bgp internal as vs external as

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Dec 10th, 2007
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So, I've been working on a lab exercise to redistribute bgp from across MPLS into an eigrp internal network. Thanks to this forum I learned that because two of my MPLS simulated devices were both using BGP AS 1 and that made them "internal" so I had to use "distribute internal". Okay so far.


Now the question is, in real life when I look at my MPLS network, each office uses BGP 1 as the AS for the local BGP process on that router, but it uses 65005 for the "neighbor".


In such a case, even though all my routers have BGP 1 for the AS, all their neighbors would be 65005...meaning they are not internal as's to themselves?


I'm trying to understand how 'global' or 'local' the AS number is. It stands to reason they can't be too far reaching as there are so few numbers to pick from, compared to the number of ISP's which sure must be using BGP....

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Collin Clark Mon, 12/10/2007 - 08:33
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Your ISP is probably doing AS-Override. At the PE-CE edge all local AS Numbers are removed from the AS-PATH. This prevents the CE router from seeing all but the provider AS number, thus eliminating the need for each site requiring a different AS number. It also allows the Customer to use the same AS number at each site.


HTH and please rate.

jean.christophe Tue, 12/11/2007 - 04:19
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Hi,


As you mentioned, there is two types of Autonomous System number: Public and Private.


Public AS number is used by ISPs and multihomed companies.


This public AS number is used by BGP to advertise public network to the Internet and make routing decision.


A Private AS number (AS64512 through to AS65535) are not allowed to be used for public Internet connection.


Now, indeed, a BGP session between routers belonging to the same Autonomous System is called: internal BGP session.


This is used to exchange routing information in the backbone of a provider.


While a BGP session between routers belonging to different Autonomous System is called: external BGP session.


This is used to exchange routing information between ISPs.


HTH


Christophe

suelange Wed, 12/12/2007 - 07:20
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my thanks to you both, indeed it does help some. I'm still muddled about when "redistribute internal" would be necessary on my BGP connection.


If my router, which is connected to the MPLS has BGP1, and it connects to PE whose BGP is 65005, then to get my BGP routes to be carried across that link to another router on the other side whose BGP is 1...


Am I using redistribute internal because the ultimate goal is to get routes from A (BGP1) into B (also bgp 1) or am I only concerned with getting routes from A (BGP1) to the neighbor in this case PE, (BGP 65005)?


I'm still confused about how far-reaching the AS is. Once there is an external AS in the pathway between two same numbered AS's...are they no longer internal to each other or is the AS unrelated to physical connectivity?


I'm trying to decide if to ask my ISP to change our local BGP AS numbers so that they are not the same. In my tests, this causes some issue to have them all the same, that I don't see if I just change them to all be external. But in my lab I don't have true PE equipment in the way so I don't have a clear picture of how that could change things.

Collin Clark Wed, 12/12/2007 - 08:02
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All part of the fun in dealing with an ISP! You're limited in what can be done/seen. All you need to worry about is advertising the route(s) from each site. What happens in the cloud is their responsibility and as customers we're blind to a lot of things. I personally don't like redistributing from IGP to BGP. Rarely do I need to advertise all those internal routes. I usually use the network command. I do redistribute (with route maps) from BGP to IGP however.


HTH

suelange Wed, 12/12/2007 - 08:39
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thank you!


Yeah I need to get my internal routes across the MPLS cloud to the routers on the other side so that users there can find servers/services here. So I'm doing the distribute from bgp into eigrp. BGP and EIGRP are both active on the routers, EIGRP active only on switch.


At least thats what I was trying to do in my lab but that might not be the best way. I'm basing much of my design on various text books used to study for ccnp and sometimes the books give you samples but don't tell you why they did what they did. It's more a matter of teaching you the commands but not much about good design.... :-(

Collin Clark Wed, 12/12/2007 - 09:35
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Yeah, when I labbed it up, I was running into stuff I could not emulate. I was lucky enough to get two sites online before our migration. I was able to really tweak things and get everything working right with all the security in place before going live.

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