We have a fairly similar design for our Metro Ethernet network.
Our primary method of protection is STP(MST). I've been thinking about
this, and I can't come up with a reason why we even really need an IGP
down to the edge PE devices? Since it's all layer2 - the core
switch/routers see all of the PE<>core links as Connected routes anyway
- what's the point of bother pushing your IGP down there? It's just more
needless routes.
That leaves you with a very small IGP in your core.
Adam Armstrong wrote:
Nathan wrote:
On Tue, Oct 21, 2008, Adam Armstrong <[EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]>> 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?
Umm. I've no idea what you're talking about now... The switch doesn't
speak LDP. It can merely participate in your IGP for its loopback
address.
Just give the switches an IP in the subnet that exists on their layer
2 domain and point their default route at one of the PEs (or do hsrp
between a couple of them).
> 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.
Well, the switches aren't important here, so if you plan to do ipv6 in
the future and aren't a huge ospf fan, have a look at isis now and
switch if you like it. It's definitely a lot easier to manage and
troubleshoot. Not to mention not having to run two versions of ospf
when you want to do ipv6!
> 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-----------/
Second option is the sensible one. Think of it as building 2 core
layer 2 domains across witch all of the PEs can talk to eachother.
During normal operation, they balance across the two domains, when a
switch or link dies, the traffic goes across the other. It's a
relatively standard design.
http://alpha.memetic.org/basic.jpg is how i would draw it.
> 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).
Well, tune your IGP so that it notices as quickly as possible and
pulls down the link.
You want as few routes as possible in IGP (so just links and
loopbacks), but i guess you already knew that! :)
adam.
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