Router ID on IPv6-Only

heasley heas at
Thu Sep 8 16:48:19 UTC 2022

Thu, Sep 08, 2022 at 08:13:33AM -0700, Randy Bush:
> > During some IPv6 numbering discussions at work today, someone had a
> > question that I hadn't really considered before. How to choose 32-bit
> > router IDs for IPv6-only routers.
> arbitrary 32 bit number unique in the autonomous system.  even in an
> ipv4 world it does not need to match any configured interface address.

A question Dorian and I discussed but never answered is, how are open
collisions handled if two speakers, presumably an external AS, happen
to have the RID?  Which would be possible if an AS chose their RIDs
randomly or intentionally used low numbers (eg: 0/24) to attract traffic
as a result of the BGP path selection process.

Based on the value of the BGP Identifier, a convention is established
for detecting which BGP connection is to be preserved when a
collision occurs.  The convention is to compare the BGP Identifiers
of the peers involved in the collision and to retain only the
connection initiated by the BGP speaker with the higher-valued BGP

Easily answered by testing, and could yield fun results (bugs).  I
presume both would (should) close and reconnect later, possibly repeatedly
before success, thereby delaying session establishment.

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