Cisco NM-8B-U - HW ROUTERS L-M User Manual page 4

Feature Overview
Layer 2 Interface Modes
Switchport mode access puts the interface into nontrunking mode. The interface will stay in access mode
regardless of what the connected port mode is. Only access VLAN traffic will travel on the access port and
untagged (802.3).
Switchport mode trunk puts the interface into permanent trunking mode.
Table 1
Feature
Interface mode
Trunk encapsulation
Allowed VLAN range
Default VLAN (for access ports)
Native VLAN (for 802.1Q trunks)
Spanning Tree Protocol (STP)
STP port priority
STP port cost
When you connect a Cisco switch to a device other than a Cisco device through an 802.1Q trunk, the
Cisco switch combines the spanning tree instance of the VLAN trunk with the spanning tree instance of
the other 802.1Q switch. However, spanning tree information for each VLAN is maintained by Cisco
switches separated by a cloud of 802.1Q switches that are not Cisco switches. The 802.1Q cloud
separating the Cisco switches that is not Cisco devised, is treated as a single trunk link between the
switches.
Make sure that the native VLAN for an 802.1Q trunk is the same on both ends of the trunk link. If the
VLAN on one end of the trunk is different from the VLAN on the other end, spanning tree loops might
result. Inconsistencies detected by a Cisco switch mark the line as broken and block traffic for the
specific VLAN.
Disabling spanning tree on the VLAN of an 802.1Q trunk without disabling spanning tree on every
VLAN in the network can potentially cause spanning tree loops. Cisco recommends that you leave
spanning tree enabled on the VLAN of an 802.1Q trunk or that you disable spanning tree on every VLAN
in the network. Make sure that your network is loop-free before disabling spanning tree.
Layer 2 Interface Configuration Guidelines and Restrictions
Follow these guidelines and restrictions when configuring Layer 2 interfaces:
In a network of Cisco switches connected through 802.1Q trunks, the switches maintain one instance of
spanning tree for each VLAN allowed on the trunks. 802.1Q switches that are not Cisco switches, maintain
only one instance of spanning tree for all VLANs allowed on the trunks.
Cisco IOS Release 12.2(2)XT, 12.2(8)T, and 12.2(15)ZJ
4
16- and 36-Port Ethernet Switch Module for Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series
Default Layer 2 Ethernet Interface Configuration
Default Value
switchport mode access / trunk
switchport trunk encapsulation dot1q
VLANs 1-1005
VLAN 1
VLAN 1
Enabled for all VLANs
128
100 for 10-Mbps Ethernet interfaces
19 for 10/100-Mbps Fast Ethernet interfaces
19 for Gigabit Ethernet interfaces operated in 100-Mb mode
4 for Gigabit Ethernet interfaces operated in 1000-Mb mode