BGP is an external routing protocol used primarily by Internet Service Providers (ISPs). Unless you work for an ISP today or in the future, you may have little or no prior exposure to BGP. Understanding BGP is a great addition to your skill set – and you have to know the basics well to pass the BSCI exam.

Note that I said “the basics”. BGP is a very complex protocol, and when you pursue your CCIE, you’ll see what I’m talking about. As with all things Cisco, though, when broken down into smaller pieces, BGP becomes quite understandable. You will need to know the basics of BGP as presented in this chapter to pass your BSCI exam – so let’s get started.

BGP Defined:

“An Internet protocol that enables groups of routers (called autonomous systems) to share routing information so that efficient, loop-free routes can be established. BGP is commonly used within and between Internet Service Providers (ISPs).”

There are a couple of terms in there that apply to the protocols you’ve mastered so far in your studies. The term “autonomous system” applies to IGRP and EIGRP as well as BGP; you’ll be indicating a BGP AS in your configurations just as you did with IGRP and EIGRP. And we’re always looking for efficient, loop-free routes, right? As it did with IGRP and EIGRP, “autonomous system” simply refers to a group of routers that is managed by a single administrative body. An autonomous system will use an Interior Gateway Protocol (IGP) such as OSPF or EIGRP to route packets inside the AS; outside the AS, an Exterior Gateway Protocol (EGP) such as BGP will be used.

BGP shares some characteristics with some routing protocols you’ve already studied. BGP supports VLSM, summarization, and CIDR. Like EIGRP, BGP will send full updates when two routers initially become neighbors and will send only partial updates after that. BGP does create and maintain neighbor relationships before exchanging routes, and keepalives are sent to keep this relationship alive.

BGP has some major differences from the IGPs we’ve studied to this point. You’ll hear BGP referred to as a path-vector protocol. As opposed to distance-vector protocols that exchange relatively simple information about available routes, BGP routers will exchange extensive information about networks to allow the routers to make more intelligent routing decisions. This additional BGP path information comes in the form of attributes, and these path attributes are contained in the updates sent by BGP routers. Attributes themselves are broken up into two classes, well-known and optional.

BGP also keeps a routing table separate from the IP routing table.

Well take a look at BGP attributes in future BSCI tutorials. In the meantime, keep studying!

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Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

Before you get into the more advanced features of BGP, you should have the fundamentals down cold, and one of those fundamentals is knowing the BGP adjacency states. This will allow you to successfully analyze and troubleshoot BGP peer relationships.

In the following example, a BGP peering is being created between R1 and R3.

R1(config-router)#neighbor 172.12.123.3 remote-as 200

Default minimum time between advertisement runs is 30 seconds

The output here can be a little misleading the first time you read it. The first highlighted line shows 172.12.123.3 is a BGP neighbor, is located in AS 200, and is an external link, indicating that the neighbor is in another AS entirely. The second highlighted line shows the BGP state as Active. This sounds great, but it actually means that a BGP peer connection does not yet exist with the prospective neighbor. Before we continue with this example, let’s look at the different BGP states:

Idle is the initial state of a BGP connection. The BGP speaker is waiting for a start event, generally either the establishment of a TCP connection or the re-establishment of a previous connection. Once the connection is established, BGP moves to the next state.

Connect is the next state. If the TCP connection completes, BGP will move to the OpenSent stage if the connection does not complete, BGP goes to Active.

Active indicates that the BGP speaker is continuing to create a peer relationship with the remote router. If this is successful, the BGP state goes to OpenSent. You’ll occasionally see a BGP connection flap between Active and Connect. This indicates an issue with the physical cable itself, or with the configuration.

OpenSent indicates that the BGP speaker has received an Open message from the peer. BGP will determine whether the peer is in the same AS (iBGP) or a different AS (eBGP) in this state.

In OpenConfirm state, the BGP speaker is waiting for a keepalive message. If one is received, the state moves to Established, and the neighbor relationship is complete. It is in the Established state that update packets are actually exchanged.

So even though the show ip bgp neighbor output indicated that this is an Active neighbor relationship, that’s not as good as it sounds. Of course, the reason the peer relationship hasn’t been established is that we haven’t configured R3 yet!

R3(config)#router bgp 200

Foreign host: 172.12.123.1, Foreign port: 11007

The peer relationship between R1 and R3 has been established!

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Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

BGP isnt just one of the biggest topics on the BSCI exam, its one of the largest. BGP has a great many details that must be mastered for BSCI success, and those of you with one eye on the CCIE must learn the fundamentals of BGP now in order to build on those fundamentals at a later time.

Path attributes are a unique feature of BGP. With interior gateway protocols such as OSPF and EIGRP, administrative distance is used as a tiebreaker when two routes to the same destination had different next-hop IP addresses but the same prefix length. BGP uses path attributes to make this choice.

The first attribute considered by BGP is weight. Weight is a Cisco-proprietary BGP attribute, so if youre working in a multivendor environment you should work with another attribute to influence path selection.

The weight attribute is significant only to the router on which it is changed. If you set a higher weight for a particular route in order to give it preference (a higher weight is preferred over a lower one), that weight is not advertised to other routers.

BGP uses categories such as “transitive”, “non-transitive”, “mandatory”, and “optional” to classify attributes. Since weight is a locally significant Cisco-proprietary attribute, it does not all into any of these categories.

The weight can be changed on a single route via a route-map, or it can be set for a different weight for all routes received from a given neighbor. To change the weight for all incoming routes, use the “weight” option with the neighbor command after forming the BGP peer relationships.

R2(config)#router bgp 100

R2(config-router)#neighbor 100.1.1.1 remote-as 10

R2(config-router)#neighbor 100.1.1.1 weight 200

Learning all of the BGP attributes, as well as when to use them, can seem an overwhelming task when you first start studying for your BSCI and CCNP exams. Break this task down into small parts, learn one attribute at a time, and soon youll have the BGP attributes mastered.

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

BGP is totally different from any protocol you learned to earn your CCNA certification, and one of the differences is that BGP uses path attributes to favor one path over another when multiple paths to or from a destination exist.

Notice I said “to or from”. In earlier free BGP tutorials, I discussed the BGP attributes “weight” and “local preference”. These attributes are used to favor one path to a destination over another; for example, if BGP AS 100 has two paths to a destination in AS 200, these two attributes can be set in AS 100 to favor one path over another. But what if AS 100 wants to inform the routers in AS 200 as to which path it should use to reach a given destination in AS 100?

Thats where the BGP attribute “Multi-Exit Discriminator”, or MED, comes in. The MED value can be set in AS 100 to tell AS 200 which path it should use to reach a given network in AS 100.

As with many BGP attributes, the MED can be set with a route-map. What you need to watch is that there is no “set med” value in route maps. To change the MED of a path, you need to change the metric of that path. Lets say that there are two entry paths for AS 200 to use to reach destinations in AS 100. You want AS 200 to use the 100.1.1.0/24 path over the 100.2.2.0/24 path. First, identify the two paths with two separate ACLs.

R1(config)#access-list 22 permit 100.1.1.0 0.0.0.255

R1(config)#access-list 23 permit 100.2.2.0 0.0.0.255

Next, write a route-map that assigns a lower metric to the more-desirable path.

R1(config)#route-map PREFER_PATH permit 10

R1(config-route-map)#match ip address 22

R1(config-route-map)#set metric 100

R1(config-route-map)#route-map PREFER_PATH permit 20

R1(config-route-map)#match ip address 23

R1(config-route-map)#set metric 250

Finally, apply the route-map to the neighbor or neighbors.

R1(config-route-map)#router bgp 100

R1(config-router)#neighbor 22.2.2.2 route-map PREFER_PATH out

The key points to keep in mind is that while many BGP attributes prefer a higher value, the MED is basically an external metric – and a lower metric is preferred, just as with the protocols youve already studied to earn your CCNA certification.

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

One of the major differences between BGP and the other protocols youve studied to date is that BGP uses attributes to describe paths, and to influence the selection of one path over the other.

In this free tutorial, were going to take a look at the Local Preference attribute and compare it to the Cisco-proprietary BGP attribute “weight”.

The Local Preference (LOCAL_PREF) attribute is used to influence how traffic will flow from one Autonomous System (AS) to another when multiple paths exist. For example, if AS 100 has two different paths to a destination network in AS 200, the LOCAL_PREF attribute can be used to influence the path selection.

The major difference between the Weight and LOCAL_PREF attributes is that when the LOCAL_PREF attribute is changed, that change is reflected throughout the AS. The new LOCAL_PREF value will be advertised to all other routers in the AS, as compared to the Weight attribute, which is locally significant only. If you change the Weight for a path on one router in an AS, the other routers in the AS will not learn of the change.

A route-map can be used to change a local preference value. For example, if you want to change the local preference value to 200 for the path advertisement 10.2.2.0/24 coming in from neighbor 10.1.1.1, there are three steps involved. First, write an ACL matching the remote network you want to change the local preference for.

R1(config)#access-list 5 permit 10.2.2.0 0.0.0.255

Second, write a route-map setting the local preference to 200. This will double the default value of 100, and the path with the highest local preference will be the preferred path.

R1(config)#route-map PREFER_PATH permit 10

R1(config-route-map)#match ip address 5

R1(config-route-map)#set local-pref 200

Finally, apply the route-map to routes that are being received from 10.1.1.1.

R1(config)#router bgp 100

R1(config-router)#network 10.1.1.1 route-map PREFER_PATH in

R1 will then advertise this new local preference value to all other routers in AS 100 – all of its iBGP neighbors.

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

BGP is unlike any protocol you learned during your CCNA studies, and even the similarities are a little bit different!

BGP forms neighbor relationships, much like EIGRP and OSPF do. The interesting thing with BGP is that potential neighbors, or “peers”, do not need to be directly connected and can use their loopback interfaces to form the peer relationships.

It may well be to your advantage to use loopbacks to form peer relationships rather than the actual interface facing the potential neighbor. This can be done because BGP uses static neighbor statements rather than any kind of dynamic neighbor discovery process.

Consider a router that has two paths to a BGP speaker. The interfaces are numbered like this:

Router1: Serial0, 172.1.1.1 /24, Serial2, 179.1.1.1 /24, loopback0, 1.1.1.1 /32.

Router2: Serial0, 172.1.1.2/24, Serial2 179.1.1.2/24, loopback0, 2.2.2.2 /32.

We could configure Router1 like this:

router bgp 200

neighbor 172.1.1.2 remote-as 200

In this case, BGP would automatically use 172.1.1.1 as the source for the TCP connection that has to be set up with the neighbor before updates can be exchanged; this address is known as the best local address. However, if the remote peers serial0 interface is shut down or goes down for another reason, the peer relationship would be lost even though Router2 is still available.

Instead of using one of the physical interfaces, we can use the loopbacks on each router to establish the TCP-based peer connection. The configurations would look like this:

Router1:

router bgp 200

neighbor 2.2.2.2 remote-as 200

neighbor 2.2.2.2 update-source loopback0

Router2:

router bgp 200

neighbor 1.1.1.1 remote-as 200

neighbor 1.1.1.1 update-source loopback0

In this case, losing one of the physical connections does not necessarily mean the BGP peering is lost; as long as the routers have a valid path to each others loopback addresses, the BGP peer relationship will stay in place. And better yet, we avoid the dreaded “single point of failure”!

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

BGP paths contain attributes, while no protocol you studied for the CCNA carried. BGP Attributes are used to choose the best path when multiple loop-free paths exist, as well as give you other specific information about the paths. This additional information includes the autonomous systems that are along the path to a given destination, what the next-hop IP address is, and much more.

Before we examine the specific attributes, we need to understand the categories used to differentiate BGP attributes. Some attributes are required, some arent; some attributes will be carried between routers, where others will not.

The first category is the well-known mandatory attribute. As youd expect, these attributes are required and will be understood by all BGP speakers. Mandatory attributes include the origin code, AS_Path, and next-hop.

Well-known discretionary attributes dont have to be present, but if they are , all BGP speakers will understand their meaning. BGP attributes that fall into this category are the MED, local preference, and atomic aggregate.

Optional transitive attributes may not be fully understood by all BGP speakers, but the attributes are sent between routers as paths are exchanged. The aggregator and community attributes fall into this category.

Finally, we have the optional nontransitive attribute. If a BGP speaker does not understand this attribute, the speaker will not forward the attribute. The Originator ID and Cluster ID are optional nontransitive attributes.

Theres one important BGP attribute that was left out of this list; indeed, if youre working in an all-Cisco environment, it may be the most important attribute of all. The weight attribute is Cisco-proprietary, so if youre working in a multivendor environment, this attribute is of limited value. However, the weight attribute is the first attribute considered when BGP is deciding between valid, loop-free paths, so its an attribute we have to keep in mind. The weight attribute doesnt really fit in any of the four BGP classes we talked about earlier in the article.

If you dont know what these attributes do yet, thats okay. Well examine each of these attributes in more detail in the next part of this free BGP tutorial. Keep studying!

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

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 todays 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. Youve got well-known mandatory, well-known discretionary, transitive, and non-transitive. Then youve 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. Lets 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 wont 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.

Choose route with lowest origin code. Internal paths are preferred over external paths, and external paths are preferred over paths with an origin of “incomplete”.
Lowest multi-exit discriminator (MED)

External BGP routes preferred over Internal BGP routes.

If no external route, select path with lowest IGP cost to the next-hop router for iBGP.

Choose most recent route.

Choose lowest BGP RID (Router ID).

If you dont know what these values are, or how theyre configured, dont panic! The next several parts of this BGP tutorial will explain it all. So spend some time studying this order, and in part II of this free BGP tutorial, well look at each of these values in detail. Keep studying!

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

Whenever I teach distance-vector protocols in my Ultimate CCNA Boot Camp or Fast Track classes, I make sure my students understand the many rules of distance-vector routing thoroughly. After that, we move on to OSPF and link-state routing.

And what do I tell my students before we move on to OSPF and link-state? “Take all that great stuff you just learned about distance-vector routing and put it aside, because none of it applies here!”

That’s about the time I get astonished looks and a few things thrown at me.

What’s this got to do with BGP? Well, when you start studying BGP, you have to put a lot of preconceptions aside. It sounds like just another routing protocol, like RIP or OSPF, but it’s not.

So… before we start studying BGP and looking at some of its many features, let’s take a look at what BGP is and what it’s not.

What BGP Does And Who Uses It

If you’re in a position where you’re used to seeing or working with routing protocols such as EIGRP and OSPF, you’re probably now working with BGP. BGP is not a protocol you’re going to configure at the office LAN. BGP is used to connect autonomous systems, which are very large “collections” of networks. (Those of you who have studied IGRP and EIGRP are familiar with the concepts of an AS.)

As opposed to routing protocols such as EIGRP and OSPF, BGP is an exterior routing protocol. BGP is not used to find a specific network instead, it’s used to find the AS where that given network can be found.

A good way to look at it is that networks are contained in Autonomous Systems BGP helps you find the AS where a given network is found. Routing protocols such as OSPF and EIGRP take it from there.

BGP Terminology

You’ll be happy to know that BGP has a terminology all its own. Well, you might not be happy about it, but it does! I’m not going to throw a bunch of terms at you right now, since these terms are better introduced to you when you can see what they do. Having said that, here are a couple of basic BGP terms that will help you make the transition from IGPs to BGP.

aggregation – This is just the BGP term for summarization. You’re familiar with route summarization from your CCNA studies, and if not, it’s time for a review from my Ultimate CCNA Study Guide. I told you you’d be using those skills for a long time!

IGP – Interior Gateway Protocol. These are routing protocols that run within an Autonomous System, such as OSPF and EIGRP.

EGP – Exterior Gateway Protocol. Remember from your CCNA studies that EIGRP routes are indicated by the letter “D” in your routing table? Ever wonder why? EGP, that’s why. Run show ip route and take a look at the routing table key. EGP was BGP’s predecessor, and is still in the routing table. EGP was there before EIGRP, so that’s why “E” in the routing table doesn’t indicate an EIGRP route.

We’ll be adding to this list a great deal in the next few weeks.

Now that you’ve got a grasp on the differences between BGP and the IGPs you’ve been working with up to this point, it’s time to start looking at some basic BGP concepts and configurations. Look for Part II of this BGP tutorial over the next few days!

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!