608
Chapter 11
Troubleshooting Switched Ethernet
VLAN Design Issues and Troubleshooting
Although VLANs must adhere to most of the basic network design rules, there are
a number of new issues for administrators to consider with Catalyst switches.
First, the network diameter should be less than eight switches. This limi-
tation is mostly related to spanning tree concerns; however, it is also a good
rule of thumb for manageability.
Second, VLANs must be numbered within certain limitations, and each
VLAN needs to adhere to MTU considerations. Although a large MTU is
desirable for FDDI and Token Ring, the Ethernet MTU limitation of 1500 is
recommended for all interfaces. This is partly due to the Catalyst backplane
and the conversions that are needed between different Physical layers.
The default configuration of the switch includes the following VLANs, as
shown in Table 11.6.
When troubleshooting switches and routers, administrators should consider
each element in the network by using a layered approach. For example, configur-
ing a bridge to link two ISL trunks could cause spanning-tree problems. In
addition, there are two spanning-tree protocols available on the switch: IEEE and
DEC. Failure to use the same protocol will again cause spanning-tree issues.
In addition, general routing rules apply to VLANs and the RSM module.
For example, a default router is still required on all devices, and all VLANs
must have a router to go from one VLAN to another.
T A B L E 1 1 . 6
The Default Switch VLAN Configuration
VLAN Name
Type of VLAN
MTU
ISL VLAN ID
802.10 VLAN
ID (SAID)
Default
ethernet
1500
0001
100001
Fddi-default
fddi
1500
1002
101002
Token Ring
default
token-ring
4472
1003
101003
Fddinet-de-
fault
fddi-net
1500
1004
101004
Trnet-default
tr-net
4472
1005
101005
Copyright ©2000 SYBEX , Inc., Alameda, CA
www.sybex.com