BGP vs OSPF bs EIGRP (EGP vs IGP)

[Umesh Shetty]

Hi All,

Whenver there is a question asked on wht BGP is perferred on the internet as opposed to other routing protocols one reason I usually here is that BGP can process huge number of routes and SPF and DUAL cannot. Can someone please help me understand in detail why is it difficult for the SPF or DUAL process to process huge number of routes which BGP does so easily ?

Thanks in Advance

Regards

Umesh Shetty       

[mvsheik123]

Hi Umesh,

Iam not sure on number of router parts but one reason- due to its best path selection algorithm (including loop avoidance) mechanism existance when compared to other IGP. lets see if experts can shed more light on your query.

Thx

MS

[Joseph W. Doherty]

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Posting

OSPF and EIGRP likely do more "computation" per "route" than BGP and they probably have much worse exponential computation growth, so it's a scalability issue.

For details of why there's a disparity, you would need to analyze the path selection algorithm and what their mathematical computation work formula is.  For instance, there's a huge difference between N squared vs. N log N as N increases.

[Umesh Shetty]

Hi Joseph,

Can you please help me with a few more stromg reasons that would justify the use of BGP over other IGP's on the internet.

Thanks in Advance

Regards

Umesh Shetty 

[vmiller]

EIGRP is proprietary, so we can just rule that out.

BGP assumes some level of physical connectivity, and at least by some folks considered a routing by policy mechanisim as opposed to a routing by link state/cost.

OSPF would struggle to scale, for example how would you define area0 area xxx for something as dynamic as the internnet

[Joseph W. Doherty]

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The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

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Posting

EIGRP is proprietary, so we can just rule that out.

Not as it was.  See http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6554/ps6599/ps6630/qa_C67-726299.html

[Joseph W. Doherty]

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

Other than BGP scalability, the other notable feature of BGP is route policy management.  IGPs like OSPF and EIGRP pretty much treat all subnets alike, they just compute what the consider the best path between networks.  BGP, though, lends itself to "doing things" with routes.  What you might accept, what you'll pass along, how you will weight them, specific paths for specific networks, etc.

It's not uncommon to see even fairly large OSPF/EIGRP networks without any route policy management, but it's fairly common to see some kind of route policy management with BGP, except in the simplest usages.