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5.2.1. EBGP Configuration Guidelines and Example ASN Scheme

5.2.1. EBGP Configuration Guidelines and Example ASN Scheme​

The diagram below illustrates an example of an ASN allocation scheme. The following is a list of guidelines that can be used:

  • EBGP single-hop sessions are established over direct point-to-point links interconnecting the network nodes, no multi-hop or loopback sessions are used, even in the case of multiple links between the same pair of nodes.

  • Private Use ASNs from the range 64512-65534 are used to avoid ASN conflicts.

  • A single ASN is allocated to all of the Clos topology's Tier 1 devices.

  • A unique ASN is allocated to each set of Tier 2 devices in the same cluster.

  • A unique ASN is allocated to every Tier 3 device (e.g., ToR) in this topology.

                                ASN 65534
                               +---------+
                               | +-----+ |
                               | |     | |
                             +-|-|     |-|-+
                             | | +-----+ | |
                  ASN 646XX  | |         | |  ASN 646XX
                 +---------+ | |         | | +---------+
                 | +-----+ | | | +-----+ | | | +-----+ |
     +-----------|-|     |-|-+-|-|     |-|-+-|-|     |-|-----------+
     |       +---|-|     |-|-+ | |     | | +-|-|     |-|---+       |
     |       |   | +-----+ |   | +-----+ |   | +-----+ |   |       |
     |       |   |         |   |         |   |         |   |       |
     |       |   |         |   |         |   |         |   |       |
     |       |   | +-----+ |   | +-----+ |   | +-----+ |   |       |
     | +-----+---|-|     |-|-+ | |     | | +-|-|     |-|---+-----+ |
     | |     | +-|-|     |-|-+-|-|     |-|-+-|-|     |-|-+ |     | |
     | |     | | | +-----+ | | | +-----+ | | | +-----+ | | |     | |
     | |     | | +---------+ | |         | | +---------+ | |     | |
     | |     | |             | |         | |             | |     | |
   +-----+ +-----+           | | +-----+ | |           +-----+ +-----+
   | ASN | |     |           +-|-|     |-|-+           |     | |     |
   |65YYY| | ... |             | |     | |             | ... | | ... |
   +-----+ +-----+             | +-----+ |             +-----+ +-----+
     | |     | |               +---------+               | |     | |
     O O     O O              <- Servers ->              O O     O O

                 Figure 4: BGP ASN Layout for 5-Stage Clos
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