Network Administrator Knowledgebase

May 15 2008   1:55AM GMT

BGP

Michael Khanin Michael Khanin Profile: Michael Khanin

BGP is the protocol that binds the Internet together. It is what sends one packet across the globe in a few milliseconds and allows you to send email, . Typically, you will see Cisco routers that will handle this sort of heavy lifting and that is the IOS that we will review briefly.

ASNStarting point
In order to have a BGP connection you will need to have an ASN(Autonomous System Number). You can get one of these through ARIN (American Registry for Internet Numbers). BGP uses ASN’s like VLAN id’s or a higher level view of subnetting. There are private ASN’s if you are planning on using BGP for internal purposes only. The private BPG range is AS64512 through to AS65535.

IOSConfiguration info
Here is the basic output of two connections to two different autonomous systems from one Cisco router.

router bgp 64512
no synchronization
bgp log-neighbor-changes
bgp dampening
network 3.3.3.0
neighbor 1.1.1.1 remote-as AS64513
neighbor 1.1.1.1 description Provider 1 >>Provider 1 Support Line<<
neighbor 1.1.1.1 password 7 09823490822093482F
neighbor 1.1.1.1 update-source Loopback1
neighbor 1.1.1.1 version 4
neighbor 1.1.1.1 route-map Provider1 out
neighbor 2.2.2.2 remote-as AS64514
neighbor 2.2.2.2 description Provider 2 >>Provider 2 Support Line<<
neighbor 2.2.2.2 password 7 09823490822093482F
neighbor 2.2.2.2 update-source Loopback2
neighbor 2.2.2.2 version 4
neighbor 2.2.2.2 weight 50
Let’s walk through the configuration a bit. Here are the same commands but with comments added in at various places

! This line is telling the BGP router which ASN it should advertise.
! A Cisco BGP router can only administer one ASN at a time.
router bgp 64512

! Log the changes when the neighbor goes up and down.
! This way you can see if the other BGP router that you are peering with is stable.
bgp log-neighbor-changes

! This is the network that you are advertising via BGP
network 3.3.3.0
! This is the ASN of your ISP or peered BGP network.
neighbor 1.1.1.1 remote-as AS64513
! While you don’t technically need this line it is important that you use it
! for your own clarification
neighbor 1.1.1.1 description Provider 1 >>Provider 1 Support Line<<
! Encrypted password for transferring your BGP data back and forth with your peer.
! Encrypting your BGP data will ensure that no one hijacks you routers.
neighbor 1.1.1.1 password 7 0934099082282F8234
! Using a loop back interface will ensure that the BGP peer always sees one
! interface that is in the network that you are advertising as being up.
! This will always keep the peering with your ISP up.
neighbor 1.1.1.1 update-source Loopback1
! The version of BGP that you are using. Version 4 is the most widely used and most recent.
neighbor 1.1.1.1 version 4
neighbor 1.1.1.1 route-map Provider1 out
As a final note, BGP is a powerful protocol with lots of features and options. However, most ISP’s don’t support the full suite of options that BGP provides so don’t expect to use all of them in order to shape your traffic.

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