BGP PROTOCOL TUTORIAL IN PDF
Łukasz Sturgulewski cittadelmonte.info BGP is a path vector or an advanced distance vector routing protocol. 2. This document contains five Border Gateway Protocol (BGP) case studies. Allows scalable, informed route selection. ○ Border Gateway Protocol. ♢. How routing gets done on the Internet today. 2. CSE – Lecture Border Gateway.
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A Routing Protocol used to exchange routing BGP. □ Path Vector Protocol. □ Incremental Updates. □ Many options for policy BGP Basics. What is BGP? • Border Gateway Protocol. • A Routing Protocol used to exchange routing information between different networks. – Exterior gateway protocol. Updated material may be found at cittadelmonte.info 1. - Border Gateway Protocol -. Border Gateway Protocol (BGP). BGP is a standardized exterior.
Service providers working with IP networks are very clear that the Border Gateway Protocol, or BGP , is the most complex and difficult-to-configure internet protocol. Its emphasis on security and scalability makes it essential, however. This tutorial gives you a detailed look at how BGP works, and offers simple and advanced BGP troubleshooting options, so your BGP-enabled routers can exchange information securely with several hundred thousand IP prefixes and keep the internet running. If you have to explain what BGP is to someone new to the service provider environment, the best definition is that it's the routing protocol that makes the internet work. As the address allocation in the internet is not nearly as hierarchical as the telephone dialing plan, most of the routers in service providers' core networks need to exchange information about several hundred thousand IP prefixes. BGP is still able to accomplish that task, which is proof it's a highly scalable routing protocol. BGP routing information is usually exchanged between competing business entities in the form of internet service providers ISPs in an open, hostile environment -- the public internet.
These could be large networks from service providers who are connected to each other.
BGP for Beginners
Within each autonomous system we are running an Interior gateway protocol. The routers that are connecting the different autonomous systems to each other are running BGP. Now if you want to see what a real BGP router looks like, you can use one of the many looking glass servers that are found on the internet, try the following one by telnetting to this address: These values are the autonomous systems that you have to go through in order to reach network 1.
Pretty neat huh?
Metric, LocPrf Local Preference and Weight are the attributes that BGP uses to influence routing decisions, you can forget about those for now but I will write another article on them in the future.
Take a look at the following picture: Let me explain you what we have here: We are using the following ip addressing scheme within the autonomous system: We have advertised the networks in BGP and life is good. However in this scenario there is a problem. AS will advertise How does the router on the right side in AS know about this information? An Internet routing table consists of roughly Excellent so now both routers are running IBGP and they can exchange routing information.
So are we done? Is this going to work? AS sends a packet meant for AS has learned about the The router on the left side in AS will send it towards the router on the right side by using the router on the top or bottom in AS What do you think these routers will do? Does OSPF know about the So do you always have to configure IBGP? You only have to do this when your autonomous system is a transit AS. A transit AS means that your autonomous system can be used to pass traffic to another autonomous system.
In the upcoming articles I will be writing about BGP attributes, this will show you how BGP will choose a certain path and how we can influence routing decisions. So what do you think? Does this make sense to you?
You could also advertise 2 default routes with a different cost or metric, if one router fails the other will take over. I got a really cool document which explains this, I think I should update my article to add this information. My mail ID is azhar. Once you are running BGP you know about all those prefixes, which AS they belong to and through which paths you need to go in order to get there.
Andrea, One major reason why a company will use eBGP is for incoming traffic. This way the internet will now see that there are 2 possible paths to their IP address rather than just the path from ISP 1. This happens all the time with many companies that purchase dedicated internet access circuits. Rene, I really appreciate your effort to share knowledge and experience.
Great, well done! From a beginner point of view this article is awesome. I guess that the other articles will get into more details such as the need to use loopbacks to identify the iBGP routers and their associated configurations…. This article is awesome. In all of these cases, it's best if your upstream ISP can help you resolve the problem -- which is, at this point, beyond the scope of technical BGP troubleshooting.
The same process has to happen in reverse direction -- or, at least, the default route has to be announced to the customer.
The networkwide BGP troubleshooting is thus composed of three steps:. Have we received the prefix? The only tool that can help you identify the problem is the debugging facility on your edge router -- as you normally don't have access to the other BGP neighbor.
BGP tutorial: The routing protocol that makes the Internet work
When doing BGP debugging, be aware a BGP neighbor can send you several hundred thousand routes, so you have to ensure the debugging output produced by the troubleshooting session does not overwhelm the router.
Your debugging tool will thus not show you an IP prefix until it has actually changed, or you've cleared the BGP session with your neighbor. Some BGP routers have the ability to store a separate copy of all routes sent by a neighbor into a parallel BGP table.
With the parallel per-neighbor table, you can pinpoint exactly what the neighbor has sent you the content of the parallel table and the routes that have passed your input filters the contents of the main BGP table.
But, of course, the parallel per-neighbor table consumes a large amount of memory. Is the IP prefix propagated across our network? Even when an edge router receives an IP prefix via BGP, it may not be propagated to the other end of your network.
As every router between every pair of edge routers has to run BGP -- otherwise, the traffic could be dropped inside your network -- the number of BGP sessions could become excessively large. The following diagram illustrates the BGP sessions needed in a small four-router network. With these rules in hand, you have to step through the graph of BGP sessions in your network, checking every BGP router on the way and ensuring the route-reflector rules are not violated -- and, using the rules, the BGP prefixes get from every edge router to all other routers.
Another common reason an IP prefix is not propagated across your network is the external subnets on the edge of your network are not advertised to your core routers. The IP next-hop of an external route is thus always the IP address of a router one hop beyond the edge of your autonomous system.
Is the prefix sent to external neighbors? As the last step in troubleshooting BGP route propagation, you have to check whether the IP prefixes transported across your network are announced to your external BGP peers. Simple approach article. Is the traffic traversing the network? Even if your BGP route propagation works flawlessly, the IP packets may not be able to traverse your network.
BGP for Beginners
The most common cause of a black hole in your network is a router in the transit path that does not run BGP and consequently has no idea how to route the received IP packet toward the destination network.
IP routing works hop by hop. Even though the ingress edge router knows exactly which egress edge router to use and how to get there, it cannot pass that information to the intermediate routers. All of them must, therefore, run BGP as well. To identify a black hole in your network, perform a traceroute from your customer's network to a destination in the internet. The last router responding to the traceroute is one hop before the black hole.
Even though all core routers in your network have to run BGP, the internal BGP sessions don't have to follow the physical structure of the network. Five reasons you need BGP complexity in your network. Using BGP as data center routing protocol. Where and how to deploy BGP in large networks. Please check the box if you want to proceed. APIs are becoming the easiest way to customize services to fit business communications needs.
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Your password has been sent to: Please create a username to comment. In understanding how BGP works, what kind of troubleshooting approach did you take to solving problems?
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