17 Mar
2016
17 Mar
'16
4:28 p.m.
RFC4360 defines extended communities, but I don't
know much more about them.
So far not I haven't seen much usage of communities within AMPR BGP networks.
Of course the used routers must support it as well...
Which usages would you like to coordinate?
My question is if I need to coordinate... I think probably not.
The usage is to mark routes with an origin, just like Jann mentioned in his
reply: we have a central gateway with VPN support and we can setup a VPN from
routers in the country that are interlinked by radio links, where of course we
would like the traffic to flow over the radio links unless they have failed, in
which case the routing via VPN would be an alternative. For BGP to know which
routes are radio routes and which are VPN routes, a "community" is the BGP name
for an attribute that you can add to each separate route, at its origin.
Then every router can examine this attribute and assign a lower preference value
to the route. Routes with a lower preference are not used when a route to the
same destination with default or higher preference is available.
Since these are arbitrary numbers, I use the first
decimal part of the 32bit
address as the high word (so that the 42 + country is conserved).
Just an idea, and it works...
e.g. 42226:0
Yes, that would be a possibility. Right now I have used 44137 (our net is 44.137)
but a numbering like that would be possible too.
Of course this conflicts with the common practice to use the 16-bit AS as the first
16 bits, because we don't own any of those two AS numbers. Jann's proposal of
using
a 16-bit private AS avoids this conflict, but it is incompatible with 32-bit AS.
However, in practice there is no problem because we will never see community
values from an AS like 42226 or 44137 on our isolated network.
(where we are using private AS numbers only)
Again, for clarity, our network is BGP routed on internet, but this is a completely
separate thing. BGP at the internet side is run by our ISP who advertises our
44.137.0.0/16 network on internet (under agreement with ARDC) using their own
AS number, receive the data and send it over an ethernet link to our gateway,
where we relay it to our radio network which also uses BGP, using private AS numbers
and those community values we are discussing, but there is no BGP traffic
"across" the gateway, only IP traffic to/from internet.
So my guess is that any community value set up using a convenient numbering scheme
to reduce the conflicts with cross-border schemes is sufficient, a more elaborate
scheme like the IP address or AS number coordination is not required.
(note that HamnetDB has a registration for AS numbers, but not for community values)
Rob