Traceroute Facility For Ip Multicast; Multicast Multipath - Alcatel Router User Manual

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Chapter : Multicasting

Traceroute Facility for IP Multicast

Multicast Multipath

Router User Guide
Displaying IGMP group
membership information
Displaying IGMP
interface configuration
Clear Command
Clearing IGMP group
membership information
With multicast distribution trees, tracing from a source to a multicast destination is difficult, since
the branch of the multicast tree on which the destination lies is unknown. The technique used by the
traceroute tool to trace unicast network paths will not work for IP multicast because traceroute
(ICMP) responses are specifically forbidden for multicast traffic. Thus, you have to flood the whole
tree to find the path from one source to one destination. However, walking up the tree from
destination to source is easy, as most existing multicast routing protocols know the previous hop for
each source. Tracing from destination to source involves only routers on the direct path.
To request a traceroute (which does not have to be the source or the destination), send a traceroute
query packet to the last-hop multicast router for the given destination. The last-hop router turns the
query into a request packet by adding a response data block containing its interface addresses and
packet statistics, and then forwards the request packet using unicast to the router that it believes is the
proper previous hop for the given source and group. Each hop adds its response data to the end of the
request packet, then unicast forwards it to the previous hop. The first hop router (the router that
believes that packets from the source originate on one of its directly connected networks) changes
the packet type to indicate a response packet and sends the completed response to the response
destination address. The response may be returned before reaching the first hop router if a fatal error
condition such as "no route" is encountered along the path.
Multicast traceroute uses any information available to it in the router to try to determine a previous
hop to forward the trace towards. Multicast routing protocols vary in the type and amount of state
they keep; multicast traceroute tries to work with all of them by using whatever is available. For
example, if a DVMRP router has no active state for a particular source but does have a DVMRP
route, it chooses the parent of the DVMRP route as the previous hop. If a PIM-SM router is on the
(*,G) tree, it chooses the parent towards the RP as the previous hop. In these cases, no
source/group-specific state is available, but the path may still be traced.
Alcatel's version 8.0 and higher for Ties supports the following PIM related feature—a "traceroute"
facility for IP multicast, as defined in draft-ietf-idmr-traceroute-ipm-05.
The multicast multipath feature in version 8.0 and higher allows load balancing on multicast traffic
across equal cost paths. Equal cost multipath routing is useful when multiple equal cost routes to the
same destination exist. These routes can be discovered and be used to provide load balancing among
redundant paths. Commonly used methods for multipath forwarding are Round-Robin and Random.
While these methods do provide a form of load balancing, but variable path MTUs, variable
latencies, and debugging can limit the effectiveness of these methods.
The following methods have been developed to deal with the load balancing limitations of the
Round-Robin and Random methods:
show ip igmp groups {all | <interface-name>} [detail]
show ip igmp interface {all | <interface-name>}
clear ip igmp groups [interface <name>] [group-addr
<addr>] [source-addr <source-addr> ]