Switch – Can default vlan 1 traffic goes to native vlan 2

CDP (LLDP) lists how the each port is configured. Cisco, in their infinite wisdom, thinks access ports in different VLANs are an error. They aren't, but it could be a misconfiguration. Disable CDP and the switch will shut up about it.

It seems that your problem is with Multicast traffic. Switches that default to IGMP snooping have a problem with multicast traffic across multiple switches. Cisco has documents, e.g. Multicast Does Not Work in the Same VLAN in Catalyst Switches, which explain the problem and solutions.

The problem is basically that switches with IGMP snooping will listen to IGMP messages in order to learn which switch interfaces are requesting traffic for which IGMP groups. If a switch doesn't have any hosts connected, it will simply drop multicast traffic. The IGMP requests will not span across switch trunks, except in specific scenarios.

Problem

Multicast traffic does not seem to pass across Catalyst switches, even in the same VLAN. Figure 1 depicts a typical scenario:

Figure 1 – Network Setup with Multicast Source and Receivers

The multicast source is connected to Switch 1, which is a Catalyst 6500 Switch with Supervisor Engine 720 that runs Cisco IOS Software. Receiver 1 is connected to Switch 1, and Receiver 2 is connected to Switch 2. Switch 2 is a Catalyst 3750. There is a Layer 2 link, either access or trunk, between Switch 1 and Switch 2.

In this setup, you find that Receiver 1, which is on the same switch as the source, gets the multicast stream with no problems. However, Receiver 2 does not get any multicast traffic. This document aims to resolve this issue.

An explanation of the problem with a link to the solutions:

Understand the Problem and Its Solutions

By default, the Catalyst switches have IGMP snooping enabled. With IGMP snooping, the switch snoops (or listens) for IGMP messages on all the ports. The switch builds an IGMP snooping table that basically maps a multicast group to all the switch ports that have requested it.

Assume that, without any prior configuration, Receiver 1 and Receiver 2 have signaled their intentions to receive a multicast stream for 239.239.239.239 that maps to the L2 multicast MAC address of 01.00.5e.6f.ef.ef. Both Switch 1 and Switch 2 create an entry in their snooping tables for these receivers in response to the IGMP reports that the receivers generate. Switch 1 enters port Gigabit Ethernet 2/48 in its table, and Switch 2 enters port Fast Ethernet 1/0/47 in its table.

Note: At this point, the multicast source has not started its traffic, and none of the switches knows about the switch mrouter port.

When the source on Switch 1 starts to stream multicast traffic, Switch 1 has "seen" the IGMP report from Receiver 1. As a result, Switch 1 delivers the multicast out port Gigabit Ethernet 2/48. But, since Switch 2 "absorbed" the IGMP report from Receiver 2 as part of the IGMP snooping process, Switch 1 does not see an IGMP report (multicast request) on port Gigabit Ethernet 2/46. As a result, Switch 1 does not send any multicast traffic out to Switch 2. Therefore, Receiver 2 never gets any multicast traffic, even though Receiver 2 is in the same VLAN but merely on a different switch than the multicast source.

The reason for this issue is that IGMP snooping is not really supported on any Catalyst platform without an mrouter. The mechanism "breaks down" in the absence of an mrouter port. If you want a fix for this solution, you must have the switches somehow learn or know of an mrouter port. The Solutions section of this document explains the procedure. But how does the presence of an mrouter port on the switches remedy the situation?

Basically, when the switches learn or statically know about an mrouter port, two critical things occur:

• The switch "relays" the IGMP reports from the receivers to the mrouter port, which means that the IGMP reports go toward the multicast router. The switch does not relay all the IGMP reports. Instead, the switch sends only a few of the reports to the mrouter. For the purpose of this discussion, the number of reports is not important. The multicast router only needs to know if there is at least one receiver that is still interested in the multicast downstream. In order to make the determination, the multicast router receives the periodic IGMP reports in response to its IGMP queries.
• In a source-only multicast scenario, in which no receivers have yet "joined" in, the switch only sends the multicast stream out its mrouter port.

When the switches know their mrouter port, Switch 2 relays out the IGMP report that the switch received from Receiver 2 to its mrouter port. This port is Fast Ethernet 1/0/33. Switch 1 gets this IGMP report on the switch port Gigabit Ethernet 2/46. From the perspective of Switch 1, the switch has received merely another IGMP report. The switch adds that port into its IGMP snooping table and begins to send out the multicast traffic on that port as well. At this point, both the receivers receive the requested multicast traffic, and the application works as expected.

But how do the switches identify their mrouter port so that IGMP snooping works as it is expected to work in a simple environment like this? The Solutions section provides some answers.

Cisco lists five different possible solutions to the problem:

1. Enable PIM on the Layer 3 Router/VLAN Interface
2. Enable IGMP Querier Feature on a Layer 2 Catalyst Switch
3. Configure Static Mrouter Port on the Switch
4. Configure Static Multicast MAC Entries on All the Switches
5. Disable IGMP Snooping on All the Switches