Hi authors, all
As an individual contributor, please find below some feedback on the policy.
I'm wondering if we could address the conflict on a per FEC/Prefix basis rather
than on a per IGP advertisement basis.
If so, this may avoid the discussion between the Quarantine vs ignore policy.
The problem that we need to solve, is to find the SID for a prefix (P1).
The algo could be:
- Find all SIDi advertised for the prefix P1
// identification of Prefix conflicts
- For each SIDi find all the prefix Pij associated with SIDi
// identification of SID conflicts
// as a result, we get a list of SIDi - Pij for P1
Get the best as per the preference algorithm.
If best Pij == P1
then use SIDij for P1
else return no SID / no SID available
for this prefix P1
Note that it would probably be better for the preference algo to put the SID
tie-brake before the prefix tie-break as with the prefix tie-break, we suffer
from the conflict twice (Prefix - SID mapping, then SID- prefix mapping) which
increase the diversity and hence the chance of not finding a valid entry. But
for the below examples, I used the preference algo from draft-ietf-*-00
Below are examples, running this policy on typical configuration error cases.
Examples
3.4.4. Network operation
Consider the following simple network example:
1. 100 nodes: R1 to R100;
2. IP Loopbacks are from 192.0.2.1 to 192.0.2.100:
3. SID are from 1 to 100;
4. R1 to R50 are SR capable and advertised their own SID using
Prefix-SID sub-TLV;
5. R51 to R100 are SR non-capable, running LDP and their SID are
advertised by two redundant Mapping Server MS1 and MS2;
6. As the number of nodes which are SR capable are expected to
increase and as in real deployment their Loopback addresses would
no the contiguous, the Mapping servers advertisement covers all
Loopbacks: (192.0.2.1/32, 1, 100);
Subsequent sections evaluate the consequences of a single
configuration error, for all conflict resolution options.
3.4.4.1. Example 1: SID configured on 1 node conflict with SID
configured on another node
Following a typo during configuration, R2 is configured with a SID of
12. That SID conflicts with the Prefix-SID advertised by R12 and the
Mapping Server Advertisement for R12.
Note: both MS advertisement are the same, so we only consider one in the
below analysis.
All prefix but R2 and R12, a single SID is advertised and hence selected.
For R2, the algo evaluates a conflict between the following advertisments:
R2 - SID2 - R2 (MS, MS)
R2 - SID12 - R12 (prefix SID, MS)
R2 - SID12 - R2 (prefix SID, prefix SID)
Best one is R2 - SID12 - R2 (smaller range (prefix SID),smaller range
(prefix SID))
==> SID12 is selected for R2.
For R12, the algo evaluates a conflict between the following advertisments:
R12 - SID12 - R12 (prefix SID, prefix SID)
R12 - SID12 - R2 (prefix SID, prefix SID)
R12 - SID12 - R12 (prefix SID, MS)
R12 - SID12 - R2 (MS, prefix SID)
R12 - SID12 - R12 (MS, MS)
Best one is R12 - SID12 - R2 (smaller range (prefix SID),smaller range
(prefix SID), smaller starting adresse (R2))
R12 <> R2 ==> R12 has no SID.
3.4.4.2. Example 2: SID configured on 1 node conflict with SID
configured on the Mapping Server
Following a typo during configuration, R2 is configured with a SID of
52. That SID conflicts with the Mapping Server advertisements of MS1
and MS2 for the loopback of R52.
Note: both MS advertisement are the same, so we only consider one in the
below analysis.
All prefix but R2 and R52, a single SID is advertised and hence selected.
For R2, the algo evaluates a conflict between the following advertisments:
R2 - SID52 - R2 (prefix SID, prefix SID)
R2 - SID52 - R52 (prefix SID, MS)
R2 - SID2 - R2 (MS, MS)
Best one is R2 - SID52 - R2 (smaller range (prefix SID),smaller range
(prefix SID))
==> SID52 is selected for R2.
For R52, the algo evaluates a conflict between the following advertisments:
R52 - SID52 - R52 (MS, MS)
R52 - SID52 - R2 (MS, prefix SID)
Best one is R52 - SID52 - R2 (smaller range (prefix SID))
R52 <> R2 ==> R52 has no SID.
3.4.4.3. Example 3: wrong configuration of a MS
Following a typo during configuration, MS1 is configured
(192.0.2.0/32, 1, 100). (i.e. 192.0.2.0 instead of 192.0.2.1). That
advertisement conflicts with the Mapping Server advertisements of MS2
and the Prefix-SID advertised by R1...R50.
We have a conflict for all routers except R100.
For LDP only routers R51 to R99 we have a conflict between both MS
advertisement.
For R52, the algo evaluates a conflict between the following advertisments:
R52 - SID52 - R52 (MS2, MS2)
R52 - SID52 - R51 (MS2, MS1)
R52 - SID53 - R52 (MS1, MS1)
R52 - SID53 - R53 (MS1, MS2)
Best one is R52 - SID52 - R51 (smaller starting address)
R52 <> R51 ==> R52 has no SID.
For SR routers, R1 to 50, we have a conflict between both MS advertisement
and Prefix SID advertisements.
For R2, the algo evaluates a conflict between the following advertisments:
R2 - SID 2 - R2 (Prefix SID, Prefix SID)
R2 - SID 2 - R2 (Prefix SID, MS2)
R2 - SID 2 - R1 (Prefix SID, MS1)
R2 - SID 2 - R2 (MS2, MS2)
R2 - SID 2 - R2 (MS2, Prefix SID)
R2 - SID 2 - R1 (MS2, MS1)
R2 - SID 3 - R2 (MS1, MS1)
R2 - SID 3 - R3 (MS1, MS2)
R2 - SID 3 - R3 (MS1, Prefix SID)
Best one is R2 - SID 2 - R2 (Prefix SID, Prefix SID)
R2 == R2 hence R2 use SID2.
Regards,
Bruno
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