Using Multicast in Your Network
Fixed Switch Configuration Guide 19-7
Generation ID gen id: 1331801871
10.5.40.0/255.255.255.0 [2] Uptime: 66704 , expires: 0
via neighbor: 10.5.50.1 version: 3
Generation ID gen id: 1331805217
10.5.50.0/255.255.255.0 [0] Uptime: 66704 , expires: 0
via neighbor: direct version: 3
10.5.51.0/255.255.255.0 [0] Uptime: 66714 , expires: 0
via neighbor: direct version: 3
10.5.52.0/255.255.255.0 [0] Uptime: 66716 , expires: 0
via neighbor: direct version: 3
10.5.60.0/255.255.255.0 [0] Uptime: 3615 , expires: 0
via neighbor: direct version: 3
10.5.70.0/255.255.255.0 [3] Uptime: 66705 , expires: 0
via neighbor: 10.5.50.1 version: 3
Generation ID gen id: 1331805217
192.168.200.0/255.255.255.0 [0] Uptime: 66721 , expires: 0
via neighbor: direct version: 3
Route Reports
DVMRP-enabled devices send route report packets to adjacent DVMRP devices every 60 seconds.
When a DVMRP device receives one, it checks to verify that the report is from a known neighbor
before processing.
The first time a device sees its own address in a neighbor’s probe packet, it sends a unicast copy of
its entire routing table to the neighbor to reduce start-up time.
The route report packet contains data about all networks/routes of which the sending device is
aware. This information is used to determine the reverse path back to a particular multicast
source. Every DVMRP device keeps a separate metric associated with each route. This metric is
the sum of all interface metrics between the device originating the report and the source network.
DVMRP devices accept route reports for aggregated source networks in accordance with classless
inter-domain devices (CIDR). This means that, if a prune or graft is received on a downstream
interface for which the source network is aggregated, then a prune or graft should be sent
upstream (to the multicast source).
If a DVMRP device has a large number of DVMRP routes, it will spread route reports across the
route update interval (60 seconds) to avoid bottlenecks in processing and route synchronization
issues.
For the purpose of pruning, DVMRP needs to know which downstream routes depend on the
device for receiving multicast streams. Using poison reverse, the upstream router maintains a
table of the source network and all downstream devices that are dependent on the upstream
device.
Mroute Table
DVMRP-enabled devices use the mroute table to maintain a source-specific forwarding tree.
When a DVMRP device is initialized, it assumes the role of the designated forwarder for all of its
locally attached networks. Before forwarding any packets, all devices use IGMP to learn which
networks would like to receive particular multicast group streams. In the case of a shared
network, the device with a lower interface metric (a configurable value), or the lower IP address
will become the designated forwarder.
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