Re: [Isis-wg] [spring] Handling same SID mapped to different prefixes and vice versa cases

[Peter Psenak <[e-mail]>]

Hi Pushpasis,

On 10/9/15 12:33 , Pushpasis Sarkar wrote:
> Hi Peter,
>
> On 10/9/15, 11:34 AM, "Peter Psenak" <[e-mail]> wrote:
>
>
>> Hi Pushpasis,
>>
>>>>
>>>> Node N1 in area 1 advertises prefix P1 with SID X. ABR1 advertises P1 to
>>>> area 0.
>>>> Node N2 in area 2 advertises prefix P2 with SID X, ABR2 advertises P2 to
>>>> area 0.
>>>>
>>>> Node 3 in area 0 get these two prefixes P1 and P2 from two different
>>>> ABRs. How does it know that the originators for P1 and P2 are different?
>>>> Do you expect the ABR1 and ABR2 to detect the collision and withdraw SID
>>>> advertisement to area 0? If yes, that may become source of all sorts of
>>>> problems.
>>> [Pushpasis] Nodes in area 0 can see that for the same prefix there are two prefix-sid advertisements from ABR1 and ABR2 but another advertisement (not re-advertisement) from Node 3.
>>
>> there is no advertisement from Node 3 in this case. Node 3 is an
>> internal router in area 0 (non-ABR). How does node 3 figures out if the
>> prefixes P1 and P2 are sourced by the same node or not? Unless you carry
>> the originator-id with each prefix, I don't see how you can do that.
>
> [Pushpasis] I overlooked that there were two different prefixes P1 and P2 in your example. In that case I will take back my last comment and re-formulate the proposal. For the situation that you mentioned above, here is what how it should be dealt with..
> 1. ABR1 and ABR 2 re-advertises the prefixes P1 and P2 into Area 0.
> 2. ABR1 will learn from ABR2’s advertisement in Area 0 that the same index is associated with P1 originated by N1 in Area 1 and and a different prefix P2 (re)originated from another node ABR2 in Area 0. It should then raise a syslog error notifying that this seems to be misconfiguration.
> 3. Likewise, ABR2 will learn from ABR1’s advertisement in Area 0 that the same index is associated with P2 originated by N2 in Area 2 and and a different prefix P1 (re)originated from another node ABR1 in Area 0. It should then raise a syslog error notifying that this seems to be misconfiguration.
> 4. And then none of the ABRs should withdraw the prefix-sid-advertisements (I take back my last comment). Things should be left as is till the operator rectifies the configuration.

the question is what Node 3 does? If it assumes that thees are coming 
from same originator, it may end up sending the traffic wrong way, which 
a serious issue.

>
> The basic rule for any node to detect a anomaly and take action should be as follows…
> A. Multiple prefix originated with same but from same originator.. No need to throw error

we do not seem to have a consistent way to detect the same originator, 
unless we advertise originator with every prefix.

> B. Multiple prefix originated with same index but from different originator..  Throw syslog.. Use the index advertised with best-originator(s) of the prefix for MPLS ingress/transit routes.

there is no best-originator, that is the whole point.

> C. Same prefix originated with different index from same originator.. Throw syslog.. But don’t program MPLS ingress/transit routes for it… Ideally this scenario is almost next to impossible unless any implementation allows assoicating multiple sids with a singe prefix.

we should not make the behavior implementation specific.

> D. Same prefix originated with different index from same originator.. Throw syslog.. Use the index advertised with best-originator(s) of the prefixfor MPLS ingress/transit routes.
>
> Hoping I have covered all possible situation with the above set of rules.

the question really is if the above is worth the benefit you get.
It seems that simplicity may be a better option here.

thanks,
Peter


>
> Thanks
> -Pushpasis
>
>
>>
>> thanks,
>> Peter