Subject: IDMR questions: unicast routing asymmetries; MBR questions

From: [email-masked]
Date: Tue, 18 Feb 1997 11:42:37 -0800 (PST)

I (still) have a few questions about the effect which 
"global" unicast routing asymmetries have on various 
multicast routing protocols.

e.g.:

AS1 <-------------------> AS4
^                          ^
|                          |
|                          |
|                          |
v                          v
AS2 <-------------------> AS3

Suppose the default unicast route from AS1 -> AS3 
is via AS2, but the unicast route from AS3 -> AS1
is via AS4.  Locally, the routes are all symmetric, 
[The routes AS[m] <-> AS[n] are all symmetric]
but globally, they are not.  My belief is that this
is (now) perfectly normal (in the unicast world), 
and has been for some time, and anyone wanting to 
can use traceroute/etc. easily to find examples.

My question is, is this a problem for native 
(non-encapsulated) multicast?

Wny?  Which protocols?  (All?)

What are some of the proposed methods to deal with it?
(Besides making it go away, which I don't think is
a realistic option.)

Do dual-redundant MBRs affect this?

Is any of this written down anywhere?  [It does not seem
to be directly addressed by "draft-thaler-interop-00.txt .ps",
although that may be because I lack the necessary
combination of knowledge and cleverness to extrapoate
the information therein to answer these questions...]

---

In a somewhat related vein, if routing-domains/regions/AS's,
each running one of the various multicast routing protocols 
[e.g. PIM-SM, CBT, MOSPF], are glued together via an 
"IDMR routing protocol" [today, this is DVMRP], 
is it the case that *all* externally-visible sources 
[that is, visible in this "global" "L2" "IDMR" routing domain] 
have to be internally propagated-through/managed-by 
each routing region/domain?  For example, suppose an 
AS/region is running PIM-SM.  Is it correct that (today) 
the RP-set routers each have to carry state about 
*all* the external groups that the transit-area MBRs
[multicast border router] for that region know about?

In other words, suppose that systems in AS1 and AS3 above 
are sending/receiving a particular multicast group.  Is
it correct that RP-set routers internal to AS2 and AS4
will have to know about the group [they will know that 
no subnet in their region needs the group]?  Or, can
the knowledge of such groups be confined to the MBRs?
(Today).  

What implications are there, if any, for the configuration
of transit-areas/IX's/NAPs relative to both backbone
carrier networks, and, leaf AS's?  What about AS's
that have multiple connections to different backbone
networks?

Are there (current, active, not withdrawn) proposals for 
(IDMR) hierarchical multicast routing?

[As usual, please pardon any/all ignorant questions herein.]

---

  Hugh LaMaster, M/S 258-5,     Email:       Please send ASCII docs to:
  NASA Ames Research Center     Internet:    [email-masked]
  Moffett Field, CA 94035-1000  Or:          [email-masked] 
  Phone:  415/604-1056          Disclaimer:  Unofficial-personal *opinion*.