Transcript SWITCH10S05L02.pptx

Implementing High
Availability
Implementing a Highly Available Network
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—5-1
Layer 2 Distributed VLANs on Access
Switches
 Not a recommended design — slow convergence
 Use only if Layer 2 VLAN spanning flexibility is required
 Requires STP convergence for uplink failure and recovery
 More complex because the STP root and HSRP should match
 May be required for WLAN (standalone APs)
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—5-2
Layer 2 Local VLANs on Access
Switches
 Recommended design, tried and true
 VLANs present on one access switch only
 Does not require STP convergence for uplink failure recovery
 Requires a distribution-to-distribution link for route summarization
 Can map Layer 2 VLAN numbers to Layer 3 subnets for ease of
use and management
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—5-3
Layer 3 Access-to-Distribution
Interconnection
 Best option for fast convergence, and easy to implement
 Uses equal-cost Layer 3 load balancing on all links
 Does not require STP for convergence
 Does not require HSRP (FHRP) configuration
 Does not support VLAN spanning distribution switches
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—5-4
Daisy-Chaining Access Layer Switches
 Primary and secondary HSRP is active after failure.
 Outbound traffic is sent from both HSRP instances.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—5-5
Daisy-Chaining Access Switch Issues
 Potential for black holes if no alternative path is provided
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—5-6
StackWise Technology Access Switches
 StackWise Technology
eliminates the daisy-chain
issue:
– Loopback links are not
required.
– A Layer 2 link in the
distribution is not required.
 StackWise switch provides
redundancy.
 Uplinks can be on different
switches within stack.
 Modular chassis–based
switches can also eliminate
the daisy-chain issue.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—5-7
Avoiding Too Little Redundancy
 Looped figure-8 topology for VLANs spanning access switches
 Blocking on uplink from Access-b.
 Initially forwarding traffic from both access switches
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—5-8
Impact of Uplink Failure
1. Traffic from Access A is dropped until HSRP goes active on
Distribution B.
2. Blocking link on Access B takes 50 seconds to move to
forwarding.
3. After STP converges, an HSRP preempt causes another
transition. Access B is used as transit for Access A traffic.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—5-9
Summary
 Achieving redundancy can be achieved at Layer 2 and at Layer 3,
by providing additional redundant paths between devices.
 StackWise Technology can be used to join several physical
switches into one virtual switch.
 Redundancy is a balance between too much redundancy, which
adds complexity to the network structure, and too little
redundancy, which creates single points of failure.
 When uplinks fail, convergence path as well as convergence time
must be taken into account to evaluate the impact of the failure on
the network infrastructure.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—5-10
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—5-11