North American Network Operators Group

Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical

Re: Notes on design of IPv6 BGP multihoming with special subroute attributes (was - Re: Shim6 vs PI addressing)

  • From: Iljitsch van Beijnum
  • Date: Thu Mar 02 08:23:16 2006

On 2-mrt-2006, at 13:44, william(at) wrote:

2.	In my current thinking on how to achieve ASN based IDR, we
	would not need ASNs for every organization that multihomes,
	only for each organization that provides transit.  This
	would greatly reduce some of the current and future demand
	for ASNs.
Yes, we wouldn't want to run out of AS numbers just now we're creating 4.29 billion new ones...

My thinking was that its a big waste of memory (in the global bgp table) to announce every IPv6 route in full in particular for cases when its sub-allocation and aggregate is already being announced.
Yes, it would be cool if the routers or route servers could automatically detect this and clean up the routing table. Unfortunately:

A --- B
/ \
\ /
C --- D

If X uses but A also announces, then A or B could decide to suppress the /24. However, Y will see the /24 through D and C but not through B and A, so Y will now send all of its traffic to X through C and D.

But it maybe possible to do limited bgp multi-homing by having such /48 and similar routes included as attributes of the main route, i.e.
A100:1000::/32 route would appear with extended attributes like
Subroutes: 0010/16 (2)
Some years ago, I suggested doing this by adding a bitmap to the aggregate route: a single bit is enough to convey holes in the aggregate, with two or three bits you can also do some traffic engineering. This will get you from a /16 aggregate to individual / 24s with 32 bytes (1 bit per more specific) or a /32 to /48s with 8 kilobytes.

Such an approach does depend on relatively tight packing of end-users that share the same ISPs, though.

All these approaches (especially second one) however certain problems when
you have to consider route security & authorization (i.e. SIDR/SBGP space)
IDR security doesn't come cheap anyway: be prepared to double or quadruple your router's memory and install crypto hardware.