Avoiding Traffic Black-Holes for Route Aggregation in IS-IS

When running the Intermediate System to Intermediate System (IS-IS) routing protocol in a highly symmetric network such as the Leaf-Spine or Fat-Tree network, the Leaf nodes (e.g., Top of Rack switches in datacenters) are recommended to be prevented from receiving other nodes’ explicit routes in order to achieve scalability, as proposed in [IS-IS-SL-Extension], [IS-IS-Overhead-Reduction], [RIFT], and [OpenFabric]. In particular, each Leaf node SHOULD simply maintain a default (or aggregated) route (e.g., 0.0.0.0/0) in its routing table, of which the next hop SHOULD be an Equal Cost Multi Path (ECMP) group including all Spines nodes that the Leaf node connects to. However, such a setup would cause traffic black-holes or suboptimal routing if link failure happens in the network, since the Leaf nodes are not aware of any topology information. To solve this problem, this document introduces INFINITE cost to IS-IS LSPs. When link failure happens between a Spine node and a Leaf node, the Spine node SHOULD advertise all prefixes attached to the Leaf node, whose costs SHOULD be set to be INFINITE, to every other Leaf node it connects to. On receiving the prefixes (with INFINITE cost), each Leaf node SHOULD add the prefixes to its routing table, thus avoiding traffic black-holes and suboptimal routing.

This section illustrates why link failure would cause traffic black-hole or suboptimal routing when Leaf nodes only maintain default (or aggregated) routes.

Figure 1 shows a Spine-Leaf topology example where Leaf1 to Leaf4 are connected to Spine1 to Spine3, and prefixA and prefixB are attached to Leaf4. To achieve scalability, as proposed in [IS-IS-SL-Extension], [IS-IS-Overhead-Reduction], [RIFT], and [OpenFabric], Leaf1 to Leaf4 SHOULD NOT receive explict routes from each other nor the Spine nodes. Instead, each of them maintains a default (or aggregated) route (e.g., 0.0.0.0/0) in the routing table, of which the next hop is an ECMP group including Spine1, Spine2, and Spine3. Flows from one Leaf node to another are shared among Spine1, Spine2, and Spine3 based on the well known 5-tuple hashing. However, such a setup would cause traffic black-hole or suboptimal routing when link failure happens in the network. For example, if the link between Spine3 and Leaf4 is broken, Leaf1, Leaf2, and Leaf3 could not get aware of the failure. As a result, these Leaf nodes will still send a portion of traffic destined for prefixA or prefixB toward Spine3, which makes the traffic be discarded at Spine3, causing traffic black-hole. On the other hand, if there are a set of links or a higher tier of switches interconnecting Spine1, Spine2, and Spine3, the traffic will be steered to other spine nodes or the higher-tier switches by Spine3, causing suboptimal routing. Therefore, this document introduces INFINITE cost to IS-IS LSPs to solve this problem.

This document introduces the INFINITE cost to IS-IS LSPs, whose value is to be determined. When link failure happens between a Spine node and a Leaf node, the Spine node SHOULD 1) encode all prefixes attached to the Leaf node into the IP Reachability TLV, 2) set the cost of the prefixes to be INFINITE, 3) append the IP Reachability TLV to the IS-IS LSP, and 4) send the LSP to every other Leaf node it connects to. When a Leaf node receives the prefixes (with INFINITE cost) advertised by a Spine node, it SHOULD install each of the prefixes into its routing table, of which the next hop SHOULD be set an ECMP group including all Spine nodes it connects to except the one who advertises the prefix. For example, if the link between Spine3 and Leaf4 in Figure 1 is broken, Spine3 SHOULD advertise prefixA and prefixB to Leaf1, Leaf2, and Leaf3, by sending them an IS-IS LSP containing the IP Reachability TLV. The cost of prefixA and prefixB SHOULD be set INFINITE. On receiving the LSP, Leaf1, Leaf2, and Leaf3 SHOULD install prefixA and prefixB into their routing tables, and the next hop of prefixA or prefixB SHOULD be set an ECMP group including Spine1 and Spine2. For instance, the routing table of Leaf1 before and after the link failure is shown in Figure 2 and Figure 3, respectively. Note that the mechanism described above could achieve minimal signaling latency, which helps to avoid black-hole or suboptimal routing rapidly when link failure happens.

TBD.