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BGP concept question


I have a BGP concept related query. What is the special thing there in BGP which makes it capable of handling lakhs of routes. Can any other IGP like OSPF/ISIS tuned to handle any such high volume routes comparable to BGP?

thanks and regards

Ashish Panda


Re: BGP concept question

Hi there,

The main idea behind making a routing protocol scalable is how it converges and accordingly how many routes it can handle.

In BGP, Scalability is more important than fast convergence --> So BGP was designed to batch updates --> the flood of updates within the batch interval time is saved, and at the end of the interval only the remaining results are forwarded as the routing updates (batching = minimun time between updates) --> If a network flaps several times within the batch interval only the state of the network at the end of the interval is sent in the routing update --> The amount of updates is controlled via the batching feature --> avoiding an uncontrolled flood of updates --> enhance scalability in the cost of convergence.


Mohammed Mahmoud.

Re: BGP concept question

Hello Ashish,

With unlimited CPU and memory resources any routing protocol can handle any number of routes. But reality is limited.

So OSPF and ISIS can handle 200000 routes, BUT the resources required are much larger than with BGP, because the requirement towards an IGP is usually fast convergence.

But this is only one apect. Mainly BGP offers a lot of features and parameters allowing to filter unwanted stuff and to influence routing decisions. This requirement stems from the fact that BGP is used between AS, which do not always have a consistent routing policy.

OSPF and ISIS are based on an identical database within an area - no filtering possible and only limited capabilities to influence path selection.

Bottom line: different requirements in IP routing lead to different solutions.

Regards, Martin

Re: BGP concept question

One more major different, IGP routing protocols always care to find the best path through the fastest link (Lower number of hops, Lower cost) because IGP mainly used in LANs which by nature

using 10M links speed in worst cases, and this nature will not be effective in environment like internet.

also IGP routing protocols metrics are very limited to used to influence routing decision while BGP have rich metrics (Attributes) which give you the ability to do what you need under the BGP process without the needs to use more commands under all interfaces, Look for the BGP attributes and image the power you have :) finally IGP protocols have very limited filtering options on the other hand BGP has wonderful filtering options.

Please rate helpful posts.

Best Regards,

Mounir Mohamed

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