Troubleshooting VLANs and Trunking, Best Cisco CCIE Security Training Institute in Delhi Gurgaon

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To effectively troubleshoot switches, you must know how to identify and resolve VLAN
performance issues, assuming you have identified and resolved any port connectivity and
autonegotiation issues. Figure 2-39 shows the flow for troubleshooting VLANs and trunks.
Figure 2-39 Troubleshooting VLANs
Native VLAN Mismatches
The native VLAN that is configured on each end of an IEEE 802.1Q trunk must be the
same. Remember that a switch receiving an untagged frame assigns the frame to the native
VLAN of the trunk. If one end of the trunk is configured for native VLAN 1 and the other
Troubleshoot
Physical Layer
Port Connectivity
Issues
Troubleshoot
VLAN and Trunking
Issues
Troubleshoot
VTP
Issues
Troubleshoot
Spanning-Tree
Issues
Do local and
peer native
VLANs match?
Do local and
peer trunk modes
match?
Does each
VLAN correspond
with a unique
IP subnet?
Does
inter-VLAN
traffic have a
functional Layer 3
process?
Troubleshooting Switched Networks 81
end is configured for native VLAN 2, a frame sent from VLAN 1 on one side is received on
VLAN 2 on the other. VLAN 1 “leaks” into the VLAN 2 segment. There is no reason this
behavior would be required, and connectivity issues will occur in the network if a native
VLAN mismatch exists.
Trunk Mode Mismatches
You should statically configure trunk links whenever possible. However, Cisco Catalyst
switch ports run DTP by default, which tries to automatically negotiate a trunk link. This
Cisco proprietary protocol can determine an operational trunking mode and protocol on a
switch port when it is connected to another device that is also capable of dynamic trunk
negotiation. Table 2-15 outlines DTP mode operations.
VLANs and IP Subnets
Each VLAN will correspond to a unique IP subnet. Two devices in the same VLAN should
have addresses in the same subnet. With intra-VLAN traffic, the sending device recognizes
the destination as local and sends an ARP broadcast to discover the MAC address of the
destination.
Two devices in different VLANs should have addresses in different subnets. With inter-
VLAN traffic, the sending device recognizes the destination as remote and sends an ARP
broadcast for the MAC address of the default gateway.
Inter-VLAN Connectivity
Most of the time, inter-VLAN connectivity issues are the result of user misconfiguration.
For example, if you incorrectly configure a router on a stick or Multilayer Switching (Cisco
Express Forwarding), then packets from one VLAN may not reach another VLAN. To avoid
misconfiguration and to troubleshoot efficiently, you should understand the mechanism
used by the Layer 3 forwarding device. If you are sure that the equipment is properly
configured, yet hardware switching is not taking place, then a software bug or hardware
malfunction may be the cause.
Table 2-15 DTP Mode Examples
Configuration Parameter Description
Dynamic Auto Creates the trunk link based on the DTP request from the
neighboring switch. Dynamic Auto does not initiate the
negotiation process; therefore, two switches set to Dynamic
Auto do not form a trunk link.
Dynamic Desirable Communicates to the neighboring switch via DTP that the
interface would like to become a trunk if the neighboring
switch interface is able to.
82 Chapter 2: Medium-Sized Switched Network Construction
Another type of misconfiguration that affects inter-VLAN routing is misconfiguration on
end-user devices such as PCs. A common situation is a misconfigured PC default gateway.
Too many PCs having the same default gateway can cause high CPU utilization on the
gateway, which affects the forwarding rate.