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5.2.4. External Connectivity

5.2.4. External Connectivity​

A dedicated cluster (or clusters) in the Clos topology could be used for the purpose of connecting to the Wide Area Network (WAN) edge devices, or WAN Routers. Tier 3 devices in such a cluster would be replaced with WAN routers, and EBGP peering would be used again, though WAN routers are likely to belong to a public ASN if Internet connectivity is required in the design. The Tier 2 devices in such a dedicated cluster will be referred to as "Border Routers" in this document. These devices have to perform a few special functions:

  • Hide network topology information when advertising paths to WAN routers, i.e., remove Private Use ASNs [RFC6996] from the AS_PATH attribute. This is typically done to avoid ASN number collisions between different data centers and also to provide a uniform AS_PATH length to the WAN for purposes of WAN ECMP to anycast prefixes originated in the topology. An implementation-specific BGP feature typically called "Remove Private AS" is commonly used to accomplish this. Depending on implementation, the feature should strip a contiguous sequence of Private Use ASNs found in an AS_PATH attribute prior to advertising the path to a neighbor. This assumes that all ASNs used for intra data center numbering are from the Private Use ranges. The process for stripping the Private Use ASNs is not currently standardized, see [REMOVAL]. However, most implementations at least follow the logic described in this vendor's document [VENDOR-REMOVE-PRIVATE-AS], which is enough for the design specified.

  • Originate a default route to the data center devices. This is the only place where a default route can be originated, as route summarization is risky for the unmodified Clos topology. Alternatively, Border Routers may simply relay the default route learned from WAN routers. Advertising the default route from Border Routers requires that all Border Routers be fully connected to the WAN Routers upstream, to provide resistance to a single-link failure causing the black-holing of traffic. To prevent black-holing in the situation when all of the EBGP sessions to the WAN routers fail simultaneously on a given device, it is more desirable to readvertise the default route rather than originating the default route via complicated conditional route origination schemes provided by some implementations [CONDITIONALROUTE].

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