Foundation Summary

The Foundation Summary provides a convenient review of many key concepts in this chapter. If you are already comfortable with the topics in this chapter, this summary might help you recall a few details. If you just read this chapter, this review should help solidify some key facts. If you are doing your final prep before the exam, the following lists and tables are a convenient way to review the day before the exam.

Table 16-5 lists the attributes supported by IOS.

Table 16-5. BGP Attributes
Attribute NameCategoryDescription
AggregatorOptional, transitiveRouter ID and AS of router that summarized. Not used in path selection.
AS_PathWell-known, mandatoryList of AS route has passed through. Prefer shortest path.
Atomic aggregateWell-known, discretionarySummary includes multiple AS. Not used in path selection.
Cluster IDOptional, nontransitiveOriginating cluster. Not used in path selection.
CommunityOptional, transitiveRoute tag. Not used in path selection.
Local preferenceWell-known, discretionaryMetric for external paths, for internal neighbors. Prefer highest.
Multiple Exit Discriminator (MED)Optional, nontransitiveInforms external peers which path to take into the autonomous system. Prefer lowest.
Next HopWell-known, mandatoryExternal peer in neighboring AS. Not used in path selection.
OriginWell-known, mandatoryPrefer BGP routes (i) over EGP (e) over redistributed (?).
Originator IDOptional, nontransitiveIdentifies Route Reflector. Not used in path selection.
WeightOptional, not communicated to peersAdministrative Cisco attribute. Prefer highest.

The complete path selection process, summarizing the descriptions in this section, is displayed in Table 16-6.

Table 16-6. BGP Path Selection
0. SynchronizedTRUEUse only routes that meet the synchronization requirement
1. WeightHighestAdministrative override
2. Local PreferenceHighestUsed internally to pick path out of AS
3. Self originatedTRUEUsed to prefer paths originated on this router
4. AS-PathShortestMinimize AS-hops
5. Origini<?Prefer stability
6. MEDLowestUsed external to come in
7. ExternalEBGP<IBGPHot potato routing
8. IGP costLowestLook for more information
9. EBGP PeeringOldestPrefer stability
10. RIDLowestExternal path preferred over internal path