2.2 Inter-VLAN Routing, CDP, and LLDP

5. Inter-VLAN Routing with a Layer 3 Switch

5.1 What Is an SVI?

An SVI (Switch Virtual Interface) is a logical Layer 3 interface associated with a VLAN. On a multilayer switch, the SVI can own the default-gateway address for that VLAN. With ip routing enabled, the switch routes between its active SVIs and other Layer 3 interfaces using the routing table.

Figure 5 - A multilayer switch routes between VLAN 10 and VLAN 20 through SVIs.

5.2 Layer 3 Switch Configuration

enable

configure terminal

hostname DIST-SW1

!

vlan 10

name USERS

vlan 20

name SERVERS

!

interface Vlan10

description USERS-DEFAULT-GATEWAY

ip address 192.168.10.1 255.255.255.0

no shutdown

!

interface Vlan20

description SERVERS-DEFAULT-GATEWAY

ip address 192.168.20.1 255.255.255.0

no shutdown

!

ip routing

end

The VLANs must exist. The SVI line protocol also depends on VLAN activity on many Cisco platforms: at least one Layer 2 port in that VLAN generally needs to be operational and participating, directly or through a trunk, for the SVI to become fully up/up. The command ip routing enables IPv4 forwarding between Layer 3 interfaces on switches that support it.

5.3 Access-Switch Uplink

interface GigabitEthernet1/0/24

description TRUNK-TO-DIST-SW1

switchport mode trunk

switchport trunk allowed vlan 10,20

6. Verify and Troubleshoot SVI Routing

6.1 Core Verification Commands

show ip interface brief

show vlan brief

show interfaces trunk

show ip route

show ip arp

show mac address-table

show interfaces Vlan10

show interfaces Vlan20

show running-config | section interface Vlan

The routing table should contain connected routes for both VLAN subnets. If Vlan10 and Vlan20 are up/up and ip routing is enabled, a packet between those connected networks can be routed locally by the multilayer switch, subject to policy and host configuration.

6.2 Routed Port versus SVI

An SVI represents a VLAN and can serve many Layer 2 ports in that VLAN. A routed physical port is converted from Layer 2 switchport behavior to Layer 3 behavior (commonly with no switchport) and receives an IP address directly. For inter-VLAN default gateways, SVIs are usually the relevant design; routed ports are common for point-to-point links between Layer 3 devices.

Example routed uplink on a multilayer switch

interface GigabitEthernet1/0/48

no switchport

ip address 10.255.0.1 255.255.255.252

no shutdown

7. Router-on-a-Stick vs Layer 3 Switch SVIs

CCNA questions often ask you to identify the method from configuration or topology. A router sub interface with encapsulation dot1Q strongly indicates Router-on-a-Stick. interface Vlan10 plus ip routing on a multilayer switch indicates SVI-based routing.

7.1 Gateway Addressing Rules

The gateway IP must belong to the same subnet as the hosts in that VLAN and must be unique. For VLAN 10 192.168.10.0/24, 192.168.10.1/24 is a common gateway choice. A host configured as 192.168.10.20/24 with gateway 192.168.20.1 will not use that remote-subnet address as a valid directly reachable default gateway in a normal Ethernet design.

8. Cisco Discovery Protocol (CDP)

8.1 Purpose and Scope

CDP is a Cisco-proprietary Layer 2 discovery protocol used by Cisco devices to advertise identity and interface information to directly connected neighbors. Because it operates over the local link, it can be useful even when IP addressing is incomplete or routing is broken. It does not discover devices multiple Layer 3 hops away.

Figure 8 - CDP helps a Cisco device identify directly connected Cisco neighbors and their attachment ports.

8.2 What CDP Can Tell You

Neighbor device ID / hostname.

Local interface used to reach the neighbor.

Remote port ID.

Platform/model information and capability codes.

Holdtime remaining.

With detailed output, management IP address and additional advertised information such as software version or interface-related details, depending on device/platform.

This makes CDP especially valuable during cabling validation and incident response. If SW1 expects DIST-SW1 on Gi1/0/48 but show cdp neighbors reports a different switch or interface, the issue may be physical patching rather than routing configuration.

8.3 CDP Timers

Common Cisco defaults are an advertisement interval of 60 seconds and a holdtime of 180 seconds. The holdtime is how long a received neighbor entry is retained without a refresh. Values can be changed globally; exact supported commands can vary by platform.

CDP verification commands

show cdp

show cdp neighbors

show cdp neighbors detail

show cdp interface

show cdp traffic

9. Configure, Limit, and Interpret CDP

9.1 Enable or Disable CDP

Enable CDP globally

configure terminal

cdp run

end

Disable CDP globally

configure terminal

no cdp run

end

Disable CDP on one interface

configure terminal

interface GigabitEthernet1/0/10

no cdp enable

end

On many Cisco IOS / IOS XE devices, CDP is enabled globally and on supported interfaces by default. Operational policy may disable it on untrusted/external-facing interfaces to avoid advertising infrastructure information where it is not needed.

9.2 Read show cdp neighbors

SW1# show cdp neighbors

Device ID  Local Intrfce  Holdtme Capability Platform Port ID

DIST-SW1   Gig 1/0/24     143     R S I      C9K      Gig 1/0/1

R1         Gig 1/0/23     165     R          ISR      Gig 0/0/0

10. Link Layer Discovery Protocol (LLDP)

10.1 Purpose and Standard

LLDP is the IEEE 802.1AB vendor-neutral Layer 2 discovery standard. Like CDP, it exchanges information only with directly connected neighbors, but its standardized TLV-based format makes it useful in mixed-vendor environments.

Figure 10 - LLDP provides standardized neighbor discovery across multi-vendor infrastructure.

10.2 LLDP TLVs

LLDP advertisements contain Type-Length-Value (TLV) elements. Mandatory/basic information includes items such as chassis identification, port identification, and time-to-live, while optional TLVs can advertise system name, system description, capabilities, management addresses, and vendor or application extensions.

10.3 LLDP Timers

Common Cisco defaults are a transmit interval of 30 seconds and a holdtime of 120 seconds, with a short reinitialization delay after LLDP is re-enabled. Treat these as common IOS / IOS XE defaults rather than universal values across every vendor/device.

11. Configure and Verify LLDP

11.1 Global Enablement

Enable LLDP globally

configure terminal

lldp run

end

Disable LLDP globally

configure terminal

no lldp run

end

11.2 Per-Interface Transmit / Receive Control

configure terminal

interface GigabitEthernet1/0/24

lldp transmit

lldp receive

end

Disable LLDP on a specific interface

configure terminal

interface GigabitEthernet1/0/10

no lldp transmit

no lldp receive

end

LLDP provides separate transmit and receive controls on many Cisco platforms. This can be useful when policy permits learning neighbor information but not advertising local details, or vice versa.

11.3 Verification Commands

show lldp

show lldp neighbors

show lldp neighbors detail

show lldp interface

show lldp traffic

As with CDP, the first operational question is whether the expected neighbor appears on the expected local interface. If LLDP is globally disabled, or transmit/receive is disabled on one end, the neighbor entry may be missing even when the Ethernet link itself is up.

12. CDP vs LLDP

Both protocols are primarily operational discovery tools. They help engineers map the physical topology, identify ports, and validate direct neighbors. Neither protocol substitutes for VLAN/trunk verification, routing-table analysis, ARP/MAC inspection, or end-to-end testing.

12.1 LLDP-MED and Voice Devices

LLDP-MED (Media Endpoint Discovery) extends LLDP for media endpoints such as IP phones. Depending on the environment, it can advertise network policy information including voice VLAN-related parameters. Cisco IP phones may also use CDP to learn voice VLAN information in Cisco-centric deployments. For CCNA, focus on the role of discovery protocols rather than memorizing every extension TLV.