Subject: Re: [RPSEC] another example of peering topology check not catching

From: Sandy Murphy <[email-masked]>
Date: Mon, 14 Mar 2005 10:56:14 -0500 (EST)

>Suppose B also advertises the /24 to both A 
>and C.
>...
>Now, for whatever reason, C decides not to readvertise the /25, but only 
>the /24. What path is the traffic coming from beyond A and C going to take? 
>Possibly A, correct? Isn't this also against B's policy?

No, I would say not, since B was advertising the /24 to both A and C.

Why would you say this was against B's policy?  Sure looks like B
intended this to happen.

--Sandy

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From: Russ White <[email-masked]>
Date: Mon, 14 Mar 2005 11:04:13 -0500 (Eastern Standard Time)

> Why would you say this was against B's policy?  Sure looks like B

It's B's policy not to accept traffic on that link unless it's link to A is 
down. This is part of the problem with policy, it's hard to define what you 
mean when you say "policy," and it's also hard to express every possible 
meaning in routing itself.

If we could express all the policies we wanted to in routing, then why do 
we have communities, and MED, and other such things? Why don't people just 
advertise what they mean, and not worry about all this other stuff? Because 
the routing information carried in BGP updates, by themselves, don't carry 
enough "policy" to sufficiently express what people mean.

:-)

Russ

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