Jack, that's a good question and this is my understanding, so that could be
wrong.
I'm now aware of ANY ISP that would redistribute BGP into anything. The
reasons are:
1. You loose a lot of information (BGP metrics such as AS path, communities,
etc)
2. Lots of computation to convert any protocol, specially on the Internet
where the routing tables change every second
My understanding is that they basically use IS-IS or OSPF so the BGP routers
know how to reach the neighbors. Basically, on a Core-Distribution
architecture, BGP would sit on the Distribution where the customers are
connected and the Core router would run IS-IS or OSPF basically routing only
UUNet addresses so that BGP routers (Distribution) can reach each other.
Basically, that's how it flows:
Customer/Peers connect to Distribution routers and speak BGP. BGP routers
know how to reach each other (neighbors statement on the bgp config)
connecting to core routers that run IS-IS or OSPF. On a simplistic way, you
can think that BGP knows that to reach destination x needs to go through
neighbor A. IS-IS or OSPF are incharged of find out/define how to get to A,
so for instance IS-IS and OSPF would be responsible on provide multiple
path, load balance, etc on their core. In other words, if you connect to ISP
A in San Francisco and you want to send a packet to ISP B in NY, and ISP A
and B connect in NY, this is how things would work: you would send the
packet to ISP A in SF. ISP A would look into the BGP tables and find out
what is the best path. Best path would be iBGP to the BGP router from ISP A
in NY (I'm considering both BGP routers in SF and NY are BGP neighbors). The
path inside ISP A backbone to reach the router in NY is determined by IS-IS
or OSPF. When the packet reach ISP A's router in NY, this router will look
what is the best BGP path to reach ISP B and send the packet.
Again, that's my understanding and I could be very wrong here.
I don't want to increase the level of conplexity here, but I talked about
multiple path, load balance, etc. I was refering to IP. I'm sure most ISPs
that still use ATM would also use ATM to load balance traffic between links,
etc.
Does it help? :-)
Guilherme
-----Original Message-----
From: Jack Walker [mailto:[EMAIL PROTECTED]]
Sent: Thursday, November 16, 2000 3:41 PM
To: [EMAIL PROTECTED]
Subject: Re: IS-IS use??
I am not familiar with the ISP enviroment at all.
When we say UUNet uses IS-IS on their core, do we mean that they
redistribute their BGP routes from their edge routers into IS-IS and
redistribute back into BGP and the far end edge routers?
Which means the edge routers are running BGP to learn customers' routes and
redistribute these routes into IS-IS to router across the UUNet core? Just
want to verify that I understand this correctly.
Thanks
Jack
"Priscilla Oppenheimer" <[EMAIL PROTECTED]> wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> At 09:38 AM 11/16/00, Spolidoro, Guilherme wrote:
>
> >UUNet for example uses IS-IS on their core while the rest (or the
majority)
> >of the ISPs use OSPF. I wonder why UUNet chosed for IS-IS instead of
OSPF.
> >Maybe somebody on the list has an answer?
> >
> >Today I would chose OSPF over IS-IS because:
> >
> >- much more vendors support OSPF compared to IS-IS
> >- it's my perception that OSPF is the direction chosen by IETF,
>
>
> A few years ago a bunch of people wore T-shirts to an IETF meeting that
> said, "IS-IS=0." They did this to bug Radia Perlman. &;-) It didn't work.
> These days the IETF seems to do a lot of work on both IS-IS and OSPF. For
a
> while it looked like we could get by without knowing IS-IS. I don't think
> that's true anymore. The pendulum has swung back in its favor.
>
> Priscilla
>
>
>
> ________________________
>
> Priscilla Oppenheimer
> http://www.priscilla.com
>
> _________________________________
> FAQ, list archives, and subscription info:
http://www.groupstudy.com/list/cisco.html
> Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
>
_________________________________
FAQ, list archives, and subscription info:
http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
_________________________________
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
