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BGP is one of the most complex topics you’ll study when pursuing your CCNP, if not the most complex. I know from personal experience that when I was earning my CCNP, BGP is the topic that gave me the most trouble at first. One thing I keep reminding today’s CCNP candidates about, though, is that no Cisco technology is impossible to understand if you just break it down and understand the basics before you start trying to understand the more complex configurations.

BGP attributes are one such topic. You’ve got well-known mandatory, well-known discretionary, transitive, and non-transitive. Then you’ve got each individual BGP attribute to remember, and the order in which BGP considers attributes, and what attributes even are… and a lot more! As with any other Cisco topic, we have to walk before we can run. Let’s take a look at what attributes are and what they do in BGP.

BGP attributes are much like what metrics are to OSPF, RIP, IGRP, and EIGRP. You won’t see them listed in a routing table, but attributes are what BGP considers when choosing the best path to a destination when multiple valid (loop-free) paths exist.

When BGP has to decide between such paths, there is an order in which BGP considers the path attributes. For success on the CCNP exams, you need to know this order. BGP looks at path attributes in this order:

Highest weight (Cisco-proprietary BGP value)

Highest local preference (LOCAL_PREF)

Prefer locally originated route.

Shortest AS_PATH is preferred.
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Like TCP, BGP is connection-oriented. An underlying connection between two BGP speakers is established before any routing information is exchanged. This connection takes place on TCP port 179. As with EIGRP and OSPF, keepalive messages are sent out by the BGP speakers in order to keep this relationship alive.

Once the connection is established, the BGP speakers exchange routes and synchronize their tables. After this initial exchange, a BGP speaker will only send further updates upon a change in the network topology.

The IGP protocols that use Autonomous Systems, IGRP and EIGRP, require prospective neighbors to be in the same AS. This is not true with BGP. Routers can be in different Autonomous Systems and still exchange routes. The BGP neighbors do not have to be directly connected, and often are not, but do need to be able to reach the IP addresses they use in their neighbor statements.
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