سؤال بخصوص الـ OSPF DIR/BDR

[Cisco Noob]

السلام عليكم ورحمة الله وبركاته

أنا مشترك جديد معاكم إن شاء الله أفيد وأستفيد.

لاحظت أنه فيه كثير من الأخوة الذين نجحوا في إمتحان الـ CCNA، ما شاء الله هذا شئ يسعدني والله، أتمنى منهم ومن كل من يعرف أن يرد على سؤالي لأنه حيّرني صراحة وصارلي كم يوم بفكر فيه وما قدرت أوصل لنتيجة مقنعة.

السؤال بخصوص إختيار الـ DR والـ BDR أثناء إستعمال بروتوكول الـ OSPF

إذا عندنا 3 روترات

Rtr1 192.168.0.1 Physical Address

Rtr2 192.168.0.6 Physical Address

Rtr3 1.1.1.1 Loopback Address

أي منهم سيكون الـ DR وأي منهم سيكون الـ BDR إذا كانت الـ Priority متساوية، نقول 1 مثلاً.

الذي أعلمه أنه في الروتر نفسه إذا كان فيه عنوان فيزيكال ولوب باك فإن اللوب باك يأخذ أولوية سواء كان الاي بي صغير أصغر من الفزيكال أو أكبر، وهو يصبح الـ RID يعني Router ID ولكن أنا سؤال هو على السغمنت (نتورك)، هل هذه القاعدة نفسها تعمل، يعني الروتر الذي عنده لوب باك بعنوان أصغر ياخذ أولوية على روتر بعنوان فزيكال أكبر؟

إن كانت نفس القاعدة تطبق فهذا يعني أن الـ Rtr3 هو الذي سيكون الـ DR، إذا لا فإذن الـ Rtr2 هو الـ DR

[amna_ali]

OSPF DR and BDR election is based on the higher Router ID if priority is equal.
The router ID = the highest loopback address
If there is no loopback address router ID = the highest physical address.

In your question
Rtr 1 ID = 192.168.0.1
Rtr 2 ID = 192.168.0.6
Rtr 3 ID = 1.1.1.1

Therefore DR = 192.168.0.6
BDR = 192.168.0.1

Notice that DR and BDR election process is not repeated when new address is configured. For example if DR = 192.168.0.6 and BDR 192.168.0.1 and you configure new loopback address in RT1 = 192.168.255.255 the DR and BDR will not change until you clear OSPF process or restart the router. Loopback addresses are used because they are logical addresses that they don’t shut down unlike physical addresses.

[lumark_s]

In this CASE

The DR is Rtr3 as its got the highest loopback address

The BDR is Rtr2 as its got the second high Physical address

and yes this is the rule for electing the DR & the BDR

as amna_ali said, this process wont be repeted if a new loopback address found on the segment, we should clear the OPSF process inorder to force the new election to happen

[Cisco Noob]

Thanks to sister Amna and Lumark for their reply.

Sister Amna is saying one thing and Lumark says another thing

sis Amna:

Therefore DR = 192.168.0.6
BDR = 192.168.0.1

Lumark:

The DR is Rtr3 as its got the highest loopback address

The BDR is Rtr2 as its got the second high Physical address

This is a very basic question but it got me a little confused.

I believe that sister Amna is correct, but the only thing that's making me doubt her answer is due to the fact that loopback addresses advertise themselves as hosts with a /32 over the segment which is different then the normal physical addresses.

I still believe that the sister is correct.

Would appreciate your elaboration.

Many thanks.

[amna_ali]

I just wanted to clear out that the DR is elected based on highest ROUTER ID not highest loopback address. In this case the router has the highest router ID is Rtr2 not Rtr3.

I refer to section -3.4.4 'Selecting the DR and BDR' - CISCO Network Academy Program BCSI v5.0

To elect a DR and BDR, the routers view the OSPF priority value of the other routers during the hello packet exchange process and then use the following conditions to determine which router to select:
· The router with the highest priority value is the DR.
· The router with the second-highest priority value is the BDR.
· The default for the priority is 1. In the case of a tie, the router ID is used. The router with the highest router ID becomes the DR. The router with the second-highest router ID becomes the BDR.

Note: The highest IP address on an active interface is normally used as the router ID. However, you can override this selection by configuring an IP address on a loopback interface or using the router-id router configuration command

[Cisco Noob]

I just wanted to clear out that the DR is elected based on highest ROUTER ID not highest loopback address. In this case the router has the highest router ID is Rtr2 not Rtr3.

I refer to section -3.4.4 'Selecting the DR and BDR' - CISCO Network Academy Program BCSI v5.0

To elect a DR and BDR, the routers view the OSPF priority value of the other routers during the hello packet exchange process and then use the following conditions to determine which router to select:
· The router with the highest priority value is the DR.
· The router with the second-highest priority value is the BDR.
· The default for the priority is 1. In the case of a tie, the router ID is used. The router with the highest router ID becomes the DR. The router with the second-highest router ID becomes the BDR.

Note: The highest IP address on an active interface is normally used as the router ID. However, you can override this selection by configuring an IP address on a loopback interface or using the router-id router configuration command

Jazakom Allah Khayr sister, and thanks for the clarification

[lumark_s]

but you saying that Rtr3 got the loopback address as its ID, and in OSPF election if there is a loopback address then it will be used rather than the physical address and its taking in concedreration the router ID in top of all those elections

thats what you saying Cisco Noob

[Cisco Noob]

Lumrak

I asked a question whether the loopback ip address takes precedence over the physical address over the segment and not on the box (router)

To elect a router ID or as they call it RID, the loopback address takes precedence no matter what IP address the physical one is.

Now, once the RID is elected on the routers, OSPF will decide which one among them is the DR by comparing their IP addresses given that the priority is the same and not 0, the router with the highest RID (ip address) wins the process and it becomes the DR, the second highest one becomes the BDR.

I hope this is clear now my dear brother.

[lumark_s]

Dear i understand the theory behinde OSPF and the election process but what i cant understand in ur example is that you said that Rtr3 ID is the loopback address right? then you said which one gonna be elected as a DR & BDR, so look to your example and tell me what is it !!!?? if you say that the Rtr3 is using his loopback address as a router ID and Rtr1 and Rtr2 both using thier Physical interfaces IP address as their router ID, and based on tge fact that OSPF election more toward any loopback if the Priority is equal, which one gonna be elected as DR and which one gonna be elected as a BDR ??can i know

[Cisco Noob]

Rtr2 DR
Rtr1 BDR

[المهندس الرائد]

ما خرجنا باتفاق بين الطرفين ..

مين الصح يا جماعة ؟؟

[ah8384]

السلام عليكم

حسب فهمي للOSPF, فإن كلام أخونا lumark_s هو الصحيح.

Rtr3 = DR
Rtr2 = BDR

اخوكم عبدالمجيد:frnd"

[Cisco Noob]

أشكر جميع الأخوة والأخوات الذين شراكوا في هذا الموضوع، أرجو من الأخ Lumark وكل من يتفق معاه أن يعطينا مصدر يستندون عليه كما فعلت الأخت Amna_Ali

تحياتي لكم.

[amna_ali]

Also can I ask brothers ah8384 and Lumark what are the routers IDs for the three routers in their opinion?