OSPF DR

Answered Question
Sep 25th, 2010
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I was asked this question during an interview. Just would like to verify whether my answer is correct.

Q: How many DRs in an area?

A: 1 DR per vlan or per subnet.

Thx,

Jingyi

Correct Answer by Peter Paluch about 6 years 8 months ago

Jingyi,


First of all, a DR for a multiaccess network can be elected only from among routers that are directly connected to that network. The routers that are not directly connected to that multiaccess network have nothing to do with that or with DR elections for that segment. Therefore, we say that the DR is not a function of the entire router but rather a function of each of its interface. A router can be DR in one multiaccess network, a BDR at the same time in some other network, and in yet some other network, it may be DROTHER (i.e. not DR nor BDR). Okay?


Now, I assume that you are asking about the following topology:


Ethernet <---> Router1 <---> Point-to-Point <---> Router2 <---> Ethernet


If there are 5 VLANs on each Ethernet then there are 10 VLANs in total. Each of these VLANs has to have its own DR. As the Ethernet VLAN is considered to be a multiaccess-type network, there will be 10 DRs in this area (note that both the Router1 and Router2 will be DRs for the VLAN they are connected to because they are the only router on that VLAN so far so they will be automatically elected as DR). Both Router1 and Router2 will be 5-fold DRs - for each VLAN they are connected to.


However, if the situation was different, like this:


Router1 <---> Common backbone network

Router2 <---> Common backbone network

Router3 <---> Common backbone network


and all other interconnections in this network, including to routers not displayed here, were only point-to-point and no other multiaccess network was present, there would be only a single DR in this area because there is only a single multiaccess network here. The DR would be elected from among Routers 1-3 first depending on their priority and the according to their Router ID (the highest wins).


Best regards,

Peter

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Peter Paluch Sat, 09/25/2010 - 23:13
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  • Cisco Employee,

Hello,


I am sorry but this is not correct. DR and BDR routers are elected on each multiaccess type of network which is by default Ethernet, Frame Relay, ATM, 802.11a/b/g/n, X.25 and possibly others. There are as many DRs in an area as how many multiaccess segments are located in the area. The DR has special responsibilities in OSPF regarding the multiaccess network to which it is connected:


  1. It topologically represents the multiaccess segment. As each router in link-state protocol describes itself and its connections to neighboring objects (other adjacent routers and networks), the same must be done for the multiaccess network itself. However, a "network segment" itself does not speak OSPF so some router must act on behalf of that segment, and this role is assigned to the DR.
  2. The DR also orchestrates the exchange of routing information on a segment and helps to synchronize the link-state database of new routers that come onto a segment and need to have their databases synchronized for the first time.


The link you have provided is unfortunately confusing the term "multiaccess network" with the term "area" and is actually misleading.


Best regards,

Peter

shelley_wang Sun, 09/26/2010 - 16:37
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Hi Peter,

Thanks so much for your detailed explanation. To enhance understanding, I would like to give an example.

An area is consisted of 2 Ethernet networks, which are interconnected by 1 point-to-point network. If there are 5 vlans inside each Ethernet network, will there be 5 DRs?

Thx,

Jingyi

Correct Answer
Peter Paluch Sun, 09/26/2010 - 23:50
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  • Cisco Employee,

Jingyi,


First of all, a DR for a multiaccess network can be elected only from among routers that are directly connected to that network. The routers that are not directly connected to that multiaccess network have nothing to do with that or with DR elections for that segment. Therefore, we say that the DR is not a function of the entire router but rather a function of each of its interface. A router can be DR in one multiaccess network, a BDR at the same time in some other network, and in yet some other network, it may be DROTHER (i.e. not DR nor BDR). Okay?


Now, I assume that you are asking about the following topology:


Ethernet <---> Router1 <---> Point-to-Point <---> Router2 <---> Ethernet


If there are 5 VLANs on each Ethernet then there are 10 VLANs in total. Each of these VLANs has to have its own DR. As the Ethernet VLAN is considered to be a multiaccess-type network, there will be 10 DRs in this area (note that both the Router1 and Router2 will be DRs for the VLAN they are connected to because they are the only router on that VLAN so far so they will be automatically elected as DR). Both Router1 and Router2 will be 5-fold DRs - for each VLAN they are connected to.


However, if the situation was different, like this:


Router1 <---> Common backbone network

Router2 <---> Common backbone network

Router3 <---> Common backbone network


and all other interconnections in this network, including to routers not displayed here, were only point-to-point and no other multiaccess network was present, there would be only a single DR in this area because there is only a single multiaccess network here. The DR would be elected from among Routers 1-3 first depending on their priority and the according to their Router ID (the highest wins).


Best regards,

Peter

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