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See our User Agreement and Privacy Policy. See our Privacy Policy and User Agreement for details. Published on Aug 28, SlideShare Explore Search You. Submit Search. Successfully reported this slideshow. We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime. Halabi Bgp4 Case Studies Tutorial. Upcoming SlideShare. Like this document? Why not share! Embed Size px. Start on. Show related SlideShares at end.

WordPress Shortcode. Full Name Comment goes here. Are you sure you want to Yes No. No Downloads. Views Total views. Actions Shares. Embeds 0 No embeds. No notes for slide. Halabi Bgp4 Case Studies Tutorial 1. It is targeted to the novice as well as the experienced user. Anautonomous system is a set of routers under a single technicaladministration. Routers in an AS can use multiple interior gatewayprotocols to exchange routing information inside the AS and an exteriorgateway protocol to route packets outside the AS.

Two BGP speakingrouters form a TCP connection between one another peer routers andexchange messages to open and confirm the connection parameters. BGP routers will exchange network reachability information, thisinformation is mainly an indication of the full paths BGP AS numbers that a route should take in order to reach the destination network.

Thisinformation will help in constructing a graph of ASs that are loop freeand where routing policies can be applied in order to enforce somerestrictions on the routing behavior. Fromthen on incremental updates are sent as the routing table changes. The version number will change whenever BGP updates thetable due to some routing information changes. Keepalive packets are sentto ensure that the connection is alive between the BGP peers andnotification packets are sent in response to errors or specialconditions.

It is necessary to ensure reachability for networks within an AS beforesending the information to other external ASs. The next step in the configuration process is to define BGP neighbors.

The neighbor definition indicates which routers we are trying to talk towith BGP. The next section will introduce you to what is involved in forming avalid peer connection. The TCP connection is essential in orderfor the two peer routers to start exchanging routing updates. Two BGP speaking routers trying to become neighbors will first bring upthe TCP connection between one another and then send open messages inorder to exchange values such as the AS number, the BGP version they arerunning version 3 or 4 , the BGP router ID and the keepalive hold time,etc.

After these values are confirmed and accepted the neighborconnection will be established. Any state other than established is anindication that the two routers did not become neighbors and hence theBGP updates will not be exchanged. The neighbor command used to establish a TCP connection is:neighbor ip-address remote-as numberThe remote-as number is the AS number of the router we are trying toconnect to via BGP.

It is essential that the two IP addresses used in the neighbor command ofthe peer routers be able to reach one another. One sure way to verifyreachability is an extended ping between the two IP addresses, theextended ping forces the pinging router to use as source the IP addressspecified in the neighbor command rather than the IP address of theinterface the packet is going out from.

A special case for loopback interfaces is discussed later. It is important to reset the neighbor connection in case any bgpconfiguration changes are made in order for the new parameters to takeeffect. IBGP routers do not have to be directly connected, as long asthere is some IGP running that allows the two neighbors to reach oneanother. Anything other than state established indicates that the peers arenot up. You should also note the BGP is version 4, the remote router ID highest IP address on that box or the highest loopback interface in caseit exists and the table version this is the state of the table.

Anytime new information comes in, the table will increase the version and aversion that keeps incrementing indicates that some route is flappingcausing routes to keep getting updated. Normally the loopback interface is used to make surethat the IP address of the neighbor stays up and is independent of aninterface that might be flaky.

In the case of EBGP, most of the time thepeer routers are directly connected and loopback does not apply. If the IP address of a loopback interface is used in the neighbor com-mand, some extra configuration needs to be done on the neighbor router.

The command used to indicate a loopback interface is:neighbor ip-address update-source interfaceThe following example should illustrate the use of this command. Loopback Interface 1 RTA will do so by adding the update-source int loopbackconfiguration neighbor In this case EBGP multihop is used to allowthe neighbor connection to be established between two non directly con-nected external peers. The following example gives a better illustration ofEBGP multihop. RTA needs to indicate that it will be using ebgp-multihop.

On the otherhand, RTB is indicating a neighbor that is directly connected Some IGP or static routing should also be configured in order to allowthe non directly connected neighbors to reach one another. This is a workaround in order to achieveload balancing between two EBGP speakers over parallel serial lines.

Innormal situations, BGP will pick one of the lines to send packets on andload balancing would not take place. By introducing loopback interfaces,the next hop for EBGP will be the loopback interface.

Static routes itcould be some IGP also are used to introduce two equal cost paths toreach the destination. RTA will have two choices to reach next hop In the BGP context, route map is a method used tocontrol and modify routing information.

This is done by defining condi-tions for redistributing routes from one routing protocol to another orcontrolling routing information when injected in and out of BGP. The for-mat of the route map follows:route-map map-tag [[permit deny] [sequence-number]]The map-tag is just a name you give to the route-map.

Multiple instancesof the same route map same name-tag can be defined. The sequence numberis just an indication of the position a new route map is to have in thelist of route maps already configured with the same name.

For example, if I define two instances of the route map, let us call itMYMAP, the first instance will have a sequence-number of 10, and thesecond will have a sequence number of When applying route map MYMAP to incoming or outgoing routes, the firstset of conditions will be applied via instance If the first set ofconditions is not met then we proceed to a higher instance of the routemap.

The conditions that we talked about are defined by the match and setconfiguration commands. Each route map will consist of a list of matchand set configuration. The match will specify a match criteria and setspecifies a set action if the criteria enforced by the match command aremet.

For example, I could define a route map that checks outgoing updates andif there is a match for IP address 1. The above can be illustrated by the followingcommands:match ip address 1. If the match criteria are met and we have a deny then the route will notbe redistributed or controlled and we break out of the list. If the match criteria are not met and we have a permit or deny then thenext instance of the route map instance 20 for example will be checked,and so on until we either break out or finish all the instances of theroute map.

If we finish the list without a match then the route we arelooking at will not be accepted nor forwarded. Filtering on the outbound is OK. In the above example if a route matches the IP address If thereis no match then we go down the route map list which says, set everythingelse to metric 5.

It is always very important to ask the question, whatwill happen to routes that do not match any of the match statementsbecause they will be dropped by default.

Example 2:Suppose in the above example we did not want AS to accept updatesabout Since route maps cannot be applied on the inbound whenmatching based on an ip address, we have to use an outbound route map onRTC:RTC router bgp network There are multiple ways to send network information using BGP. I will gothrough these methods one by one. With this command we are not trying to run BGP on a certaininterface, rather we are trying to indicate to BGP what networks itshould originate from this box.

The mask portion is used because BGP4 canhandle subnetting and supernetting. A maximum of entries of thenetwork command are accepted. The network command will work if the network you are trying to advertiseis known to the router, whether connected, static or learned dynamically.

An example of the network command follows:RTA router bgp 1network Note that we need the static route to get the router to generate This sounds scary because now you are dumping all of yourinternal routes into BGP, some of these routes might have been learnedvia BGP and you do not need to send them out again.


BGP4 Case Studies/Tutorial

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Internet Routing Architectures, 2nd Edition

Internet Routing Architectures, Second Edition, explores the ins and outs of interdomain routing network designs with emphasis on BGP-4 Border Gateway Protocol Version 4 --the de facto interdomain routing protocol. Using a practical, example-oriented approach, this comprehensive resource provides you with real solutions for ISP connectivity issues. You will learn how to integrate your network on the global Internet and discover how to build large-scale autonomous systems. You will also learn to control expansion of interior routing protocols using BGP-4, design sound and stable networks, configure the required policies using Cisco IOS Software, and explore routing practices and rules on the Internet. Formerly, he held technical leadership positions with four Internet service providers Qwest, GTE Internetworking, Genuity, and internetMCI , where he was responsible for network and product architecture, routing design, peering, and other business- and policy-related issues. He is an acknowledged expert in Internet architecture and routing protocols.





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