Troubleshoot Voice, Video and Multicast

I have had a hard time figuring out how they are going to test us for voice troubleshooting when the only real command they cover is auto qos and and the MQC. As a result I’m going to concentrate on the definitions.

Voice Definitions
Gatekeeper — provides bandwidth management through call admission control (CAC).
Gateway — ensures interoperability between VOIP and the public switched telephone network (PSTN).
Jitter (delay variation) — When consecutive packets experience different amounts of delay. Data applications tend to be much more forgiving of jitter than voice and video.
Delay — There are multiple types of delay in a network. Some are standard or fixed and some are variable in their affects, the TSHOOT book describes delay as propagation delay, the time it takes to get a bit from one end of a link to the other.
Drops — Congested packets overflow a buffer.

Cisco Phone Boot Process
1. Power, PoE
2. Load firmware from flash.
3. Catalyst switch informs the phone it’s voice VLAN.
4. DHCP for ip address and TFTP server.
5. Downloads configuration using TFTP.
6. Registers with call agent or Call Manager.

QoS Metrics for Video

QoS Metric

HQ Audio & Video PC Video Conference Video Surveillance VOIP
One-Way Delay

150 ms

200 ms max

500 ms

150-200 ms1, 2

10 ms

10 ms

10 ms

30 ms

0.05 % max

0.05 % max

0.5 % max

1.0 % max 3

1 — ONT Certification Guide p.62
2 — Cisco DocWiki
3 — Enabling VOIP

Class D IP address in the range through Source sends one packet stream to the multicast address and all hosts that have joined that group receive that packet.

Internet Group Management Protocol (IGMP)
Hosts join a multicast group by sending an IGMP join message to router, which then knows to send multicast messages out that interface. IGMP snooping allows a switch to learn which interfaces desire multicast traffic by listening for IGMP traffic between routers and hosts. This stops the switch from flooding multicast traffic out all ports.

IGMP Version 1 — Hosts join a multicast group by sending a membership report to its local router. Every 60 seconds the querier router sends a messages to all-hosts to ensure that there is a host on that network segment that is still in the group. IGMPv1 does not have a mechanism for hosts to leave a group, and it takes three query intervals (3 minutes) to stop sending multicast traffic to a segment.

IGMP Version 2 — Adds the ability for routers to query a specific multicast group, elect a querier for a segment and allows a host to send a leave group message to the all routers address All routers start as queriers, however, if a router hears a query from another router, the router with the highest IP address on the segment becomes the querier for that segment.

Reverse Path Forwarding (RPF) — Verifies that multicast traffic flows away from the source or root and is flowing toward the branch or host.

Protocol Independent Multicast (PIM) — Allows multicast to build distribution trees regardless of the unicast routing protocol which is running such as EIGRP or OSPF.

PIM Dense Mode (PIM-DM) — Uses a source distribution tree. At first all routers receive traffic for the group, but if no host joins using IGMP the router sends a prune message so that unnecessary traffic does not continue. Most often used when recipients are on every subnet, densely populated.

PIM Sparse Mode (PIM-SM) — Uses a shared tree with a root router or rendezvous point (RP) that is not necessarily the multicast source but is usually centrally located on the network. All multicast streams go through this router, hence the name share tree or shared distribution tree. A router only joins the tree when a host has joined the multicast group. It is built opposite of dense mode, the tree is built from the leaves to the root, it is only when a host joins a multicast group that the router forwards the membership report to the RP.

PIM Sparse-Dense Mode — Allows a router to use sparse or dense-mode or both at the same time. Dense mode is used to flood RP discovery and announcement messages so that the client can find the RP and use the RP to find the multicast server.

Multicast Configuration

switch(config)# ip igmp snooping
switch(config)# ip igmp snooping vlan x

router(config)# ip multicast-routing
router(config)# ip pim {dense-mode | sparse-mode | sparse-dense-mode}
router(config)# ip pim version {1 | 2}

Multicast Troubleshooting
ip igmp join-group — Let’s a router join a group in order to test.
sh ip igmp group — Shows the groups a router has joined.
sh ip igmp interface — Shows IGMP information for each interface.

sh ip mroute
ping multicast address
sh ip pim rp
sh ip rpf

sh ip igmp group

R1#sh ip igmp groups 
IGMP Connected Group Membership
Group Address    Interface                Uptime    Expires   Last Reporter   Group Accounted     Loopback1                00:06:35  00:02:18       Loopback1                00:06:35  00:02:16 

sh ip igmp interface

R1#sh ip igmp interface 
Loopback1 is up, line protocol is up
  Internet address is
  IGMP is enabled on interface
  Current IGMP host version is 2
  Current IGMP router version is 2
  IGMP query interval is 60 seconds
  IGMP querier timeout is 120 seconds
  IGMP max query response time is 10 seconds
  Last member query count is 2
  Last member query response interval is 1000 ms
  Inbound IGMP access group is not set
  IGMP activity: 2 joins, 0 leaves
  Multicast routing is enabled on interface
  Multicast TTL threshold is 0
  Multicast designated router (DR) is (this system)
  IGMP querying router is (this system)
  Multicast groups joined by this system (number of users):

sh ip mroute

R1#sh ip mroute 
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,
       L - Local, P - Pruned, R - RP-bit set, F - Register flag,
       T - SPT-bit set, J - Join SPT, M - MSDP created entry,
       X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,
       U - URD, I - Received Source Specific Host Report,
       Z - Multicast Tunnel, z - MDT-data group sender,
       Y - Joined MDT-data group, y - Sending to MDT-data group
Outgoing interface flags: H - Hardware switched, A - Assert winner
 Timers: Uptime/Expires
 Interface state: Interface, Next-Hop or VCD, State/Mode

(*,, 00:21:25/stopped, RP, flags: DCL
  Incoming interface: Null, RPF nbr
  Outgoing interface list:
    Serial1/1, Forward/Dense, 00:21:25/00:00:00
    Serial1/0, Forward/Dense, 00:21:25/00:00:00
    Loopback1, Forward/Dense, 00:21:25/00:00:00

ping multicast address


Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to, timeout is 2 seconds:

Reply to request 0 from, 12 ms
Reply to request 0 from, 16 ms
Reply to request 0 from, 16 ms
This entry was posted in Routing. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s