[c-nsp] question about service provider network design

[Dan Armstrong]

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 <lists at memetic.org 
>> <mailto: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?
> 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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