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Home > Archive > CCNP > September 2001 > A question about PIM (BSMCN)
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A question about PIM (BSMCN)
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| In Cisco Press Certification Guide page 376,It says that "No designated route exsits when running PIM in dense mode". But In page 377, there is a list of the command "show ip pim neighbor", We can see the PIM mode is "Dense",and there is a DR exists.
Anyone can explain it? Thanks. | |
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| I don't have the same book as yourself but have found this on the cisco web site at :
http://www.cisco.com/univercd/cc/td...tm#xtocid821170
The relevant bit being :
show ip pim neighbor
To list the PIM neighbors discovered by the Cisco IOS software, use the show ip pim neighbor EXEC command.
Use this command to determine which routers on the LAN are configured for PIM.
Sample Display
The following is sample output from the show ip pim neighbor command:
Router# show ip pim neighbor
PIM Neighbor Table
Neighbor Address Interface Uptime Expires Mode
198.92.37.2 Ethernet0 17:38:16 0:01:25 Dense
198.92.37.33 Ethernet0 17:33:20 0:01:05 Dense (DR)
198.92.36.131 Ethernet1 17:33:20 0:01:08 Dense (DR)
198.92.36.130 Ethernet1 18:56:06 0:01:04 Dense
10.1.22.9 Tunnel0 19:14:59 0:01:09 Dense
This too confirms what you have seen.
Digging further I found the following :
http://www.cisco.com/univercd/cc/td...t3/1cdmulti.htm
and it states :
Multicast routers elect a PIM designated router (DR) for the LAN (subnet). The DR is the router with the highest IP address. The designated router is responsible for sending IGMP host-query messages to all hosts on the LAN. In sparse mode, the designated router also sends PIM Register and PIM Join messages toward the RP router.
So I understand that a DR IS used on both sparse mode and dense mode for host queries (explaining why it is in the show ip pim neighbor output) but in sparse mode it has the added responsibility of sending PIM register/join messages to the RP.
Could the book be confusing DR with RP as these are only used in sparse mode???
Hope this helps.
Mat P. | |
| depamo 2001-09-12, 1:38 pm |
| PIM in Dense or Sparse mode will require a DR, Designated Router. Now in Dense mode you will not have any RP's. If you have a single RP in Dense Mode, it will try to force Sparse Mode.
Book doesn't segragate these too well. You will get a DR election on a multicast domain where you have multiple PIM enabled router interfaces. The purpose here is a master for sending IGMP queries.
You will only get a RP when you assign one with the ip pim commands or enable AutoRP. An RP is used to designate a centrally located information source on all multicast groups and their locations to optimize the information moving through the network. |
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