Overview

VLAN trunking on a core router is critical for scalable enterprise and carrier-grade networks. This FAQ addresses both pre-sales architectural concerns (throughput, optic compatibility, backplane capacity) and post-sales troubleshooting (CLI configuration, stacking interface errors, power failover validation). All answers are structured for immediate technical decision-making.

Frequently Asked Questions

Q1: What is the maximum switching capacity and packet forwarding rate of this core router when fully loaded with VLAN trunks?

The maximum switching capacity is 12.8 Tbps with a packet forwarding rate of 8.4 Bpps (billion packets per second) for 64-byte frames. This line-rate non-blocking performance is maintained across all configured VLAN trunks simultaneously, provided the backplane fabric operates at 2.4 GHz per slot. Actual throughput depends on enabled L3 protocols; for pure VLAN trunking (802.1Q), the switch fabric achieves line-rate on all ports up to the maximum forwarding table size of 512,000 MAC entries.

Q2: Which optical modules (SFP, SFP+, QSFP28) are compatible for VLAN trunk uplinks, and do you support third-party optics?

All standard SFP (1G), SFP+ (10G), and QSFP28 (100G) modules compliant with MSA (Multi-Source Agreement) are hardware-compatible for VLAN trunk uplinks. The core router does not digitally encode vendor-locked EEPROM data by default, meaning third-party optics work out-of-box. However, for DOM (Digital Optical Monitoring) and advanced diagnostics, we recommend validated optics from our compatibility list. To enable unrecognized third-party modules, use the CLI command: 'service unsupported-transceiver' followed by a reload of the interface.

Q3: What is the exact CLI command sequence to create a VLAN trunk on a 10G Ethernet interface and allow specific VLANs?

The authoritative CLI sequence for VLAN trunk configuration is: first enter global configuration mode, then interface sub-mode. Execute: 'configure terminal', 'interface tengigabitethernet 1/1/4', 'switchport mode trunk', 'switchport trunk allowed vlan add 100,200,300', 'switchport trunk native vlan 99' (optional), and 'no shutdown'. To verify, use 'show interfaces trunk'. The core router defaults to 802.1Q encapsulation; no additional encapsulation command is required. Always issue 'write memory' to persist after reboot.

Q4: How does backplane capacity and per-slot bandwidth affect VLAN trunk performance compared to previous generation (Gen4 vs Gen5)?

Current generation core router provides 2.4 Tbps per slot backplane bandwidth, compared to 480 Gbps per slot in previous generation (Gen4). This translates to 5x improvement for aggregated VLAN trunk traffic. Key differences: Gen5 supports distributed L3 forwarding across all line cards simultaneously (no central bottleneck), while Gen4 relied on a single fabric module. For VLAN trunking with inter-VLAN routing, Gen5 delivers 8.4 Bpps versus Gen4's 1.8 Bpps. The backplane oversubscription ratio dropped from 3:1 to 1:1 (line-rate on all ports concurrently).

Q5: How do I troubleshoot a stacking interface failure that causes VLAN trunk flapping between two core routers?

Stacking interface failures causing trunk flapping are diagnosed by first checking physical layer: 'show stack-port 1/1 statistics' and looking for CRC or alignment errors. Then verify VLAN consistency: 'show vlan consistency' across stack members. The definitive fix sequence is: 1) Set a static stack master priority ('stack-master priority 15'), 2) Manually configure trunk allowed lists identically on both peers ('switchport trunk allowed vlan add X'), 3) Disable DTP auto-negotiation ('switchport nonegotiate'), 4) If flapping persists, reboot the standby member with 'redundancy reload shelf-peer' after saving config. Avoid using auto-VLAN trunk protocols (VTP) in core deployments.

Q6: What is the correct procedure to validate redundant power supply failover during a live VLAN trunking session?

To validate redundant power failover without traffic loss, first confirm both PSUs are in active-standby mode using 'show power | include A/B'. Then simulate a failure by pulling AC line cord of PSU-A while monitoring trunked interface counters ('show interface trunk statistics' in 1-second intervals). The core router should maintain zero packet loss on all VLAN trunks if the failover trigger threshold is set to 10ms (default). To manually test, use 'power redundancy force-switchover' which hot-swaps to PSU-B. Post-test, verify syslog entries: 'PSU failover completed, no traffic interruption'. If latency spikes exceed 50μs, replace DC capacitors on affected power distribution board.

Q7: What are the supported maximum VLANs and trunk encapsulation overhead for jumbo frames?

The core router supports up to 4094 active VLANs (802.1Q) per switch instance, with a maximum of 16,000 VLAN trunks globally. For trunk encapsulation overhead: standard 802.1Q adds 4 bytes to the Ethernet frame. When using jumbo frames (MTU 9216 bytes), the effective payload after trunk tagging is 9212 bytes. To enable, set system MTU to 9216 via 'system mtu jumbo 9216' then 'interface mtu 9216' on trunk ports. Double-tagged VLAN (Q-in-Q, 802.1ad) adds 8 bytes total; ensure all trunk links support at least 9224-byte frames to prevent fragmentation.

Q8: How do I configure redundant trunk links between two core routers using LACP (802.3ad) for failover?

To configure LACP on redundant trunk links, first create the port-channel interface: 'interface port-channel 1', 'switchport mode trunk', 'switchport trunk allowed vlan all'. Then bind physical interfaces: 'interface tengigabitethernet 1/1/1', 'channel-group 1 mode active', 'interface tengigabitethernet 1/1/2', 'channel-group 1 mode active'. LACP active mode ensures failover within 50ms. Validate with 'show lacp neighbor' and 'show etherchannel summary'. For inter-chassis redundancy across two physically separate core routers, additionally enable VPC (Virtual Port Channel) using 'vpc domain 1' and 'peer-keepalive' configuration. Without VPC, spanning-tree will block one trunk link.