[c-nsp] question about service provider network design

[Nathan]

On Tue, Oct 21, 2008, Adam Armstrong <lists at memetic.org> wrote:
> Nathan wrote:
>> - Is running OSPF on a switch at all useful when the switch is
>> connecting routers that are running MPLS, MP-BGP, and OSPF? Can it
>> provide faster detection of link loss?
>
> The routers can see each other directly at L2? Then no. It might make it
> easier to keep the switch's management loopback connected though.

Well I don't see how the LDP would keep running if the switch cut off L2.
The switch would need to speak LDP . . . which would make it an MPLS P
router, which would be cool but I'm quite sure neither 2960s or even 3550s
can do that :-) P router with eight gigabit ethernet ports running at line
speed for the price of a 2960 anyone? Seriously, what kind of beast does
that? A 7600 or 6500 I suppose, anything smaller?

Good point about the management loopback.

> Consider switching to IS-IS, assuming your kit can do it.

The switches can't, but I do think the routers can. What would the benefits
be? If I change to IS-IS, now's the time.

> Do you have a diagram?

I'm not sure that ASCII art will cut it, but I'll try . . .

First option:

 /----------SW----------WAN---------SW-----------\
 |  |  |     |                       |     |     |
PE PE PE     |                       |    PE PE PE
 |     |     |                       |     |  |  |
 \----------SW----------WAN---------SW-----------/

This way I don't have to have each PE connected to both switches in order to
communicate directly, it's only when a switch goes down that PEs only
connected to that single switch will have a problem. I'll have to place
different VLANs on top and bottom and use MST so that both links are used.
If I lose the ethernet link on a WAN link, MST notices immediately and
reroutes traffic.

Second option:

 /----------SW----------WAN---------SW-----------\
 |  |  |                                   |  |  |
PE PE PE                                  PE PE PE
 |  |  |                                   |  |  |
 \----------SW----------WAN---------SW-----------/



> When you say WAN, what do you mean? A long distance ethernet circuit? Or a
> Serial/Pos/etc?

Thay are seen as gigabit ethernet (copper or fiber), but they run over the
national backbone of bigger fish than I.  They are probably AToM
pseudowires. Unfortunately that means that when one goes down (not often,
maybe once or at most twice a year) I don't always lose the ethernet link
(and I suppose I might get one-way communication only).

-- 
Nathan