Failure detection is only the first step in BGP reconvergence - and if
you have two links to different ISPs, reconvergence times will be in the
order of minutes anyway, so tuning down fault detection from "30s" to "1s"
will not make failover instant anyway.
Very interesting discussion ... sorry for late reading, but I was in
long haul transit :)
Few observations ... There are few components ...
- preparation
- detection
- reaction
Let me explain.
- preparation ... it really means that you should have a backup path in
place via different exit point (or multiple local paths what may not be
easy). If your other exit is via different ASBR I do recommend at the
current state of the technology to use Diverse-Path on RR to send backup
path towards the best path's ASBR. This is shipping feature in cisco
*well yes I am biased .. I have designed that one :)* When all routers
talk add-paths you could switch to that.
- detection ... reducing hold time in BGP is a bad idea. if the goal is
to converge in 1-3 sec max I recommend (if BFD is not an option - even
single side BFD) use a very unknown IOS feature called "Object
Tracking". It is a piece of excellent code written by cisco EMEA
developers which can invalidate your next hop based on periodic ping
(every time X) to peering ASBR. Very cool tool.
- reaction ... yes PIC is the best way. You preprogram to FIB backup
paths then just at the failure time switchover to backup not per net but
per next hop speed at any BGP switching node ... assuming no intra-as
tunneling. If there is tunneling you just need to switch once.
Best,
R.
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