Cisco IOS XR 3.4.0 through 3.9.1, when BGP is enabled, does not properly handle unrecognized transitive attributes, which allows remote attackers to cause a denial of service (peering reset) via a crafted prefix announcement, as demonstrated in the wild in August 2010 with attribute type code 99, aka Bug ID CSCti62211.
https://exchange.xforce.ibmcloud.com/vulnerabilities/61443
http://mailman.nanog.org/pipermail/nanog/2010-August/024837.html