OSPF DR/BDR and broadcast multi-access real-world examples

Answered Question
Feb 18th, 2010

CCNA tought us OSPF DR/BDR using an example with three routers connected to a switch. Also, the curriculum insinuates that there might be dozens of routers on a broadcast multiaccess segment.

Why would we want to connect all those routers to a switch?

Are there any real-world examples of such topologies?

Thanks

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Correct Answer by Giuseppe Larosa about 6 years 9 months ago

Hello John,

some service provider designs of POP  use two backbone vlans where they connect multiple devices, some are the inner core other can be used for specific services.

The result can be an high count of routers in each Vlan

I don't want to say it is a recommended design but it has been used.

The limits of this design came out when on the same broacast domain there were devices with very different speeds  from 100 Mbps to 10GE.

controlling multicast traffic on this was a real challenge.

Hope to help

Giuseppe

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Giuseppe Larosa Thu, 02/18/2010 - 08:59

Hello John,

>>yes there are examples of LANs with 30/40 OSPF routers on it

the most dramatic fact is that usually OSPF DR is the device with the greatest uptime that may be different from the desired one

Hope to help

Giuseppe

johnnyd110 Thu, 02/18/2010 - 09:16

Hi Giuseppe,

you say that there are LANs with 30/40 routers on it. I believe you, but I would like to know why would we use such topologies? What is the application? What are those routers used for?

Thanks a lot

Correct Answer
Giuseppe Larosa Thu, 02/18/2010 - 10:03

Hello John,

some service provider designs of POP  use two backbone vlans where they connect multiple devices, some are the inner core other can be used for specific services.

The result can be an high count of routers in each Vlan

I don't want to say it is a recommended design but it has been used.

The limits of this design came out when on the same broacast domain there were devices with very different speeds  from 100 Mbps to 10GE.

controlling multicast traffic on this was a real challenge.

Hope to help

Giuseppe

johnnyd110 Fri, 02/19/2010 - 01:47

Thanks Giuseppe, that was the info I needed.

I knew the DR/BDR thing in theory, but I couldn't have imagined any meaningful application.

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