Re: BGP over satellite link

[Philip Smith <pfs at cisco.com>]

At 16:45 07/04/2003 -0400, Joe Abley wrote:

>>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.

This is for the benefit of the list... ;-)

Generally the problem with eBPG-multihop is that the next hop is not 
physically attached by a direct link to the router in question. It is 
somewhere out there, in the wide blue yonder. And you have to set up some 
other sort of routing entries to tell your router how to get to this 
next-hop. Usually people will point a static route for that particular 
address at something closer. Or do worse, as is often the case.

Now, many of us all understand how this works. And know the pitfalls. And 
many ISPs also understand this, and take the attitude of "ebgp-multihop 
over my dead body". The only uses they see are for things like loadsharing 
over parallel circuits between physically connected routers, or providing 
routing information to the many route views around the world.

But there seems to be an urban myth on the go which says that 
"ebgp-multihop" is good. I encounter it lots, and I'm accused of all sorts 
of things when I try and point out the problems. Anyone who has worked in 
operations, or in a NOC, or on customer support, trying to deal with a BGP 
customer or peer who is not very experienced with BGP and has a badly 
broken eBGP-mhop configuration will very quickly start cursing Cisco for 
even having the capability. (Well, I did in my past. ;-) The problem I've 
hinted at above - providing the routing information to get to the next hop. 
When this breaks, due to bad static route, bad connectivity, poor 
configuration of IGP, etc, etc, etc, some people find that their eBGP 
multihop sometime might still work, but the path taken by traffic is 
completely unexpected. Or the BGP bounces up and down for no apparent 
reason. Or is broken and they have no idea how to restart it. Customers 
generally don't understand BGP, so they whinge at the ISP for their network 
being broken. And the ISP wastes lots of time explaining to customer that 
it's the customer network which is "broken". And round it goes. Been there 
many times. :-(

So advice, and certainly best practice as far as I and many others are 
concerned, is to use eBGP-multihop as an absolute last resort, and even 
then only if you really understand what you are doing.

>Contrary to popular opinion, you don't need lots of RAM to run BGP.

Exactly!!! Another urban myth, no doubt helped by sales people and 
resellers trying to maximise their profits. A simple basic 2501 with IP 
software will run BGP just nicely. The  full routing table won't fit on 
there, but as I go on time and again, no one needs the full routing table 
unless they are seriously in the transit business with many diverse paths 
to many different parts of the Internet. If people don't believe me, check 
out the NANOG BGP tutorials for the configuration examples, distilled from 
configs of real live ISPs.

>  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.

Exactly. I wonder how many ISPs have I worked with to multihome with BGP 
using the basic original 4M RAM 4M FLASH 2501 router...  :))

>Simplicity and consistency are good. Complexity is best avoided.

Agreed with Joe 100%.

philip
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