Layer 2 Access with Layer 3 Distribution
- Each access switch has unique Vlan
- No Layer 2 loops
- Layer 3 on link between Distribution switches
- No Spanning-Tree blocked Links
- Fast convergence, only dependent on FHRP
Some Best Practices:
- Use rapid-pvst (802.1w) or MST (802.1s) instead pvst+ (802.1d)
- tune FHRP and STP root bridge to load balance uplinks
- Tune CEF load-balancing
- summarize to the core
Layer 2 Access with Layer 2 and Layer 3 Distribution
- Some Vlans span multiple access switches
- Layer 2 loops
- Layer 2 and Layer 3 on link between Distribution switches
- Spanning-Tree Blocked links
One solution : Use MEC (Multi-Chassis EtherChannel) => VSS(6500) or VPC(Nexus)
Routed Access Design Best Practices:
- Use routed point-to-point link (quick convergence versus L2)
- ECMP to avoid convergence ( quick reroute)
- Tune CEF L3/L4 load balancing hash (for best load balancing on equal costs multipath)
- Build triangle not square for quickly convergence (No need to recalculate new path)
- Insure redundant L3 path to avoid black holes
- Summarize distribution to Core to limit black hole (Requires a link between the distribution switches)
Constrains:
- Can’t span Vlans across multiple wiring closet switches
- IP addressing
Daniel LARDEUX 