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.
| Attribute Name | Category | Description |
|---|---|---|
| Aggregator | Optional, transitive | Router ID and AS of router that summarized. Not used in path selection. |
| AS_Path | Well-known, mandatory | List of AS route has passed through. Prefer shortest path. |
| Atomic aggregate | Well-known, discretionary | Summary includes multiple AS. Not used in path selection. |
| Cluster ID | Optional, nontransitive | Originating cluster. Not used in path selection. |
| Community | Optional, transitive | Route tag. Not used in path selection. |
| Local preference | Well-known, discretionary | Metric for external paths, for internal neighbors. Prefer highest. |
| Multiple Exit Discriminator (MED) | Optional, nontransitive | Informs external peers which path to take into the autonomous system. Prefer lowest. |
| Next Hop | Well-known, mandatory | External peer in neighboring AS. Not used in path selection. |
| Origin | Well-known, mandatory | Prefer BGP routes (i) over EGP (e) over redistributed (?). |
| Originator ID | Optional, nontransitive | Identifies Route Reflector. Not used in path selection. |
| Weight | Optional, not communicated to peers | Administrative Cisco attribute. Prefer highest. |
The complete path selection process, summarizing the descriptions in this section, is displayed in Table 16-6.
| Order | Preference | Description |
|---|---|---|
| 0. Synchronized | TRUE | Use only routes that meet the synchronization requirement |
| 1. Weight | Highest | Administrative override |
| 2. Local Preference | Highest | Used internally to pick path out of AS |
| 3. Self originated | TRUE | Used to prefer paths originated on this router |
| 4. AS-Path | Shortest | Minimize AS-hops |
| 5. Origin | i<? | Prefer stability |
| 6. MED | Lowest | Used external to come in |
| 7. External | EBGP<IBGP | Hot potato routing |
| 8. IGP cost | Lowest | Look for more information |
| 9. EBGP Peering | Oldest | Prefer stability |
| 10. RID | Lowest | External path preferred over internal path |
