Network Convergence

Network Convergence

If a link or device failure exists, the network needs to reconverge, providing alternate paths for data traffic. Because physical devices are typically interconnected in somewhat of a ring topology for redundancy reasons, deploying Layer 2 protocols that aid in eliminating any logical loops is important

The three possible methods for Layer 2 loop avoidance are RTG, the Rapid Spanning Tree Protocol (RSTP), and the Multiple Spanning Tree Protocol (MSTP).

The RTG feature eliminates the need to configure STP on the switch. It is similar to RSTP root and alternate port, without the need for configuring RSTP. Its ideal implementation is on a switch with a dual-home connection, where one link becomes active and forwards traffic, while the other link is blocking the traffic and is a backup to the active link. It provides sub-second convergence and achieves loop prevention without a spanning tree.

RSTP provides sub-second convergence while avoiding STP slow network convergence. While STP can take 30 to 50 seconds to respond to a topology change, RSTP is typically able to respond to changes within three times the Hello message interval. (The default value is 6 seconds.)

In the past, MSTP was defined in Institute of Electrical and Electronics Engineers (IEEE) 802.1s, and it was later added into the IEEE 802.1q standard. MSTP is an extension to RSTP and further develops the creativity of VLANs. MSTP creates a separate spanning tree for each VLAN group and blocks all paths but one of the possible alternate paths within each spanning tree.

VSTP addresses the drawback of STP’s and RSTP’s inability to utilize redundant paths to forward traffic, enabling each VLAN to run its instance of the spanning tree. It is a non-standards based protocol and it inter-operates with Cisco’s PVST

Because no dynamic path optimization exists between the network tiers, a network failure can cause higher latency. You should be aware of this drawback when designing networks for latency-sensitive applications

Layer 3 at the Access Tier

Consider an alternative to the Layer 2 approach at the access tier. You can configure the access-core uplink as a Layer 3 connection, which requires servers to have the network’s default gateway pointing to the access switch. In this design, Layer 2 broadcast domains can span across multiple member switches within the same access switch Virtual Chassis. This setup implements multinode server cluster technology that requires Layer 2 connectivity among the nodes participating in these clusters. Some examples of these technologies include VMware live migration technology (VMotion), Microsoft active clusters, and other high-performance computer clusters or grid-computing applications.

The design requires the use of an interior gateway protocol (IGP) — OSPF, for example. We recommend including equal-cost multipath (ECMP) routing, which provides traffic load balancing between the network tiers. The use of ECMP on the uplink LAGs, interconnecting the access and core tiers, replaces STP and has many advantages

• Minimized Layer 2 broadcast domain,

• Ease of troubleshooting,

• Deterministic behavior for minimal packet loss,

• Automatic load balancing at a per-prefix or a per-packet level, and

• Dynamic network optimization and path selection

This approach limits the Layer 2 domain to a single Virtual Chassis. It also limits Layer 2 mobility to a set of access elements.

Network Convergence Across Access-Core Tiers

Because a single Layer 3 routing domain directs network traffic flow, any failure in the link or network device results in a topology change and network re-convergence across the entire network. Network path optimization happens automatically, resulting in network deterministic behavior, providing a consistent latency of traffic flow, and allowing easier network provisioning.