Okie guys... Lets continue with our yesterdays discussion about BGP. Today we will discuss why do ISPs need BGP ?
Alrite as we now know that public AS number and a pool a of public IP address is required to be an entity on the internet. Now every ISP has its separate AS number and pool of public IP addresses. Now when ISP1 wishes to interact with ISP2 (to exchange the routing table with other ISP) it uses BGP. BGP ensures that it exchanges only the information which is meant to be shared and that too securely.ISPs keep in sync with other ISPs on any network path changes and does all their path calculations for inter-ISP traffic on the basis of BGP metrics like AS-Path.
Now when do customers using BGP starts suffering ?
okie.. when a customer is multihomed ( Means have links from multiple ISPs to achieve failover or load sharing). Customer is connected to multiple ISPs and is continously exchanging routing information with the ISPs. In such cases if BGP is not configured properly, ISPs would consider your AS as the best path to another ISP and start using your AS as a transit AS.
So what ?
okie.. when your AS is being used as a transit AS, all your available link bandwidth would used by the transit traffic and the traffic destined to your network would start suffering.
So, when to configure BGP ?
1. When your network actually needs BGP.
2. When your Network Administrator understands the BGP metrics very well.
Alrite.. thats all for now... will carry on with BGP tweakings in my next post.