Got already a ticket at Mikrotik regarding my issue, and during this research I have found that Mikrotik increments by default atomic-aggregate=yes to any announced prefixes.
I am replacing a CER2024 with a Mikrotik, and moving some of the BGP sessions to the Mikrotik. In our setup we have multiple CER2024 that peer each-other with iBGP and Loopbacks with OSPF.
On Mikrotik I have added a few sessions, all seem to work fine until I want to send the received prefixes to our CER2024 and some IPv6 to a Dell L3 stack switch.
Both of equipments complain that there is an issue with aggregator length:
Dell 4032F:
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<187> Jun 19 00:00:16 core-stack-2 BGP[BGP Protocol]: bgpattr.c(1628) 955842 %% ERR [VRF ""] Received UPDATE from peer1011::4 invalid AGGREGATOR attribute. Aggregator AS is 25773. Aggregator ID is 0.0.0.0. Resetting peer.
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Flags : (0xd0) Optional, Transitive, Complete, Extended length
Type : (0x07) Aggregator
Length: (0x0008) 8
Error: Invalid AGGREGATOR attribute length 8
FRR/quagga are not affected and tested Cisco 7200 IOS on GNS3, but cannot replicate a a real setup with real bgp feed so I do not know for sure if the issue affects cisco or not.
RFC4271 says that the receiving peer should close the session if he receives invalid attributes, both Dell and Brocade do that, but what I do want to know until a fix is made available by Mikrotik, is anyone else encountering this issue?