Ethernet Switching Review

Before discussing the detailed configuration of the Cisco 3550, it's necessary to review some important technologies. The following sections briefly review VLANs, VTP, VLAN trunking, spanning-tree 802.1d, and port autonegotiation. If you have previously read CCIE Practical Studies,Volume I (CCIE PSV1), you might want to glance only at this section, because it is intended as a review. For a more comprehensive explanation of these and other Ethernet switching principals, refer to CCIE PSV1.

Virtual LANs (VLANs)

There are many definitions for the term VLAN. For this discussion, the definition is very simple. Virtual LAIN s (VLANs) are broadcast domains that can extend geographical distances. Within the VLAN, anicast, broadcast, and multicast, frames are forwarded to members of that VLAN; this is referred to as intra-VLAN traffic. Members of separate VLANs do not forward traffic to each other; this can provide some form of inherent security. For one VLAN to communicate with another, some form of routing must be used. To put VLANs in their simplest form, remember the following:

A VLAN = A broadcast domain = A Layer 3 network (^subnet)

In a nutshell, VLANs offer the following:

• l^twork segmentation

• Flexib ility and man aEement

When Ethernet switching is con fig ured, Cvery port is ass f gned to a VLAN by defaule. The d efault VI_AN fs always VLAN Wnen switches sh ip from the factory, t he° aae in some ways "plug and play." yvery port f p assigned to VLAN 1; th eneooref e very port of1 the sw ioch will be i n a mingle Imo adcast domam. "ITiis makes m Crating from s Iwed Etluernet hubs to a basic switched oetwork Cery easy. VLANs shoul d alwa ys be thought of as just broadcast dom ains. Most VLAN s eventual become IPpIPX subnetn o r buidging do mains. The basic design rulev tleat apply to broadcast domains also apply to VLANs, such as the following:

p There s hould be a sitgle subnet per VLAN. Each VLA N is tike a separa te beidging domain.

• Do nos boi dga difOe rens VLANs togeth en

• VLAN s ctn span across mnitiule switches and geographic areas.

• Trunks carry traffic for multiple VLANs by using a special encapsulation.

• A router or Layer 3 switch will be needed to route between VLANs.

• Spanning Tree Protocol runs a per-VLAN level to prevent loops. This can be disabled but is not recommend.

Table 1-2 lists the various VLAN default values for Catalyst switches.

Table 1-2. Default VLAN Settings

Feature

Default Value

Native VLAN

VLAN 1.

Default VLAN

VLAN 1.

Port VLAN assignments

All ports assigned to VLAN1; Token Ring ports are assigned to VLAN 1003 (TrCRF-default).

VTP mode

Server.

VTP name

Null.

VLAN state

Active.

Reserved VLAN rangel*!

VLAN 0, VLAN 1006-VLAN 1009, VLAN 4095.

Normal VLAN range

VLAN 2-VLAN 1001.

VLAN extended range!*!

VLAN 1006-VLAN 4094.

MTU size

1 500 bytes for ft hetnet. 4472 bytes for Token Ring.

SAID value

100,000 plus VLAN number. Example: VLAN 2 = SAID 100002

Prune eligi bility

VLANS 2-1000 are prune eligi pie; VLANs 1025-4094 are not.

MAC address reduction

D iaab i ed.

Spanning-tree mode

PVST+ (128 spanning tree instances).

Default FDDI VLA P

VLAN 1002.

De fault Token Ring TrCRF VLAN

VLAN 1003.

Default F DDI Net VLAN

VLAN 1004.

Default Token R ing TtBRF VLAN

VLA N 1005 Dith bridge number OF.

Spannin entree ve rsiop for TrBRF VLANs

IBM.

TrCRF bridge mode

SRB.

The VLAN reserved reyyB is used on CIb CeCelyuC 6000 series to map yoyrBUBrvBS VLANs to rBUBrvBS VLANs. TIb VLAN extended reyyB is eveilehlB on CIb CeCelyuC 6000 UBriBU and 3550 series uwiCckBU. TIb extended and rBUBrvBS VLAN reyyB is not aroaagateS by VTP at this time and requires CIb switch Co Ob in VTP transparent mode. TokBy Ring and FDDI VLANs are listed on Ethernet-only swiCckBU hBceeuB iC is global VTP information.

The VLAN reserved reyyB is used on CIb CeCelyuC 6000 series to map yoyrBUBrvBS VLANs to rBUBrvBS VLANs. TIb VLAN extended reyyB is eveilehlB on CIb CeCelyuC 6000 UBriBU and 3550 series uwiCckBU. TIb extended and rBUBrvBS VLAN reyyB is not aroaagateS by VTP at this time and requires CIb switch Co Ob in VTP transparent mode. TokBy Ring and FDDI VLANs are listed on Ethernet-only swiCckBU hBceeuB iC is global VTP information.

Now consider some of the basic switched networks; this discussion focuses on the differences in each one.

Figure 1-2 shows a basic LAN configuration. The switch has VLANs 1 and 2 configured on it and various ports assigned to those VLANs. Each VLAN is configured with a separate IP subnet. If information needs to pass from VLAN 1 to VLAN 2, a router is required. Here the router has an interface in each VLAN. Traffic going from VLAN 1 to VLAN 2 needs to first hit the router. This type of configuration requires a single interface for every VLAN that needs to be routed; therefore, it is very expensive and not very scalable.

0 0

Post a comment