iBGP

iBGP Explained

iBGP, or internal BGP, is a way for routers within the same autonomous system (AS) to coordinate their efforts in handling BGP. Every BGP router within an AS maintains an iBGP session with every other BGP router in the AS, allowing all routers to have a full view of all BGP information so they can make the best routing decisions. In service provider networks, all routers usually run iBGP, even routers that don’t connect to external ASes. Unlike eBGP sessions, iBGP sessions don’t add the router’s own AS number to the AS path or update the next hop address. Also, there is no requirement for a direct connection between two iBGP routers, and normally no filters or route maps are applied to iBGP sessions.

iBGP Main Benefits

One of the key benefits of iBGP is that it allows for more efficient routing within an autonomous system. Without iBGP, each router in the AS would need to rely solely on external BGP (eBGP) sessions with routers in other ASes to learn about available routes. This could lead to suboptimal routing decisions, especially if some routers in the AS have better eBGP connections than others.

By contrast, with iBGP, each router in the AS has a complete view of all BGP information within the AS, allowing for more informed routing decisions. iBGP also allows for better load balancing and redundancy within the AS, since all routers can see all available paths and can make decisions accordingly.

Another advantage of iBGP is that it allows for more granular control over routing policies within the AS. Since all routers have a complete view of all BGP information, it’s easier to apply filters, route maps, and other policies to control the flow of traffic within the AS. For example, an AS might use iBGP to prioritize certain types of traffic over others or to implement security policies to block certain types of traffic altogether.

iBGP is important when managing routing within an autonomous system. By allowing all routers to have a complete view of all BGP information, it enables more efficient and effective routing decisions, while also providing greater control over traffic flow and security.

iBGP Intricacies

There are some iBGP intricacies you should be aware of:

• Full Mesh: In iBGP, every router must be fully meshed with every other router in the same AS. This means that each router must establish a BGP session with every other router. As the number of routers in an AS grows, the number of iBGP sessions required can become unmanageable. To address this, networks typically use route reflectors to reduce the number of iBGP sessions that need to be maintained.
• Split-horizon: In iBGP, by default, a router will not advertise a route learned from one iBGP peer to another iBGP peer. This is because iBGP does not modify the AS path attribute, and if a router advertises a route with its own AS number in the AS path, it could cause a routing loop. To overcome this limitation, use route reflectors or configure the next-hop-self command.
• AS-Path: In iBGP, the AS path attribute is not modified. Therefore, iBGP does not provide any loop prevention mechanism. If a router receives a route with its own AS number in the AS path attribute, it will not install the route in its routing table. This means that you need to ensure that your network design does not allow for loops to occur.
• Next-hop: In iBGP, the next-hop attribute is not modified by default. This means that if a router receives a route from an iBGP peer with a next-hop address that is not reachable, the router will not install the route in its routing table. To bypass this limitation, you can use the next-hop-self command.
• Synchronization: In iBGP, the synchronization rule states that a router should not advertise a route learned from an iBGP peer to an eBGP peer unless the router has learned the same route from an IGP (Interior Gateway Protocol) or another eBGP peer. This rule ensures that all routers in the AS have the same view of the network. However, this rule can cause suboptimal routing if the IGP does not converge quickly enough.
• Path Attributes: In iBGP, all path attributes are propagated unchanged from one iBGP peer to another iBGP peer. This means that any path attributes set by an eBGP peer will be propagated to other iBGP peers. To address this, you can use the remove-private-as command.

Understanding these intricacies is crucial for designing, configuring, and troubleshooting iBGP networks.

GLOSSARY:

• Border Gateway Protocol
• JFlow Analysis