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the question is about BGP update messages, namely about the maximal amount of prefixes which can be places in an NLRI.

malocho
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1 Answers1

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No limit, other than the limit on the maximum size BGP Message. You can, in principle, mix withdrawn and reachable NLRI, and withdrawn/reachable NLRI for different AFI/SAFI... but I would not recommend it.

Chris Hall
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  • could I see somewhere an example of an NLRI with several prefixes, from example a sample from the BGPStream, thanks – malocho Mar 10 '20 at 09:17
  • The [RIPE RIS](https://www.ripe.net/analyse/internet-measurements/routing-information-service-ris) has a huge collection of real BGP Messages. See, in particular, the [Raw Data](https://www.ripe.net/analyse/internet-measurements/routing-information-service-ris/ris-raw-data). – Chris Hall Mar 10 '20 at 09:34
  • I used pybgpstream library and extracted a range of messages but have not seen a message with several prefixes. That is the reason for my question. – malocho Mar 10 '20 at 11:00
  • btw the BGP message descriptions are not intuitive for me, I have not found a good explanation yet. – malocho Mar 10 '20 at 11:01
  • Where there are multiple prefixes they all have the same Attributes. The routes advertised by a peer will (generally) have the same attributes (they are all routes to prefixes within the peer's AS). So, when a BGP session comes up, the peer's messages may contain multiple prefixes. So looking for messages from a given neighbor just after a BGP OPEN may help. Or just after a Route Refresh. The sender does not have to collect together prefixes in this way. What a sender does depends on the internal mechanics for generating updates and the timing of changes which require those updates. – Chris Hall Mar 10 '20 at 12:52
  • I just looked at libBGPStream. I have a strong feeling that each call of `get_next_record(record)` is reading the next BGP message. Then each `record.get_next_elem()` is returning one element, containing one prefix, iterating through all the prefixes in the BGP message. – Chris Hall Mar 10 '20 at 13:37