Re: REG: Multiple communities received in a single UPDATE

[Iljitsch van Beijnum <iljitsch@...]

On Mon, 12 Aug 2002, prem chand wrote:

> I have query regarding reception of multiple community values in a
> single UPDATE mesg.

> Suppose, if we received an UPDATE mesg containing NO_EXPORT and
> NO_ADVERTISE for a route, what has to be done. For each well known
> attribute, there is a seperate functionality. In this case, which
> functionality is expected to behave.

Are you building your own BGP implementation?  :-)

In this case it is quite simple: no_advertise is stricter than no_export,
so if you apply no_advertise you're doing no_export as well.

However, this isn't a very interesting example. In the real world, you'll
want to use user-defined communities. For instance, if an ISP has a
community 123:2 you can use to prepend the path twice, and a community
123:3 to prepend the path three times, and you set them both, what should
happen? Two prepends? Three? Five?

If you're implementing this on a Cisco, you'll be using a route map. The
route map either checks for the 123:2 community first and then for the
123:3 community, or the other way around. But once the first match clause
takes effect, the rest of the route map isn't used any more. So only one
community action is executed, the other is ignored.

(You may be able to check for both communities and perform some special
action if you see them both, but I'd have to check and see if this works.)

Iljitsch van Beijnum