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Think of a pc connected to switch1. Sw1 is connected to sw2(trunk port on both switches) and sw2 is connected to a router(dhcp server). Pc sends a dhcp broadcast. Does sw1 passes the broadcast through the trunk port(connected to sw2), without looking at the mac table?

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Yes, DHCP broadcast on Ethernet are sent to address FF:FF:FF:FF:FF:FF.

The 802.1AD document that define bridges (I.E. switches) operations state that (emphasis mine):

The broadcast address and other group MAC Addresses apply to the use of the MAC Service provided by a Bridged Local Area Network as a whole. In the absence of explicit filters configured via management as Static Filtering Entries, or via GMRP as Group Registration Entries (Clause 14, Clause 10, 7.9), frames with such destination addresses are relayed throughout the network

So by definition a broadcast is sent to all connected switches and host in the (V)LAN.

There's no MAC table lookup, since it is not needed.

  • So a trunk port is enough to send the broadcast to the other switch right? – user19215 Feb 17 at 20:22
  • It depends. You can restrict which VLAN are allowed to pass through a trunk port. A broadcast is propagated throughout the whole VLAN. So if the VLAN is allowed in the trunk, yes. – JFL Feb 18 at 6:59

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