Re: BGP over satellite link

[Joe Abley <jabley at isc.org>]

On Monday, Apr 7, 2003, at 15:39 Canada/Eastern, Stephane Bortzmeyer 
wrote:

> On Mon, Apr 07, 2003 at 03:35:34PM -0400,
>  Randy Bush <randy at psg.com> wrote
>  a message of 4 lines which said:
>
>>> There is something I always liked with you, Randy, it is the lot of
>>> details you give to support an opinion :-)
>>
>> your day for bad-mouthing people?
>
> Is it an explanation of the reasons why we should not run eBGP
> multi-hop? If so, I'm afraid I did not understand it.

I don't know what reasons Randy had for reacting to your diagram, but 
here are some thoughts.

> +-----------------------+                     +------------+   
> +---------+
> |  Upstream             |  Leased line        |   Entry    |---| BGP   
>   |
> |  router               |---------------------|   router   |   | 
> router  |
> +-----------------------+                     +------------+   
> +---------+
>              ^                                                     ^
>              |                                                     |
>              +-----------------------------------------------------+
>                  You'll need eBGP multihop between these two

The entry router needs to know enough about the network to know that 
the world is reachable through the "upstream router", and that the 
local net is reachable via the "BGP router". This is most certainly 
achievable; a strategic set of static routes on "entry router" will 
give it all the forwarding clue it needs, and everything will work.

The problems will come later when the ISP needs to announce some new 
network, or introduces some other new local topology. "Entry router" 
represents a new source of complexity in the routing domain; it's 
another place where things have to be kept up to date to avoid routing 
loops.

Contrary to popular opinion, you don't need lots of RAM to run BGP. The 
chances are "entry router" above has more than enough horsepower to 
announce local nets to "upstream router" and to learn a default using 
BGP. A cisco 2501 with the minimum amount of RAM necessary to load an 
IP image will do just fine.

Simplicity and consistency are good. Complexity is best avoided.


Joe


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