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Ruchir JainCustomer Support Engineer

CCIE R&S (26911)

With,

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What could possibly cause multicast to be S/W switched

Troubleshooting approach

Tools that help

Questions from the Cisco Support Community

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Hardware Switching (CEF) is disabled

Presence of “ip igmp join”

Traffic failing RPF

Traffic on first hop router for registration with RP

Any traffic in reserved range 224.0.0.X, 224.0.1.39 and 224.0.1.40.

Traffic having TTL=1

Traffic requiring fragmentation

Platform specific limitations

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??First step is to determine what is causing the high CPU

• CPU sniffer captures

• 6500/7600: Netdr capture

• 4500: Inbuilt CPU packet dump capability

• 3560/3750/ME3XXX: Inbuilt CPU packet dump utility

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Determine which groups are causing the problem

• Analyze captures and see which groups are processed in software.

• If one group has majority of traffic then select that group for further troubleshooting.

• If multiple groups are cause of problem then select one with your best judgment.

,

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Check data part of multicast traffic

• See if TTL value is 1

• Check the length of packet and compare it to MTU of interfaces

• Check if the multicast traffic is from reserved range of group addresses - 224.0.0.x

• Auto-RP flows : 224.0.1.39 and 224.0.1.40

• Check for any IP options present in traffic

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Check mroute entry (show ip mroute <group>)

Router#sh ip mroute 239.1.1.1<snip>(*, 239.1.1.1), 00:02:29/stopped, RP 4.4.4.4, flags: SPF Incoming interface: Ethernet1/0, RPF nbr 10.1.24.4 Outgoing interface list: null

(10.1.12.1, 239.1.1.1), 00:01:34/00:03:26, flags: FT Incoming interface: Ethernet0/0, RPF nbr 0.0.0.0 Outgoing interface list: Ethernet1/0, Forward/Sparse, 00:01:34/00:02:56

• Check which PIM flavor are you running

• Check if inbound and outbound interfaces are listed properly.

• Check if “ip igmp join-group” is present on any interfaces.

• If you see “registering” then some problem with registration process

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Check mroute count (show ip mroute <group>)

Router#sh ip mroute 239.1.1.1 count <snip>Group: 239.1.1.1, Source count: 1, Group pkt count: 1869 RP-tree: Forwarding: 0/0/0/0, Other: 2/1/1 Source: 10.1.12.1/32, Forwarding: 1869/31/996/248, Other: 1871/0/2

• Check if RPF is failing

• Use “show ip rpf <source_ip>” to figure out the rpf interface

• Check if route to source is learned correctly and is present in routing table

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Commands that help in troubleshooting process

• Debugs:

“debug ip pim <group_address>”“debug ip igmp <group_address>”

• Show commands:

“show ip pim rp mapping”“show ip pim rpf”“show ip pim neighbors”

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!!Other things that we can check:

• For layer 3 switches if running multicast using SVI then if IGMP snooping is disabled for a VLAN then traffic would be punted to CPU

• Platform specific limitations: these can be found on configuration guide of each platform

• Incorrect programing of forwarding information in hardware. This is specific to H/W switching platforms.

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Q & A

Thank you.