As we know BGP is an EGP type routing protocol and used for routing between multiple autonomous systems. But this requirement can also be achieve using IGP protocol then why still we need BGP?
Yes, we can do the routing between multiple autonomous system using IGP and redistribution and it also works fine. But it is only good if we are having 2 or 3 autonomous systems as well as these autonomus systems should not be too big. Because IGP protocols are not well scaled for handle the large amount of routes. Suppose if you want to receive complete global routing table from your ISP then IGP protocols can’t handle this table due to big size.
Apart of this we are also having many other true benefits of BGP.
Benefits
If we are having two links from same ISP and want to do separate using policies then it can be done easily using BGP.
If we are having multiple links from different ISPs and want to use all links as active / active then we can do it with BGP.
If we want to connect with internet and don’t want the routing only on default route or we are having a requirement of receiving complete global table from ISP. Then BGP is well scale for it and it can handle complete global table. But our router should match the requirement like it should have 512 MB RAM. This size is recommended but I don’t think that 512 MB RAM is good idea. Because you will receive many alternate routes, path attributes dampening, communities etc. so router’ RAM and CPU should be enough.
We are also having many things in BGP to manipulate the routing and traffic which is not present on IGP table.
Generally , People thinks like: we need the BGP when we are having a link from ISP but it is not true. Because ISP is having all the flavors. You can do the request for routing on default route/static, RIP, IGRP , EIGRP , OSPF. You can do the routing with your ISP using any option. Here is no restriction as well as it works fine. But you will miss many things which can be done only with BGP.
Below is the different possible typologies:
I will describe the other benefits in my further posts.
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Thanks