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administrative weight – Cisco proprietary. Assigned to an NLRI to influence that router’s choice of “best”. Is not carried in BGP updates to other routers.
AGGREGATOR – optional transitive. A PA containing the ASN and RID of the summarizing router.
AS_CONFED_SEQ – part of AS_PATH. Contains the confederation ASNs the route has gone through in an ordered list.
AS_CONFED_SET – part of AS_PATH. Contains the confederation ASNs in an unordered list.
AS_PATH – describes the ASNs that the route has passed through. Contains four segments, AS_SEQ, AS_SET, AS_CONFED_SEQ and AS_CONFED_SET. AS_SEQ is the one we typically care about the most.
AS_PATH access list – an IOS feature that allows you to do pattern matching against the AS_PATH PA.
AS_PATH length – how many ASNs did this NLRI traverse. Each ASN in AS_SEQ counts as 1. AS_SET counts as 1, period, no matter how many ASNs are in there.
AS_PATH prepending – when a router sends a BGP update to another router, it will prepend his ASN on the front of the path. You can also artificially manipulate AS_PATH by adding additional ASNs to the AS_SEQ, probably so that the route will look less desirable to other BGP routers.
AS_SEQUENCE – the AS_PATH PA segment we’re often most concerned with in typical BGP operations. This contains an ordered list of ASNs that the route has traversed.
AS_SET – similar to AS_SEQUENCE, only the list is not ordered. This segment of AS_PATH PA is populated as a cross-section of all ASNs traversed by subnets that make up a summary route.
ATOMIC AGGREGATE – well-known discretionary. Indicates that the route is a summary.
BGP decision process – the lengthy process which BGP will follow to determine which route is best, assuming multiple routes for the same NLRI.
CLUSTER_LIST – optional nontransitive. Sort of like AS_SEQ, this PA lists the route-reflector clusters that that the route has been advertised, for purposes of loop prevention.
COMMUNITY – optional transitive. A 32-bit value that usually stores an ASN and community ID of the routers that set the PA. That information can be keyed off of by the receiving router to influence routing decisions.
LOCAL_AS – part of the COMMUNITY PA, indicates that the route should not be advertised out of the local confederation subAS. Cisco-proprietary, RFC1997 version is called “NO_EXPORT_SUBCONFED”
LOCAL_PREF – larger is better; sent throughout an ASN to indicate to all routers in that ASN the preferred exit point.
MULTI_EXIT_DISC (MED) – smaller is better; called the “BGP metric”. Router in one ASN set this value and advertise it to routers in another ASN.
Neighbor Type – describes what sort of relationship the BGP neighbor has, whether it’s eBGP, iBGP or in a confederation.
NEXT_HOP – a PA containing the next-hop router to reach a particular NLRI.
NLRI – network layer reachability information. In BGP, this is the ip prefix and prefix length one is trying to get to.
NO_ADVERT – part of the COMMUNITY PA, indicates that the route should not be advertised to anyone else.
NO_EXPORT – part of the COMMUNITY PA, indicates that the route should not be advertised outise of the ASN.
NO_EXPORT_SUBCONFED – the same as LOCAL_AS, except this is what RFC1997 calls it.
optional nontransitive – a BGP router doesn’t have to support this kind of PA. If it gets one, it should strip it before advertising the route.
optional transitive – a BGP router doesn’t have to support this kind of PA. If it gets one, it should forward it along in advertisements of the route.
ORIGIN – describes how the router made it into the BGP table.
ORIGINATOR_ID – route-reflectors use this to describe the RID of the iBGP neighbor responsible for placing the route into the ASN.
private AS – not used for the Internet. Values between 64512 and 65535.
regular expression – a symbolic language used to pattern match strings.
soft reconfiguration – allows a BGP router to apply new filters and route-maps without having to clear the entire BGP session and re-exchange updates. It uses a copy of send/received updates to accomplish this.
well-known discretionary – all BGP routers must support this type of PA. The PA may or may not be populated, just depending on whether the feature is in use or not.
well-known mandatory – all BGP routers must support this type of PA. The PA must be populated with information.
Ethan Banks writes & podcasts about IT, new media, and personal tech.
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