IS-IS Routing for Spine-Leaf Topology

This extension assumes the network is a Spine-Leaf topology, and it should not be applied in an arbitrary network setup. The spine nodes can be viewed as the aggregation layer of the network, and the leaf nodes as the access layer of the network. The leaf nodes use a load sharing algorithm with spine nodes as nexthops in routing and forwarding. This extension works when the spine nodes are inter-connected, and it works with a pure CLOS or Fat Tree topology based network where the spines are NOT horizontally interconnected. Although the example diagram in shows a fully meshed Spine-Leaf topology, this extension also works in the case where they are partially meshed. For instance, leaf1 through leaf10 may be fully meshed with spine1 through spine5 while leaf11 through leaf20 is fully meshed with spine4 through spine8, and all the spines are inter-connected in a redundant fashion. This extension can also work in multi-level spine-leaf topology. The lower level spine node can be a 'leaf' node to the upper level spine node. A spine-leaf 'Tier' can be exchanged with IS-IS hello packets to allow tier X to be connected with tier X+1 using this extension. Normally tier-0 will be the TOR routers and switches if provisioned. This extension also works with normal IS-IS routing in a topology with more than two layers of spine and leaf. For instance, in example diagrams and , there can be another Core layer of routers/switches on top of the aggregation layer. From an IS-IS routing point of view, the Core nodes are not affected by this extension and will have the complete topology and routing information just like the spine nodes. To make the network even more scalable, the Core layer can operate as a level-2 IS-IS sub-domain while the Spine and Leaf layers operate as stays at the level-1 IS-IS domain. This extension also supports the leaf nodes having local connections to other leaf nodes, in the example diagram there is a connection between 'Leaf1' node and 'Leaf2' node, and an external host can be dual homed into both of the leaf nodes. This extension assumes the link between the spine and leaf nodes are point-to-point, or point-to-point over LAN. The links connecting among the spine nodes or the links between the leaf nodes can be any type.

This extension introduces one new TLV which may be used in IS-IS Hello (IIH) PDUs, LSPs, or in Circuit Scoped Link State PDUs (CS-LSP) . It is used by both spine and leaf nodes in this Spine-Leaf mechanism.

The fields of this TLV are defined as follows: 1 octet Suggested value 150 (to be assigned by IANA) 1 octet (2 + length of sub-TLVs). 16 bits

A 4 bits value range from 0 to 15. It is used to represent the spine-leaf tier level when the 'T' bit is set. If the 'T' is cleared, this value MUST be set to zero from the sender, and it MUST be ignored on the receiver. The value 15 is reserved to indicate the tier level is unknown or not configured. Only leaf node sets this bit. If the L bit is set in the SL flag, the node indicates it is in 'Leaf-Mode'. Only Spine node sets this bit. If the R bit is set, the node indicates to the leaf neighbor that it can be used as the default route gateway. Only leaf node sets this bit on Leaf-Leaf link, in additional to the 'L' bit setting. If the B bit is set, the node indicates to its leaf neighbor that it can be used as the backup default route gateway. If set, the value in the 'Tier' field represents the spine-leaf tier level in the topology. Not defined in this document, for future extension sub-TLVs MAY be included when the TLV is in a CS-LSP. sub-TLVs MUST NOT be included when the TLV is in an IIH

If the data center topology is a pure CLOS or Fat Tree, there are no link connections among the spine nodes. If we also assume there is not another Core layer on top of the aggregation layer, then the traffic from one leaf node to another may have a problem if there is a link outage between a spine node and a leaf node. For instance, in the diagram of , if Leaf1 sends data traffic to Leaf3 through Spine1 node, and the Spine1-Leaf3 link is down, the data traffic will be dropped on the Spine1 node. To address this issue spine and leaf nodes may send/request specific reachability information via the sub-TLVs defined below. Two Spine-Leaf sub-TLVs are defined. The Leaf-Set sub-TLV and the Info-Req sub-TLV.

This sub-TLV is used by spine nodes to optionally advertise Leaf neighbors to other Leaf nodes. The fields of this sub-TLV are defined as follows: 1 octet Suggested value 1 (to be assigned by IANA) 1 octet MUST be a multiple of 6 octets. A list of IS-IS System-ID of the leaf node neighbors of this spine node.

This sub-TLV is used by leaf nodes to request more specific prefix information from a selected spine node, upon detecting one of the spine node has lost the connection to a leaf node. The fields of this sub-TLV are defined as follows: 1 octet Suggested value 2 (to be assigned by IANA) 1 octet. It MUST be a multiple of 6 octets. List of IS-IS System-IDs of leaf nodes for which connectivity information is being requested.

In cases where connectivity between a leaf node and a spine node is down, the leaf node MAY request reachability information from a spine node as described in Section 3.3.1.2. The spine node utilizes TLVs 135 and TLVs 236 to advertise this information. These TLVs MAY be included either in IIHs or CS-LSPs sent from the spine to the requesting leaf node. Sending such information in IIHs has limited scale - all reachability information MUST fit within a single IIH. It is therefore recommended that CS-LSPs be used.

Leaf nodes in a spine-leaf application using this extension are provisioned with two attributes: 1)Tier level of 0. This indicates the node is a Leaf Node. The value 0 is advertised in the Tier field of Spine-Leaf TLV defined above. 2)Flooding reduction enabled/disabled. If flooding reduction is enabled the L-bit is set to one in the Spine-Leaf TLV defined above A spine node does not need explicit configuration. Spine nodes can dynamically discover their tier level by computing the number of hops to a leaf node. Until a spine node determines its tier level it MUST advertise level 15 (unknown tier level) in the Spine-Leaf TLV defined above. When a spine node receives an IIH which includes the Spine-Leaf TLV with Tier level 0 and 'L' bit set, it labels the point-to-point interface and adjacency to be a 'Reduced Flooding Leaf-Peer (RF-Leaf)'. IIHs sent by a spine node on a link to an RF-Leaf include the Spine-Leaf TLV with the 'R' bit set in the flags field. The 'R' bit indicates to the RF-Leaf neighbor that the spine node can be used as a default routing nexthop. There is no change to the IS-IS adjacency bring-up mechanism for Spine-Leaf peers. A spine node blocks LSP flooding to RF-Leaf adjacencies, except for the LSP PDUs in which the IS-IS System-ID matches the System-ID of the RF-Leaf neighbor. This exception is needed since when the leaf node reboots, the spine node needs to forward to the leaf node non-purged LSPs from the RF-Leaf's previous incarnation. Leaf nodes will perform IS-IS LSP flooding as normal over all of its IS-IS adjacencies, but in the case of RF-Leafs only self-originated LSPs will exist in its LSP database. Spine nodes will receive all the LSP PDUs in the network, including all the spine nodes and leaf nodes. It will perform Shortest Path First (SPF) as a normal IS-IS node does. There is no change to the route calculation and forwarding on the spine nodes. RF-Leaf nodes do not have any LSP in the network except for its own. Therefore there is no need to perform SPF calculation on the RF-Leaf node. It only needs to download the default route with the nexthops of those Spine Neighbors which have the 'R' bit set in the Spine-Leaf TLV in IIH PDUs. IS-IS can perform equal cost or unequal cost load sharing while using the spine nodes as nexthops. The aggregated metric of the outbound interface and the 'Reverse Metric' can be used for this purpose.

In a data center where the topology is pure CLOS or Fat Tree, there is no interconnection among the spine nodes, and there is not another Core layer above the aggregation layer with reachability to the leaf nodes. When flooding reduction to RF-Leafs is in use, if the link between a spine and a leaf goes down, there is then a possibility of black holing the data traffic in the network. As in the diagram , if the link Spine1-Leaf3 goes down, there needs to be a way for Leaf1, Leaf2 and Leaf4 to avoid the Spine1 if the destination of data traffic is to Leaf3 node. In the above example, the Spine1 and Spine2 are provisioned to advertise the Leaf-Set sub-TLV of the Spine-Leaf TLV. Originally both Spines will advertise Leaf1 through Leaf4 as their Leaf-Set. When the Spine1-Leaf3 link is down, Spine1 will only have Leaf1, Leaf2 and Leaf4 in its Leaf-Set. This allows the other leaf nodes to know that Spine1 has lost connectivity to the leaf node of Leaf3. Each RF-Leaf node can select another spine node to request for some prefix information associated with the lost leaf node. In this diagram of , there are only two spine nodes (Spine-Leaf topology can have more than two spine nodes in general). Each RF-Leaf node can independently select a spine node for the leaf information. The RF-Leaf nodes will include the Info-Req sub-TLV in the Spine-Leaf TLV in hellos sent to the selected spine node, Spine2 in this case. The spine node, upon receiving the request from one or more leaf nodes, will find the IPv6/IPv4 prefixes advertised by the leaf nodes listed in the Info-Req sub-TLV. The spine node will use the mechanism defined in Section 3.3.2 to advertise these prefixes to the RF-Leaf node. For instance, it will include the IPv4 loopback prefix of leaf3 based on the policy configured or administrative tag attached to the prefixes. When the leaf nodes receive the more specific prefixes, they will install the advertised prefixes towards the other spine nodes (Spine2 in this example). For instance in the data center overlay scenario, when any IP destination or MAC destination uses the leaf3's loopback as the tunnel nexthop, the overlay tunnel from leaf nodes will only select Spine2 as the gateway to reach leaf3 as long as the Spine1-Leaf3 link is still down. This negative routing is only relevant between tier 0 and tier 1 spine-leaf levels in a multi-level spine-leaf topology when the reduced flooding extension is in use. Nodes in tiers 1 or greater have the full topology information.

In Spine-Leaf extension, Complete Sequence Number PDU (CSNP) does not need to be transmitted over the Spine-Leaf link to an RF-Leaf. Some IS-IS implementations send periodic CSNPs after the initial adjacency bring-up over a point-to-point interface. There is no need for this optimization here since the RF-Leaf does not need to receive any other LSPs from the network, and the only LSPs transmitted across the Spine-Leaf link is the leaf node LSP. Also in the graceful restart case, for the same reason, there is no need to send the CSNPs over the Spine-Leaf interface to an RF-Leaf. Spine nodes only need to set the SRMflag on the LSPs belonging to the RF-Leaf.

Leaf to leaf node links are useful in host redundancy cases in switching networks, and normally there is no flooding extensions are required in this case. Each leaf node will set tier level = 0 in the Spine-Leaf TLV included in hellos to leaf neighbors. LSP will be exchanged over this link. In the example diagram , the Leaf1 will get Leaf2's LSP and Leaf2 will get Leaf1's LSP. They will install more specific routes towards each other using this local Leaf-Leaf link. SPF will be performed in this case just like when the entire network only involves with those two IS-IS nodes. This does not affect the normal Spine-Leaf mechanism they perform toward the spine nodes. Besides the local leaf-to-leaf traffic, the leaf node can serve as a backup gateway for its leaf neighbor. It needs to remove the 'Overload-Bit' setting in its LSP, and it sets both the 'L' bit and the 'B' bit in the SL-flag with a high 'Reverse Metric' value.

The leaf node SHOULD set the 'overload' bit on its LSP PDU, since if the spine nodes were to forward traffic not meant for the local node, the leaf node does not have the topology information to prevent a routing/forwarding loop.

This extension creates a non-reciprocal relationship between the spine node and leaf node. The spine node will receive leaf's LSP and will know the leaf's hostname, but the leaf does not have spine's LSP. This extension allows the Dynamic Hostname TLV to be optionally included in spine's IIH PDU when sending to a 'Leaf-Peer'. This is useful in troubleshooting cases.

This metric is part of the aggregated metric for leaf's default route installation with load sharing among the spine nodes. When a spine node is in 'overload' condition, it should use the IS-IS Reverse Metric TLV in IIH to set this metric to maximum to discourage the leaf using it as part of the loadsharing. In some cases, certain spine nodes may have less bandwidth in link provisioning or in real-time condition, and it can use this metric to signal to the leaf nodes dynamically. In other cases, such as when the spine node loses a link to a particular leaf node, although it can redirect the traffic to other spine nodes to reach that destination leaf node, but it MAY want to increase this metric value if the inter-spine connection becomes over utilized, or the latency becomes an issue. In the leaf-leaf link as a backup gateway use case, the 'Reverse Metric' SHOULD always be set to very high value.

Besides using the IS-IS Reverse Metric by the spine nodes to affect the traffic pattern for leaf default gateway towards multiple spine nodes, the IPv6/IPv4 Info-Advertise sub-TLVs can be selectively used by traffic engineering controllers to move data traffic around the data center fabric to alleviate congestion and to reduce the latency of a certain class of traffic pairs. By injecting more specific leaf node prefixes, it will allow the spine nodes to attract more traffic on some underutilized links.

Losing the topology information will have an impact on some of the end-to-end network services, for instance, MPLS TE or end-to-end segment routing. Some other mechanisms such as those described in PCE based solution may be used. In this Spine-Leaf extension, the role of the leaf node is not too much different from the multi-level IS-IS routing while the level-1 IS-IS nodes only have the default route information towards the node which has the Attach Bit (ATT) set, and the level-2 backbone does not have any topology information of the level-1 areas. The exact mechanism to enable certain end-to-end network services in Spine-Leaf network is outside the scope of this document.

IPv6 Address families, Multi-Topology (MT) and Multi-Instance (MI) information is carried over the IIH PDU. Since the goal is to simplify the operation of IS-IS network, for the simplicity of this extension, the Spine-Leaf mechanism is applied the same way to all the address families, MTs and MIs.

For this extension to be deployed in existing networks, a simple migration scheme is needed. To support any leaf node in the network, all the involved spine nodes have to be upgraded first. So the first step is to migrate all the involved spine nodes to support this extension, then the leaf nodes can be enabled with 'Leaf-Mode' one by one. No flag day is needed for the extension migration.