Troubleshooting Catalyst Switches Layer 3 Chapter Summary

The following section is a summary of the major points you should be aware of in this chapter.

Catalyst Switched VLAN Interfaces Overview

1. In a switched network, VLANs separate devices into different collision domains
2. VLANs are also used to separate devices into different subnets
3. Multilayer switches support the configuration of Switched Virtual Interfaces (SVIs)
4. SVIs represent VLANs and allow the switch to serve as the default gateway for the VLAN
5. An SVI is not automatically created when a VLAN is created
6. By default, however, an SVI for VLAN 1 is automatically created by the software
7. A Switched Virtual Interface is a very resilient interface
8. In order for an SVI to be placed into the up/up state, the following conditions must be met:

1. The VLAN exists and is active in the VLAN database of the switch
2. The VLAN interface is not administratively down
3. At least one Layer 2 (access port or trunk) port exists, has a link up on this VLAN
4. At least one Layer 2 (access port or trunk) is in the STP forwarding state

Catalyst Switch MLS Overview

1. Multilayer Switching (MLS) combines Layer 2, Layer 3, and Layer 4 switching technologies
2. MLS allows switches to forward packets at wire speed using hardware
3. Cisco supports MLS for both Unicast and Multicast traffic flows
4. In MLS switching, an MLS cache, is maintained for the Layer 3-switched flows
5. The MLS cache maintains flow information for all active flows
6. The MLS cache includes entries for traffic statistics
7. After the MLS cache is created, packets for an existing flow can be Layer 3-switched
8. MLS integrates both data plane and control plane functions
9. The control plane is where routing and other control information, is stored and exchanged
10. The control plane is responsible for updating the routing table
11. The data plane is responsible for the actual forwarding of data
12. The data plane is typically populated using information derived from the control plane
13. MLS is enabled by configuring Cisco Express Forwarding (CEF) on the switch
14. CEF uses a FIB to make IP destination prefix-based switching decisions
15.  The FIB is conceptually similar to a routing table or information base (RIB)
16. The adjacency table is created to contain all connected next hops
17. An adjacent node is a node that is one hop away, i.e. directly connected

Troubleshooting Multilayer Switching

1. MLS troubleshooting requires troubleshooting at both the control and data planes
2. The following steps should be taken when troubleshooting Unicast MLS issues:

1. Verify that IP routing information for the address is correct
2. Verify that the next hop has a valid MAC address
3. Verify that the FIB next hop is the same as the RIB next hop
4. Verify the CEF adjacency table rewrite information
5. Verify FIB and adjacency table population in TCAM

Understanding and Troubleshooting HSRP

1. Hot Standby Router Protocol is a Cisco-proprietary First Hop Redundancy Protocol
2. Cisco IOS software supports two versions of HSRP: version 1 and version 2
3. By default, when HSRP is enabled in Cisco IOS software, HSRP version 1 is enabled
4. HSRPv1 supports up to 255 groups; HSRPv2 supports up to 4096 groups
5. HSRPv1 uses Multicast group address 224.0.0.2 and UDP port 1985
6. HSRPv2 uses Multicast group address 224.0.0.102 and UDP port 1985
7. The version 2 packet format uses a Type/Length/Value (TLV) format
8. HSRPv1 does not support millisecond timer values; HSRPv2 supports millisecond timers
9. HSRPv2 includes a 6-byte Identifier field
10. HSRPv1 uses virtual MAC addresses in the range 0000.0c07.acxx
11. HSRPv2 uses virtual MAC addresses in the range 0000.0C9F.F000 to 0000.0C9F.FFFF
12. The majority of HSRP issues are due to router and switch misconfigurations
13. Common HSRP problem scenarios include the following:

1. Gateway Logging Continuous HSRP State Changes
2. HSRP Gateways Not Reflecting the Correct State
3. HSRP Does Not Detect Peer Router
4. HSRP Causes MAC Violation on a Secure Switch Port

Understanding and Troubleshooting VRRP

1. VRRP operates in a similar manner to HSRP; however VRRP is an open standard
2. VRRP is defined in RFC 2338
3. VRRP sends advertisements to Multicast address 224.0.0.18, using IP protocol number 112
4. At the Data Link layer, VRRP uses MAC addresses in the range 01-00-5e-00-01xx
5. Both HSRP and VRRP use a default priority value of 100
6. By default, VRRP version 2 is enabled when VRRP is configured on a gateway
7. Configurable VRRP priorities range from 1-254; for HSRP the range is 1-255
8. VRRP priority 255 is automatically configured when an interface IP is used for a group
9. When the IP Address Owner is up, it responds to all packets that are sent to the IP address

Understanding and Troubleshooting GLBP

1. Gateway Load Balancing Protocol is a Cisco-proprietary FHRP, like HSRP
2. GLBP allows multiple gateways to actively forward packets using a single GLBP group
3. GLBP gateways communicate through Hello messages that are sent every 3 seconds
4. GLBP sends updates to the Multicast address 224.0.0.102, using UDP port 3222
5. GLBP group members elect one gateway to be the AVG for that group
6. The AVG is the gateway that has the highest priority value
7. The other gateways in the GLBP group provide backup for the AVG
8. The AVG answers all ARP requests for the virtual router address
9. The AVG assigns a virtual MAC address to each member of the GLBP group
10. Each gateway is responsible for forwarding packets that are sent to its virtual MAC address
11. These gateways are referred to as active virtual forwarders (AVFs)
12. A GLBP group allows up to four virtual MAC addresses per group
13. By default, the GLBP gateway preemptive scheme is disabled
14. A backup virtual gateway can become the AVG only if the current AVG fails
15. By default, each gateway is assigned a weight of 100

Troubleshooting Switch Supervisor Redundancy

1. Catalyst 4500 and 6500 series switches support two supervisor engines for high availability
2. The standby supervisor engine assumes primary supervisor if the following happens:

1. The primary supervisor engine fails or crashes
2. The primary supervisor engine is rebooted
3. The administrator forces a manual failover from active to standby
4. The primary supervisor engine is physically removed
5. Clock synchronization between the supervisor engines fails

1. Cisco IOS software supports three modes for redundant supervisor implementations:

1. Route Processor Redundancy (RPR)
2. Route Processor Redundancy Plus (RPR+)
3. Stateful Switchover (SSO)

1. When using RPR, the standby supervisor engine is only partially booted and initialized
2. RPR switchover generally takes between 2 and 4 minutes
3. RPR+ improves on RPR and provides failover generally within 30 to 60 seconds
4. With RPR+, the standby is initialized but not fully operational
5. RPR+ synchronizes user-entered CLI commands incrementally line-by-line
6. Following RPR+ switchover, the following events occur:

1. Traffic is disrupted until the Redundant Supervisor Engine completes the takes over
2. The switch maintains any static routes across the switchover
3. The switch does not maintain any dynamic routing protocol information
4. The switch clears the FIB Tables on switchover
5. The switch clears the CAM Tables on switchover
6. State information, such as active TCP sessions, is not maintained on switchover

1. SSO is the preferred redundancy mode for supervisor engines
2. With SSO, the redundant supervisor is fully booted and initialized
3. With SSO, supervisor engines must be synchronized
4. With SSO, configuration information and data structures are synchronized
5. SSO maintains state information between the redundant supervisor engines
6. When using SSO, the following events cause a switchover:

1. A hardware failure on the active supervisor engine
2. Clock synchronization failure between supervisor engines
3. A manual switchover

Troubleshooting Switch Performance Issues

1. One of the most telling signs of performance issues on devices is high CPU utilization
2. Cisco software-based routers use software in order to process and route packets
3. High CPU utilization typically indicates capacity issues on software-based routers
4. Catalyst 4500 and 6500 series switches are hardware-based platforms
5. High CPU utilization does not indicate capacity issues on hardware-based platforms
6. The Supervisor 720 module is comprised of three integrated core components, which are:

1. The Multilayer Switch Feature Card 3
2. The Policy Feature Card 3
3. The Switch or Switching Fabric

1. Even in hardware-based platforms, packets must be punted and processed in software
2. The reasons packets may be punted include:

1. Packets destined to the switch,  such as a Telnet session that is destined for the switch
2. Packets requiring special processing, such as packets with IP options or expired TTL
3. ACL-based features, such as ACL logging
4. Hardware resources full conditions, such as when the CAM or TCAM are full
5. Multicast traffic,  such as IGMP packets
6. Other features, such as NBAR and DHCP Snooping
7. IP version 6 packet processing