Operational Issues
From AS4
Contents |
Notation
Several new notations for ASNs were defined, but fortunately everyone has agreed to use asplain in the meantime. Unfortunately asdot has been implemented already, so we'll probably have to use both notations for a while.
| Name | Notation | 16-bit Example (AS 18508) | 32-bit Example (AS 393222) | Comments |
|---|---|---|---|---|
| asplain | Decimal integer | 18508 | 393222 | Proposed as the textual notation in RFC 5396 and widely implemented |
| asdot+ | <high order 16-bit value in decimal>.<low order 16-bit value in decimal> | 0.18508 | 6.6 | Very little support |
| asdot | 16-bit ASNs use asplain, 32-bit ASNs use asdot+ | 18508 | 6.6 | Widely implemented and used by RIRs |
| ascolon | <high order 16-bit value in decimal>:<low order 16-bit value in decimal> | 0:18508 | 6:6 | Implemented by Redback |
Communities
RFC 4893 recommends to use the Four-octet AS Specific BGP Extended Community with 32-bit ASNs, however this is an I-D on -02 revision and Quagga is the only routing implementation that supports the I-D so far.
A problem with this I-D is that communities only support uint32:uint16 format, which means you cannot set the local administrator field to be a 32-bit ASN. This breaks the BGP community prefix filtering convention used at IX route servers (DE-CIX, INEX, LINX, …).
There is no BCP on using communities right now, other than to use extended communities.
AS_CONFED_SEQUENCE in AS4_PATH
- A detailed explanation and discussion from the NANOG mailing list is here: BGP Session Teardown due to AS_CONFED_SEQUENCE in AS4_PATH
- Breaking your network with AS4/ASN32 by Andy Davidson (NANOG45; 26 January, 2009)
- IDR mailing list thread on the proposed error handling changes in RFC 4893bis
- Cisco bug IDs: CSCsx10140 (IOS), CSCsx18598 (IOS XR), CSCsx23179 (NX-OS)
- Juniper bug ID: PR417046
- OpenBGPD: discussion and patched source code
Other Issues
- The neighbor remove-private-as command only works on private 16-bit ASNs (64512-65535) until routing implementations are updated for RFC 5398 to remove other reserved ASNs
- Possible routing loops: AS path strangeness with AS4, private ASes and remove-private-as
- 4 Byte ASN with Cisco IOS Software has a great introduction and description of how 32-bit ASNs work, and explains an issue with MEDs and AS 23456 (slides 15 – 17)
