[c-nsp] four L3 devices, three ports

[Stephen Wilcox]

On 28 Sep 2007, at 20:15, Mark Kent wrote:

>
> This is the first of what could be an annnoying sequence of questions.
> Please turn on your fixed-width font viewer...
>
> Here is the scenario:
>
>   {ISP1}  {ISP2}
>
>   [7301]  [7301]
>
>   [4948]  [4948]
>
>     {servers}
>
> and the question is What's the best way to hook them up,
> using only the three built-in gige ports?
>
> Ignore the servers.  What I run now is this:
>
>   {ISP1}  {ISP2}
>     |        |
>   [7301]-a-[7301]
>     |        |
>     b        c
>     |        |
>   [4948]-d-[4948]
>
> a is ptp L3.
> b,c,d are pruned-vlan trunks
>
> I do ospf among the four L3 devices in the vlan,
> with point-to-multipoint costs set.
>
> Link a carries ibgp, and also is in the one ospf area.
>
> I did this because I have this general feeling that if you've got two
> edge routers then you should plug them in directly to each other,
> no electronics in between.  And once I decided that, it nailed
> down how to hook up the rest.  This works.
>
> But...  I've been thinking about this:
>
>   {ISP1}  {ISP2}
>     |        |
>   [7301]  [7301]
>     |   \  /  |
>     b    \/   c
>     |   / \   |
>   [4948]-d-[4948]
>
> completely with L3 numbered ptp links (possibly not d).
>
> Stupid?  Good?  Better than the square approach?
> (I know, cisco says to avoid squares).

cisco says all kind of things :D

if i understand your setup you ought to do the square as only the  
7301s carry BGP and having gaps in your BGP core could cause routing  
loops (perhaps not with the above setup, but maybe a problem in the  
future) .. i note the vlans but even so, in a failure scenario i can  
see if you dont have loops you might have traffic taking inefficient  
paths tromboning vlans etc

Steve