5.2.3. Prefix Advertisement
5.2.3. Prefix Advertisement
A Clos topology features a large number of point-to-point links and associated prefixes. Advertising all of these routes into BGP may create Forwarding Information Base (FIB) overload in the network devices. Advertising these links also puts additional path computation stress on the BGP control plane for little benefit. There are two possible solutions:
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Do not advertise any of the point-to-point links into BGP. Since the EBGP-based design changes the next-hop address at every device, distant networks will automatically be reachable via the advertising EBGP peer and do not require reachability to these prefixes. However, this may complicate operations or monitoring: e.g., using the popular "traceroute" tool will display IP addresses that are not reachable.
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Advertise point-to-point links, but summarize them on every device. This requires an address allocation scheme such as allocating a consecutive block of IP addresses per Tier 1 and Tier 2 device to be used for point-to-point interface addressing to the lower layers (Tier 2 uplinks will be allocated from Tier 1 address blocks and so forth).
Server subnets on Tier 3 devices must be announced into BGP without using route summarization on Tier 2 and Tier 1 devices. Summarizing subnets in a Clos topology results in route black-holing under a single link failure (e.g., between Tier 2 and Tier 3 devices), and hence must be avoided. The use of peer links within the same tier to resolve the black-holing problem by providing "bypass paths" is undesirable due to O(N^2) complexity of the peering-mesh and waste of ports on the devices. An alternative to the full mesh of peer links would be to use a simpler bypass topology, e.g., a "ring" as described in [FB4POST], but such a topology adds extra hops and has limited bandwidth. It may require special tweaks to make BGP routing work, e.g., splitting every device into an ASN of its own. Later in this document, Section 8.2 introduces a less intrusive method for performing a limited form of route summarization in Clos networks and discusses its associated tradeoffs.