BGP filtering PA and PI blocks

Allot of documenation i have read suggest PA blocks are less likley to be
filtered out of BGP announcements that PI blocks.
Out of curiousity how does a filter differenciate between a PA and PI block,
i would have thought an address block can only be differentiated by its
prefix length alone ?
Reply to
Glen Watson
Loading thread data ...
On 12.06.2006 15:45 Glen Watson wrote
Have for example a look at
inetnum: 195.50.106.0 - 195.50.106.255 netname: YAHOONET descr: YAHOONET country: GB admin-c: KW3969-RIPE tech-c: LTHM status: ASSIGNED PA remarks: all abuse reports to snipped-for-privacy@level3.com mnt-by: LEVEL3-MNT mnt-lower: LEVEL3-MNT mnt-routes: YAHOO-MNT source: RIPE # Filtered
The status field gives you information about PA or PI
Arnold
Reply to
Arnold Nipper
Thanks.
So do isp's actually compile a list PA an PI space using whois and put this information into thier BGP route filters ? Seems it would be a very tedious way of filtering PI announcements.
Reply to
Glen Watson
No, that would be tedious and unmanageable. ISP usually filter on prefix size and filter out long prefixes (say /24 and up) as ISP's usually advertise /16, /17 etc.. That's one way of doing it anyway.
PI space would tend to have a longer prefix and so would be more likely to be filtered by tier 1 (or maybe even tier 2) ISP's.
Chris.
Reply to
chris
On 14.06.2006 00:03 chris wrote
To be more precise they actually might use something like ftp://ftp.ripe.net/ripe/docs/ripe-380.txt which is also available for the other registries (ARIN, AfriNIC, LACNIC and APNIC)
Reply to
Arnold Nipper
The general guideline between tier 1s is to filter at the /24 and shorter level. (/23, /22, etc..)
Longer prefixes between peers will usually be dependent larger aggregate blocks to pass between peers. Since PI space is doled out at /24 and larger, it usually doesn't encounter filtering based on mask length. I believe there are a couple filtering at /21 level still. (not 100% on that)
Glen, the nsp's don't really care if it is PA or PI space and don't compile lists like that.
Grog late, I know...
Reply to
Grog
Thanks.
So if i had my own addresses that couldn't be agregated, say of prefix length /23 i could well be stuffed due to the filtering that could occur between tier 1's
I say this as we qualify for our own /23 small pa space with RIPE. If we chose to go down that route and announce our routes with bgp via 2 upstream isp's through multihoming we could get filtered.
It sounds like we may be better getting address space allocated out of one isp's aggregatable block and announce this over both isp's so that way at least we could rely on the shorter prefix aggregates getting announced everywhere.
Have i understood correctly ?
Reply to
Glen Watson
With a /23 you should be fine since /23 > /24. With your PI /23 and 2 upstreams, you should have no problems announcing your route globally. I personally think it is better to get PI space than PA if you qualify for it. If you change upstreams later on and have PA space, think of the nasty renumbering job you have in front of you.
When it comes to address space, think in terms of 32 bit addresses, so it helps to be able to think in binary (well, kinda).
larger (more total ip addresses) netblocks have shorter netmasks. smaller netblocks have longer netmasks.
Now, that should be clear as mud.. :)
Reply to
Grog

Cabling-Design.com Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.