BGP Frequently Asked Questions
From ImageStream Router Documentation
Revision as of 00:16, 18 May 2008 by RichardJune
What is BGP?
- The Border Gateway Protocol (BGP) is an exterior routing protocol. BGP version 4, described in RFC 1771, and further defined in version 5 of the BGP4 Internet Draft of January 1997, was designed for routing between Autonomous Systems. BGP Autonomous System Confederations information can be found in RFC 1965. BGP Route Reflection information can be found in RFC 1966. The BGP COMMUNITIES attribute is described in RFC1997. Although it can be used internally, BGP is primarily used to route between networks, rather within them (such as RIP, OSPF, IS-IS).
What is "multi-homing"?
- "Multi-homing" means having multiple connections to the Internet. There are two basic types of multi-homed organizations:
- An organization which has connections to provider at more than one point of presence, and has internal links between these sites;
- An organization which has a connection to provider and at least one other Internet access provider.
- If you have only a single connection to the Internet there is not a lot to be gained from using BGP. With only one gateway, your router already knows how to send all traffic not destined to one of your networks. Everyone should be conscious of keeping the Internet's routing table as small as possible. This can be best addressed by aggregating addresses using CIDR blocks. As a single-homed connection, your provider will likely choose to aggregate your networks with the networks of other single-homed customers and thus announce a single CIDR block under their ASN in order to reduce the routing table size.
What are the benefits of using BGP?
- The single word answer is "control". When you decided to multi-home it was for a reason, maybe extra capacity, redundancy, or tariffing, so in order to actually perform any of these functions you need to control how your networks are announced to your providers. If you don't exchange routes with them but rely on them announcing your routes you could find your traffic flowing down an expensive low capacity link with the other cheaper, higher capacity links mostly idle!
Does using BGP make it easier to change providers?
- Each Tier 1 backbone provider has its own procedures to handle customers that wish to use BGP. These procedures may be easier than the procedures used for staticly-routed customers, but generally each will require some sort of route registration, so there is no real time or effort saved.
- However if you are using BGP to more than one provider, you can migrate routes between the providers without their intervention (assuming any route registration has already been performed). In order to migrate the routes, you should start announcing the routes to the new provider and confirm that the new path is visible via a "looking glass". You may need to prepend your AS on the old path to make that path "longer". Having confirmed that the new provider is successfully announcing the routes you can then terminate the announcement to the old provider.