BGP

• BGP stands for Border Gateway Protocol.
• BGP uses the concept of autonomous systems. An autonomous system is a group of networks under a common administration.
• Autonomous systems run Interior Gateway Protocols (IGP) within the system. They run an Exterior Gateway Protocol (EGP) between them.
• BGP version 4 is the only EGP currently in use.
• BGP neighbors are called peers and must be statically configured.
• BGP uses TCP port 179.
• BGP is a path-vector protocol. Its route to a network consists of a list of autonomous systems on the path to that network.
• BGP’s loop prevention mechanism is autonomous system number.

When an update about a network leaves an autonomous system, that autonomous system’s number is prepended to the list of autonomous systems that have handled that update. When an autonomous system receives an update, it examines the autonomous system list. If it finds its own autonomous system number in that list, the update is discarded.

In Figure 6-1, BGP routers in AS 65100 see network 10.1.1.0 as having an autonomous system path of 65200 65300 65400.

Multihoming
Multihoming means connecting to more than one ISP at the same time. It is done for redundancy and backup in case one ISP fails and for better performance if one ISP provides a better path to often used networks. Three ways exist to receive routes from each ISP:

• Default routes from each provider—This results in low use of bandwidth and router resources. The internal network’s IGP metric determines the exit router for all traffic bound outside the autonomous system.
• Default routes plus some more specific routes—This results in medium use of bandwidth and router resources. This allows you to manipulate the exit path for specific routes using BGP, but the IGP metric chooses the exit path for default routes.
• All routes from all providers—This requires the highest use of bandwidth and router resources. It is typically done by large enterprises and ISPs. Path selection for all external routes can be controlled via BGP policy routing tools.

BGP Databases
BGP uses three databases. The first two listed are BGP-specific; the third is shared by all routing processes on the router:

• Neighbor database—This is a list of all configured BGP neighbors. To view it, use the show ip bgp summary command.
• BGP database, or RIB (Routing Information Base)—This is a list of networks known by BGP, along with their paths and attributes. To view it, use the show ip bgp command.
• Routing table—This is a list of the paths to each network used by the router, and the next hop for each network. To view it, use the show ip route command.

BGP Message Types
BGP has four types of messages:

• Open—After a neighbor is configured, BGP sends an open message to try to establish peering with that neighbor. Includes information such as autonomous system number, router ID, and hold time.
• Update—Message used to transfer routing information between peers.
• Keepalive—BGP peers exchange keepalive messages every 60 seconds by default. These keep the peering session active.
• Notification—When a problem occurs that causes a router to end the BGP peering session, a notification message is sent to the BGP neighbor and the connection is closed.

Internal and External BGP
Internal BGP (IBGP) is BGP peering relationship between routers in the same autonomous system. External BGP (EBGP) is BGP peering relationship between routers in different autonomous systems. BGP treats updates from internal peers differently than updates from external peers.

In Figure 6-2, routers A and B are EBGP peers. Routers B, C, and D are IBGP peers.
BGP Next Hop Selection
The next hop for a route received from an EBGP neighbor is the IP address of the neighbor that sent the update.

When a BGP router receives an update from an EBGP neighbor, it must pass that update to its IBGP neighbors without changing the nexthop attribute. The next-hop IP address is the IP address of an edge router belonging to the next-hop autonomous system. Therefore, IBGP routers must have a route to the network connecting their autonomous
system to that edge router. For example, in Figure 6-3, RtrA sends an update to RtrB, listing a next hop of 10.2.2.1, its serial interface. When RtrB forwards that update to RtrC, the next-hop IP address will still be 10.2.2.1. RtrC needs to have a route to the 10.2.2.0 network in order to have a valid next hop.

To change this behavior, use the neighbor [ip address] next-hop-self command in BGP configuration mode. In Figure 6-3, this configuration goes on RtrB. After you give this command, RtrB will advertise its IP address to RtrC as the next hop for networks from AS 65100, rather than the address of RtrA. Thus, RtrC does not have to know about the
external network between RtrA and RtrB (network 10.2.2.0).


BGP Next Hop on a Multiaccess Network
On a multi-access network, BGP can adjust the next-hop attribute to avoid an extra hop. In Figure 6-3, RtrC and RtrD are EBGP peers, and RtrC is an IBGP peer with RtrB. When C sends an update to D about network 10.2.2.0, it normally gives its interface IP address as the next hop for D to use. But because B, C, and D are all on the same multiaccess network, it is inefficient for D to send traffic to C, and C to then send it on to B. This process unnecessarily adds an extra hop to the path. So, by default, RtrC advertises a next hop of 10.3.3.3 (RtrB’s interface) for the 10.2.2.0 network. This behavior can also be adjusted with the neighbor [ip address] next-hop-self command.


BGP Synchronization Rule
The BGP synchronization rule requires that when a BGP router receives information about a network from an IBGP neighbor, it does not use that information until a matching route is learned via an IGP or static route. It also does not advertise that route to an EBGP neighbor unless a matching route is in the routing table. In Figure 6-3, if RtrB advertises a route to RtrC, then RtrC does not submit it to the routing table or advertise it to RtrD unless it also learns the route from some other IGP source.

Recent IOS versions have synchronization disabled by default. It is usually safe to turn off synchronization when all routers in the autonomous system run BGP. To turn it off in earlier IOS versions, use the command no synchronization under BGP router configuration mode.