CCNP Enterprise Core (350-401)
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BGP Path Attributes: Minimum Cluster List Length
With a tie on the Router ID, it falls to the minimum cluster list length to try break the deadlock. The step uses the cluster list to locate the path that has travelled the least number of iBGP advertisement hops. The cluster list is a non-transitive BGP attribute that is appended by a router reflector…
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BGP Path Attributes: Router ID / RID
With the oldest eBGP path ending in a tie, or into iBGP paths, the next step for the best path algorithm is to choose the best path from the session with the lowest Router ID of the advertising router. If it was a route reflector that advertised the route, then the originator ID is substituted…
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BGP Path Attributes: Prefer the oldest eBGP Path
If the lowest IGP metric ends in a tie, the oldest eBGP session is preferred simply for its stability.
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BGP Path Attributes: Lowest IGP Metric
If there is a tie on the eBGP over iBGP decison, lowest IGP metric is examined next. It is simply a comparison of metrics to the destination, the lowest metric to the next-hop destination is the preferred path.
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BGP Path Attributes: eBGP over iBGP
Following a MED math, the next decision is where did the path come from, in this order of preference: eBGP peers are most desired Confederation member AS peers are next iBGP peers are least desired.
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BGP Path Attributes: Multi-Exit Discriminator (MED)
Following on from checking the origin type, the next path attribute that is checked is the MED, or Multi-Exit Discriminator. Multi-Exit Discriminator utilises a 32-bit value (0 – 4,294,967,295) known as a metric. The lower the metric the more preferred a route is. The MED is set automatically to the IGP path metric during network…
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BGP Path Attributes: Origin Type
Following AS Path, origin type is evaluated next in the order of preference. The BGP attribute is well-known and is known simply as ‘origin’. Origin defines where networks originated from, how they were advertised into BGP. Networks that were advertised into BGP via a network statement in the BGP configuration are given an ‘i’ indicator,…
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BGP Path Attributes: Shortest AS Path
After AIGP Path Attributes, the next deciding factor is evaluating the AS Path of a route. The AS Path attribute typically corresponds to the number of AS hops are needed to reach a destination. The shorter the AS path, the more preferred it is. There can be times when AS paths are not relative to…
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BGP Path Attributes: Accumulated Interior Gateway Protocol
AIGP, Accumulated Interior Gateway Protocol, is an optional nontransitive path attribute that is included with advertisements throughout an AS. It is the fourth decider in a BGP best path after ‘Locally Originated via Network or Aggregate Advertisement’ Interior Gateway Protocols, OSPF or EIGRP, use the lowest path metric to identify the shortest path to a…
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BGP Path Attributes: Locally Originated via Network or Aggregate Advertisement
After Weight and Local Preference, the third decision is to determine if the route originated locally. Preference is given in this order: Was the route advertised locally Was this part of a network that has been aggregated locally Was this route learned by a BGP peer