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Re: transit across the ixs

  • From: Leo Bicknell
  • Date: Sun Feb 14 22:49:25 1999

In a previous e-mail, Randy Bush said:
> we can't decide whether to force next hop on their routes, or keep watching
> and de-peer them if they do not cease and desist.  what do others do in this
> circumstance?

[Not speaking for any employer, just for myself.]

	While I think it is rude not to set next-hop-self on transit,
I'll also point out doing so may actually hurt all involved.  Consider
a poorly constructed exchange point of two switches:

         +----------+           +----------+
TC ------+          |           |          +----TP
         | Switch 1 +-Weak Link-+ Switch 2 |
Sink-----+          |           |          +----Other
         +----------+           +----------+

TC = Transit Customer
TP = Transit Provider
Sink = Where all the traffic is going
Other = Some other network that the Sink network wants to get to.

	Now, if TP advertises the Sink to TC, and TC sends directly
to the Sink the traffic stays in switch 1.  However, if TP sets
next-hop-self the traffic is sent across the weak link to TP, then
back across the weak link to the Sink.  This may degrade the ability
for the Sink network to get to the Other network.

	I think it breaks down to your point of view:

1) I want the most efficient traffic distribution.

   These are people that tend to peer with everyone, and while they
   might dislike providers sending them stuff they didn't ask for,
   they will accept the traffic efficiency that I described above.

2) I want to control peering.

   These people tend to not peer a lot at public exchanges, and
   get flaming mad if you don't set next hop self because they
   want to see you pay the penality for the traffic, even if it
   does just all come back to them.

3) Exchange points are not for transit.

   These people don't like the concept of transit at the exchange
   at all, and find both situations distasteful for their respective

	Personally, I think it is worth the cost of a cross connect
to get transit off exchange points.  This has several advantages.
First, it gets no one upset.  Second, it gives the two parties
involved control over the connection (can you say QOS) that is not
available over an exchange point.  Third, it probably gets (some)
traffic off the exchange point, which with many having the
"weak links" is a good thing.

Leo Bicknell -
Systems Engineer - Internetworking Engineer - CCIE 3440
Read TMBG List -,

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