eBGP and iBGP

Answered Question
Jan 29th, 2008
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How do you configure a router to use eBGP on the s0/1 and iBGP on f1/0?


I have two router with a connection to two different ISP's. I'm setting up a lab and testing . I have my IGP running just fine now I'm working on the BGP side of it and i don't know how to configure the iBGP and eBGP.

Correct Answer by Kevin Dorrell about 9 years 1 month ago

You are running router bgp 120, i.e. AS 120. If you define a neighbor with "remote-as 120", then it will run iBGP with that neighbor. If you define a neighbor with remote-as anything else, then it will run eBGP with it.


As for the network statement, you need a mask as well. BGP only advertises prefixes where the address AND the mask are both correct for an existing route in the routing table, e.g. network 172.16.1.0 mask 255.255.255.0.


However, I am beginning to suspect that there is a fundamental misunderstanding here. Unlike the other routing protocols, BGP does not use the "network" statement to activate BGP on an interface, nor does it discover neighbors automatically. Neighbors have to be configured manually and individually.


Furthermore, by default, BGP does not originate any prefixes. You have to tell it explicitly what to advertise to its neighbors.


I'm sorry if I have misjudged your level of knowledge of BGP, but I'm afraid you might need to do some fundamental reading about BGP before you try configuring it.


Kevin Dorrell

Luxembourg


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massimiliano.se... Tue, 01/29/2008 - 06:17
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Hi,

When you define neighbor, automatically if neighbors are in the same autonomous system (internal BGP neighbor) then they will run iBGP. If the neighbors are in different autnomous systems (External BGP neighbors)then they will run eBGP.

The link below is "Configuring BGP"; more in particular you can see "BGP Neighbor Configuration Examples":

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800ca763.html#wp1002801

I hope this helps.

Best regards.

Massimiliano.

smitty6504 Tue, 01/29/2008 - 06:27
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I'm running a 172.16.x.x between f1/0 on R1 and f1/0 on R2. When i use "rotuer bgp 120"

that works but when i try network 172.16.x.x it says uncomplete command. I am running IOS 12.4

Correct Answer
Kevin Dorrell Tue, 01/29/2008 - 06:46
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You are running router bgp 120, i.e. AS 120. If you define a neighbor with "remote-as 120", then it will run iBGP with that neighbor. If you define a neighbor with remote-as anything else, then it will run eBGP with it.


As for the network statement, you need a mask as well. BGP only advertises prefixes where the address AND the mask are both correct for an existing route in the routing table, e.g. network 172.16.1.0 mask 255.255.255.0.


However, I am beginning to suspect that there is a fundamental misunderstanding here. Unlike the other routing protocols, BGP does not use the "network" statement to activate BGP on an interface, nor does it discover neighbors automatically. Neighbors have to be configured manually and individually.


Furthermore, by default, BGP does not originate any prefixes. You have to tell it explicitly what to advertise to its neighbors.


I'm sorry if I have misjudged your level of knowledge of BGP, but I'm afraid you might need to do some fundamental reading about BGP before you try configuring it.


Kevin Dorrell

Luxembourg


smitty6504 Tue, 01/29/2008 - 07:11
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Kevin,


I have no knowledge of BGP. I am just trying to setup the test lab. I have the BSCI book but i'm on the policy routing chapter, the next chaper is BGP.

Kevin Dorrell Tue, 01/29/2008 - 07:15
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I think you should read the BGP chapter before you start configuring in the lab. You will be in for some surprises. ;-)


Kevin Dorrell

Luxembourg


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