With the advent of new technologies as well as the ability to have multi-layer switches at the access layer, Cisco is starting to change the game. New network designs are pushing layer 3 routing out to the access switches creating a routed edge.
Most campus and data center network designs have been following the standard 3 tier design model established by Cisco. With this you have Core, Distribution, and Access tiers that comprise the 3 tiers. You would typically use layer 2 at the access, use vlans to segment traffic, spanning for loop avoidance and begin routing at the distribution tier. Pretty standard stuff. But there are some challenges to this design.
With the new routed edge designs all of that changes.
Issues with a Switched Access Layer
- Vlans must span across multiple switches
- Layer 2 Loops
- Layer 2 and 3 running
- Blocked Links
- Must configure STP root
- Must configure RootGuard, LoopsGuard, etc.
- Must set HSRP tuning to load balance uplinks
- Many moving parts increase instability
Moving to a Routed Access layer we can avoid many of these issues, have faster convergence, faster failover, improved stability, as well as increased security.
Benefits of a Routed Access Layer
- EIGRP and OSPF converges < 200 msec
- No Matching STP.HSRP/GLBP priority
- No Layer 2 Loops
- No blocked links
- Single Control Plane and known tool set – (traceroute, show ip route, etc.)
- Routed access network recovery dependant on L3 reroute
- Flows based on Equal Cost Multi-Path (ECMP)
- All fault recovery is ECMP-based (i.e, consistent and predictable)
- Allows for VRF enabled designs (improved security over VLANs)
- Relatively painless migration to L# using EIGRP (depending on address scheme)
As you can see there are some serious benefits to moving to a routed access especially if you have the capability at your access layer (i.e., 3750’s, 4948’s, or 4507’s – multilayer switches). There are still challanges to this new design. How do you management of configs, changed to ip addressing, training of personel, etc., With new technologies comes the need for new understanding of these technologies and how to support them.
With sub-200 msec of failover, especially when implementing IP telephony(VoIP) in your environment – a layer 3 access layer looks to be a must have.
What are your thoughts or commments on this new design? Have you implemented this in your environment or do you think Cisco is just trying to drive equipment sales? Leave your comment below and thanks for reading!
