BGP 4 BYTE ASN LAB OUTSTANDING

BGP 4 BYTE ASN
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The original BGP ASN number was a 2 byte (16 bits) number which gave a 665536 possible ASN 0-65535
This is are not used up on the modern internet a rfc came about to extend the 2 byte of a 4 byte
giving the potential for 4294967297 ASN number.

Interaction
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New BGP Speaker advertises to its neighbor that it uses a 4-byte ASN using BGP Capability Advertisements. If the neighbor also advertises that it uses a 4-byte ASN i.e. a New BGP Speaker, the two neighbors can inform each other of their 4-byte ASNs using OPEN messages.
If the neighbor is an Old BGP Speaker and responds that it does not support 4-byte ASN, the New BGP Speaker can still bring up the neighbor relationship, but cannot advertise its 4-byte ASN to this Old BGP neighbor. Instead, the New BGP speaker uses a reserved 2-byte ASN, 23456, called AS_TRANS (even if multiple ASNs use this). The New BGP Speaker adds this ASN to the OPEN message

A New BGP Speaker advertises the routes to its New BGP neighbors with AS_PATH carrying 4-byte ASN
when advertising to an Old BGP neighbor during which it adds the AS_TRANS ASN instead of adding its own 4-byte ASN. The router also adds a new attribute, AS4_PATH, to the route
The AS4_PATH attribute is an optional transitive attribute which carries the real AS_PATH list, carrying both 4-byte and 2-byte ASN.
When an Old BGP Speaker advertises routes with AS4_PATH and AS_PATH attributes to a New BGP Speaker, the New BGP Speaker uses both attributes to reconstruct the path: AS4_PATH for 4-byte ASNs and AS_PATH for 2-byte ASNs.


LAB
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I do not have 12.4 (24t) ios so i will need to wait till i get it to lab this