Asnumber formats

• an asnumber is shown in a show ... command like, e.g. sh ip bgp neighbors.
• an asnumber is shown as part of the configuration, i.e. when using show run.
• the string representation of an aspath is generated whenever an advertisement from a peer is received. This 'string representation' of aspaths is also shown in sh ip bgp summary and in sh ip bgp paths, and it is also what is used to match regular expressions from as-path-access-lists!

• whether the asnumber was received on the wire in 2 or 4 bytes (well, of course you can never get an asnumber > 65535 in two bytes - but the other way round).

asdot

[<higher16bits>.]<lower16bits>

The part [<higher16bits>.] is only given out if it is not zero. This has the effect of not changing asn16 asnumbers in any way and making real as4 very obvious. But you potentially have to work on your regular expressions, i.e. if you used _[0-9]+_ in the asn16 world you have now to cope with _[0-9\.]+_ or similar.

asdot+

<higher16bits>.<lower16bits>

The higher 16 Bit value is always given out.

asplain

<32bits>

This is just an unsigned 32 bit integer. As with asdot, as2 numbers are not changed, but you have to realize yourself that you crossed the 65535 and are in the true as4 world (if that matters to you). But your regexps like _[0-9]+_ have not to be changed (if they do not depend on as numbers having no more than 5 digits).

asip

<highest8bits>.<8bitsbelowthat>.<8bitsfurtherdown>.<lowest8bits>

This is looks like an ip address. This format should never be used and is only there for testing purposes.

Asdot and asdot+ are defined in draft-michaelson-4byte-as-representation-02.txt, even if they are not called such. Asdot+ is supposed to be used after the transition period, i.e. when all bgp speakers worldwide can handle as4 - something which may well be decades in the future.

The configure command for setting the asnumber format which is used is

      bgp asnum format (asdot|asdot+|asip|asplain)

and it will show up in the configuration if you do not use asdot because the is the default format and will be used when you define nothing.

Remarks

The output format of asnumbers with the advent of as4 is currently being discussed big way. I have been told that the RIRs - who are obviously the first who work with as4 - have adopted asdot, as has cisco in their implementation. Out of the network operator corner comes an energic vote for asplain. A general consensus has not been reached yet; the RIRs and cisco may well be the defining actors based on the fact that they started using something.

This was the situation when I wrote this patch. I have no strong support for one or the other - if I do not count that I have seen as4 first on RIPE meetings, and there they have been presented in asdot. Not surprisingly because the presentations were done by RIR people. But: just having seen them written one way is no reason to choose that way as the only reasonable one.

So I decided to implement all formats, and make this runtime-configurable. You have to (hard!) reset your bgp sessions if you switch, because regular expressions are influenced by this (and remember to actually have your regexps in the right format!). The decision to make asdot the default is based on the fact that this is already in use by the RIRs (which would have been reason enough) and cisco. asip is more a joke than anything else, do not take that one seriously.