Re: [Isis-wg] Proposed isis-wg documents - draft-ginsberg-isis-route-preference-00

[Uma Chunduri <[e-mail]>]

Dear Les,


My detailed response in-line [Uma]:

Quick Summary:
----------------------

1. There are no route preferences currently defined in RFC 5305. In fact Tony (both Tony's!)  took more effort to not to have any by even removing the
    I/E bit (remember BGP MED issue?).   This is applicable to MT V4 TLV 235 (RFC 5120).
    "Down bit" semantic *only* used for not creating circular looping  for TLV 135/235 as defined (unlike V6 5308).

2. Even for TLV 236/TLV 237 (V6 variants) there is no internal/external metric and hence no route preferences from those. However, yes, 
    route preferences based on "Down bit " are defined in RFC 5308 (Section 5) and hence these are applicable to MT variant TLV 237.
    The good news is it's clearly defined and  is applicable to 236 and 237.
    If you want to correct this to make this "equivalent" to V4 (135/235), actually you create the non-existent loop issue for V6 you are PRECISELY  trying to prevent.

   Essentially this draft could create problems for BOTH  V4 (135/235 through clarification)  and V6 (236/237 through correction).

   Did you also see the backward compatible problems  I am talking about both V4 and V6 once an implementation follows your
   new draft with nodes behaving per 5305 and 5308 (and 5120)??

--
Uma C.


-----Original Message-----
From: Les Ginsberg (ginsberg) [mailto:[e-mail]] 
Sent: Thursday, August 21, 2014 2:36 PM
To: [e-mail]; Christian Hopps; Uma Chunduri
Cc: Hannes Gredler; [e-mail] list
Subject: RE: [Isis-wg] Proposed isis-wg documents - draft-ginsberg-isis-route-preference-00

Let me add to what Chris and Stephane have stated.
The issue is NOT confined to new style TLVs - it could occur w old style TLVs as well since RFC 5302 defined the DOWN bit for those TLVs. 

[Uma]: I didn't see you alter preferences to the old style in this draft?? I see only for 135/235 in Section 3.3 and 236/237  
in Section 3.4 http://tools.ietf.org/html/draft-ginsberg-isis-route-preference-00#page-5 


It is important to note that even though RFC 5302 allowed for the possibility of the DOWN bit being set in L2 LSPs (see Section 3.3) it did NOT define any L2 route types with DOWN bit set to one (see Section 3.1). This was not strictly necessary because RFC 5302 also states (Section 3.3) that DOWN bit should be ignored when seen in L2 LSPs. IMO it is the subtleties of allowing DOWN bit to be set in L2 LSPs but specifying it should be ignored that has led to inconsistent implementations. This is compounded by the fact that - as Chris already pointed out - RFC 5308 did NOT ignore the DOWN bit in L2 LSPs.

[Uma]: You are trying to fix TLV 236 route preferences with DOWN bit because you see a problem with TLV 135 (V4?).
             IMO,  It doesn't make sense and Spec is very clear for both V4 and V6.


The primary point of this draft is to clarify what the correct behavior should be. In doing so we do not make ANY changes to RFC 5302/5305 - a strict interpretation of those RFCs is still correct under the rules defined in this draft. We do correct RFC 5308 so that it is consistent w RFC 5302/5305.

[Uma]: You inadvertently created problems for both V4 and V6, which you were trying to solve to start with.
             Let's see quickly how.. take the same example 
             http://tools.ietf.org/html/draft-ginsberg-isis-route-preference-00#appendix-A 

            Assume instead of 135, all nodes are using V6 TLV 236/237.
            R2 is now updated and  implementing your draft  (section 3.4 as stated) and R1 is not.
            R1 is just implementing 5308.

		============ http://tools.ietf.org/html/draft-ginsberg-isis-route-preference-00#page-6===
         		 3.4.  Order of Preference for all types of routes supported by TLVs 236
      			and 237

   		This document defines the following route preferences for IPv6 routes
   		advertised in TLVs 236 or 237.

  		 1.  L1 intra-area routes; L1 external routes

  		 2.  L2 intra-area routes; L2 external routes; L1->L2 inter-area
  		 routes; L1-L2 external routes;L2-L2 inter-area routes

   		3.  L2->L1 inter-area routes; L2->L1 external routes;L1->L1 inter-
  		 area routes
                            ============

                    R2 selects R1, because it prefers the route which doesn't have up/down bit set in "L2-L2 inter-area routes" (#2 above) as defined above.
                    R1 selects R2 as next hop due to lowest metric and it doesn't have corrected behavior of your draft for " L2-L2 inter-area routes".
                   Hence the loop.

--
Uma C.