BGP 4-Byte ASN’s

Lessons Learned: 

4-Byte BGP ASNs

0.0   – 65535.65535 notation

-0. [0-65535] denote original 2-byte ASNs

Requires backwards compatibility with old code

-4 byte ASN support negotiated during capability exchange

-“OLD” BGP speakers are sent ASdot numbers encoded as ASN 23456

-Real AS-Path encoded with optional transitive attributes AS4_AGGREGATOR as AS4-PATH

Most Devices that are running later versions of 12.4T code will support the 4-Byte AS.

The quickest clue will be under the BGP process – when you configure the AS number, if you’re not allowed to add for example:

Router bgp 1.5 – Then the code you’re running does not support the newer 4-Byte AS Numbers.

To view the 4 Byte field you can simply use # sh ip bgp command – under the AS Path you will see the 4 byte number.

Note: From the perspective of any system that only supports the two Byte AS number, essential the Local AS that it will send to its peer is AS# 23456 (or HEX 5BA0). Also any 4 –byte AS that is in the AS path will be encoded as AS# 23456.

There’s an encoding that happens on routers that do not support the 4 byte that converts the AS to 23456 – so the path might look like 4 23456 23456 23456. The real path un-encoded would look like 4 1.4 1.5 1.6, etc.

Key point is that from the device that only support only the 2-Byte AS – they need to say that the remote as is 23456.

Note: One thing to be aware of – if you’re doing EBGP multi-hop peering, we need to make sure not to introduce a “BGP race condition” in the case that the neighbors address we’re learning is also a route in BGP. By peering with neighbor that’s already advertising that same peer prefix into BGP. This could cause a BGP timeout because BGP cannot rely on itself for transport.