set higher better uses wait timer to stop preemption of current drbdr. Please explain the concept of ospf dr bdr election.
Exstart once the dr and bdr are elected, the actual process of exchanging link state information can start between the routers and their dr and. The election process will be described in a later section. If the priority of router c is changed to 255, then it will become the dr. This has to happen before we can continue with the rest of the process. Drbdr hence, ospf elects a designated and backup designated router for broadcast networks adjacencies only formed with dr and bdr lsas sent only to dr bdr listens 224. The election process is discussed later in the chapter.
Ties among routers with equal dr priorities are broken by router id, with the highest rid being preferred. If the link we are using is a multiaccess network ospf has to elect a dr designated router and bdr backup designated router. When enabling ospf on r2, it would have got the rid of 10. We can influence the dr election process by administratively. You can use this parameter to help bias one router as the dr. If priority is same, then the router with highest ip. In order to determine a dr and bdr the steps below are followed. Backup designated router an overview sciencedirect topics. If theyre in the same segment, all the routers that form part of that segment will participate in a single drbdr election process. Ilmll clsco cisco networking academy mind wide open.
In an ipv6 network, if there are no ipv4 addresses configured on the router, then the router id must be manually configured with the routerid rid router configuration command. After the end of wait period dead interval 40 sec, router considers himself as bdr and then moved to dr. As you can see r3 is still the dr, we need to reset the ospf neighbor adjacencies so that well elect the new dr and bdr. In summary, these routers are central to the ospf area or areas of routing devices. Yes great but you are using the fastethernet interface there elected dr bdr election process drbdr elected as 224. Ospf 100 interview questions ip with ease ip with ease. When an interface first becomes active on a multiaccess network, it sets the dr and bdr to 0. In this case, the bdr will immediately become the dr, and the election of the new bdr starts. Jul 11, 2017 during the dr and bdr election process, the dr will be the one with the highest interface priority. The objective of this lab exercise is for you to learn and understand how to debug ospf adjacencies. The serial links used in this lab are pointtopoint links, so no dr bdr election is performed. Ccnp route study ospf drbdr elections and how to rig.
Intially when we enable ospf on routers then they start sending hello messages to each other. By default both routers interface priority would have been 1 so the dr bdr election process tie breaker would have been the router id, and in this case r2 would have became the designated router as it has a higher router id number and r1 would have became the dbr. Once the dr and bdr are elected, the actual process of exchanging link state information can start between the routers and their dr. The bdr takes over should the dr fail, and then there is another election for the role of bdr. Igrp, eigrp, isis, rip, bgp, static or other instances of the ospf routing process are called autonomous system boundary router asbr.
By default both routers interface priority would have been 1 so the drbdr election process tie breaker would have been the router id, and in this case r2 would have became the designated router as it has a higher router id number and r1 would have became the dbr. Ospf interview questions and answers online networks solution. Uses wait timer to stop preemption of current drbdr. A simulation of the ospf election protocol shows three results. Objectives modify the ospf interface priority to influence the designated. Exstart once the dr and bdr are elected, the actual process of exchanging link state information can start between the routers and their dr. Linkstate packets there are different types of link state packets, those are. The router with the second highest ospf priority or router id will become a bdr. By default, all ios ospf routers are assigned a dr priority of 1. Lesson 40 ospf fundamentals part3 routerid and drbdr. According to the debug output on ra, which router was elected dr and which router was elected bdr.
Router a will become the dr and router d will become the bdr. Jan 06, 2014 the other routers become slaves to the master dr. If the current dr becomes unavailable, the current bdr becomes the new dr and a new election takes place to find another bdr. Thus, if a router with a higher priority is added to the network, it will not automatically supplant an existing dr. If the priority is not manually configured, there will be a tie on the default value of 1, so the router with the highest router id will be selected. Receiving a database descriptor dbd packet from a neighbor in the init state will also a cause a transition to 2way state. Configure ospfv2 interface priority to determine the dr and bdr in part 3, you will configure router interface priority to determine the drbdr election, reset the ospfv2 process, and then verify that the dr and bdr routers have changed ospf interface priority overrides all other settings in determining which routers become. The normal state for an ospf router is usually full. The serial links used in this lab are pointtopoint links, so no drbdr election is. If a new router with a higher priority is added to this network, it will become the dr.
The dr and bdr form ospf adjacencies with all of the other routers on the. Using debugging, you can identify issues that may be causing ospf to stop operating. Serial is pointtopoint access and dont need elect a focal point for updates. In the topology, r5 has been elected as the dr and r4 as the bdr due to having the highest router id values on the segment. Thus, a router that should never become the dr should always have its priority set to 0. The hello packet is also used in the election process of the designated router. Dec 08, 2017 router a will become the dr and router d will become the bdr. One major component of ospf is the usa of designated dr and backup designated routers bdr, selected through the ospf dr election. The drbdr election process not only occurs when the network first becomes active but also when the dr becomes unavailable. The devices that are neither the dr or bdr are referred to as drothers. Dbbdr election process router with the highest priority value is the dr, second highest is bdr in the event of tie router with the highest ip address on an interface become dr and second highest is bdr drbdr election can be manipulated by using routerid command. Default drbdr election process the routers in the network elect the router with the highest interface priority as the dr. If a dr loses connectivity to the network, the bdr will take its place. You can monitor the dr and bdr election process with a debug command.
Red font color or gray highlights indicate text that appears in the instructor copy only. That is because a drbdr election does not take place till the existing drbdr leaves the network. Ospf interview questions and answers online networks. Please explain the concept of ospf dr bdr election practically. Since there must always be a dr on any given subnet, if no router is dr or bdr for any given subnet, an election process takes place to select the dr first and then a second election is held for the bdr. Topology addressing table device interface ip address subnet mask ra g00 192. Dr election building the adjacency adjacencies on pointtopoint interfaces adjacencies on nonbroadcast multiaccess nbma networks avoiding drs and neighbor command on nbma pointtopoint subinterfaces selecting interface network types ospf and route summarization interarea route summarization external route summarization stub areas.
Upon the segment each router will go through an election process, to elect a dr and bdr. Ospf dr and bdr election solutions experts exchange. Always verify whether the local router is fulfilling the dr or bdr role with show. After a bdr is promoted to dr, a new bdr election occurs and the drother with the higher priority or router id is elected as the new bdr. If all of the routers on the multiaccess network have not finished booting, it is possible that a router with a lower router id becomes the dr.
Priority router with the highest wins the election. By default, 40 seconds for pointtopoint link and 120 seconds for nbmabroadcast links 4bytes designated routerthe ip address of the current dr. Once an ospf becomes active on a multiaccess network, it sets the dr and bdr values to 0. One way to force a reelection is to restart the ospf process on the current dr and bdr. Debugging ospf adjacencies is a fundamental troubleshooting skill. There are two rules used to determine who is elected. We need to clear ospf process for another drbdr election,which is not good in a production enviroment. Ospf elects a dr to be the focal point of all linkstate updates and lsas. I configured ospf on that router and i started debug message. The dr bdr election process not only occurs when the network first becomes active but also when the dr becomes unavailable. This can be a lowerend router that takes less time to boot. One major component of ospf is the usa of designated dr and backup designated routers bdr, selected through the ospf dr election process. You can influence the dr and bdr election process by manually configuring the ospf priority.