From: Jim (quangnn@hptvietnam.com.vn)
Date: Wed May 04 2005 - 12:14:26 GMT-3
Tim,
It's interesting with the case when the forwarder doesn't have a path to RP
or multicast source address.
Because it is a automatic proccess, digging deeper into this to make sure
routers never fall into the case lead to trouble of source unreachable.
And that IOS programmers are not bad guys when they insert 2 valuable fields
into the Assert Message.
i) Encoded Group Address: the Multicast IP destination address of the packet
that triggerd the Assert
ii) Encoded Unicast Source Address: the IP source address of the multicast
packet that triggered the Assert.
Beside AD, metric and IP Address, I am sure that the routers also verify
that whether the source multicast is reachable in the PIM forwarder election
proccess. Obviously, if one router does not have a path to the source
multicast, the ii) field must be empty and it never becomes DR for that
shared LAN.
I will lab it up tomorrow to confirm this,
Thanks, Jim
----- Original Message -----
From: "ccie2be" <ccie2be@nyc.rr.com>
To: "'Jim'" <quangnn@hptvietnam.com.vn>; "'Danshtr'" <danshtr@gmail.com>;
"'Cisco certification'" <ccielab@groupstudy.com>
Sent: Wednesday, May 04, 2005 7:43 PM
Subject: RE: Multicast DR...
> Hi Jim,
>
> If there are 2 pim SM routers on a Lan segment either of which could
> potentially sends joins up the shared tree to the RP, however, only 1 of
> those routers has a path to the RP, make sure the router with a path to
> the
> RP is the DR. Recall that the pim router with the highest ip address
> becomes the DR and that only the DR sends joins up the tree to the RP. If
> the DR doesn't have a path to the RP, the joins won't ever get to the RP
> and
> mcast traffic won't ever flow down the tree. To see which of the 2
> routers
> on the segment is the DR, do a show ip pim nei. If the output shows "(DR)"
> on the right side of neighbor's entry, then that nei is the DR. One way to
> change which router becomes the DR is to change the ip addresses of the
> interfaces on the lan segment such that the preferred DR has the highest
> IP
> address. A better way is to use the interface command, ip pim dr-priority
> #.
>
>
> In a DM network where 2 or more pim routers on LAN could potentially send
> mcast pkts onto the lan segment, DM pim prevents this by electing a
> forwarder - a rtr which will send the mcast traffic onto the lan. 1st,
> the
> AD of the route to the source is compared and the route with the lowest AD
> wins. If the AD's are equal, then the metric of the route is compared and
> the lowest metric wins. If there's still a tie, the ip address is the
> tiebreaker.
>
> HTH, Tim
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Jim
> Sent: Wednesday, May 04, 2005 7:20 AM
> To: ccie2be; 'Danshtr'; 'Cisco certification'
> Subject: Re: Multicast DR...
>
> Tim,
>
>
>
> Thanks for getting back to me,
>
>
>
> so which one will sends joins up the tree if 2 routers are on the
> shared-media with client?
>
>
>
> R1 R2
>
> ----|-------|-------
>
> |
>
> client
>
>
>
> the proccess of PIM forwarder election works with both pim dense & spare
> mode and it happens whenever one of the router receive multicast packets
> on
> its outgoing interface for that packet's group.
>
> When the client sends join message to R2, R2 joins shared tree and the
> traffic will flow from source to client this way. But when R2 forwards
>
> traffic with destination address as multicast packets to clients, switch
> will treats these packets as broadcast and forward to all other ports. As
> the result, R1 receives packets on its outgoing interface for that
> packet's
> group and it immediately starts the proccess of PIM forwarder election by
> sending assert message. Finally, the winner will join the shared tree and
> take the role of the forwarder for all clients on the shared media.
>
>
>
> On TCP/IP vol2 489, we have "the election of designated router on
> multiaccess networks are common to PIM-SM and PIM-DM"
>
>
>
> Jim.
>
>
>
>
>
>
> ----- Original Message -----
> From: "ccie2be" <ccie2be@nyc.rr.com>
> To: "'Jim'" <quangnn@hptvietnam.com.vn>; "'Danshtr'" <danshtr@gmail.com>;
> "'Cisco certification'" <ccielab@groupstudy.com>
> Sent: Wednesday, May 04, 2005 4:53 PM
> Subject: RE: Multicast DR...
>
>
>> Jim,
>>
>> I think your remarks are correct only if PIM Dense Mode is being used.
>>
>> If PIM Sparse Mode is used, then the rtr which sends joins up the tree
>> will
>> also forward mcast traffic to hosts.
>>
>> HTH, Tim
>>
>> -----Original Message-----
>> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
>> Jim
>> Sent: Wednesday, May 04, 2005 5:27 AM
>> To: Danshtr; Cisco certification
>> Subject: Re: Multicast DR...
>>
>> On a shared media, the router selected to forward traffic to client is
>> the
>> one
>>
>> i) who advertises lowest metric preference ( in other words, lowest AD)
>> ii) who advertises lowest metric if the metric preference is equal
>> iii) who has the highest IP address on the network if all other metric
>> are
>> the same.
>>
>> PIM bases on other protocols for its routing so you can verify DR
>> selection
>> by checking the route to Multicast Source in routing and mroute table.
>>
>> HTH, Jim
>>
>> ----- Original Message -----
>> From: "Danshtr" <danshtr@gmail.com>
>> To: "Cisco certification" <ccielab@groupstudy.com>
>> Sent: Wednesday, May 04, 2005 1:47 PM
>> Subject: Multicast DR...
>>
>>
>>> Hello
>>>
>>> Reading through multicast documentation, they are mentioning DR and
>>> queriers which both are intended for deciding which router will
>>> forward multicast packets to the recievers.
>>>
>>> But i couldn't find how the routers, on a shared media, decide which
>>> one will forward the packets from the source.
>>>
>>> Does anyone here knows the answer? I am sure there is... because it
>>> works!
>>>
>>> --
>>> Best regards,
>>> Dan
>>>
>>> _______________________________________________________________________
>>> Subscription information may be found at:
>>> http://www.groupstudy.com/list/CCIELab.html
>>
>> _______________________________________________________________________
>> Subscription information may be found at:
>> http://www.groupstudy.com/list/CCIELab.html
>>
>> _______________________________________________________________________
>> Subscription information may be found at:
>> http://www.groupstudy.com/list/CCIELab.html
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
This archive was generated by hypermail 2.1.4 : Fri Jun 03 2005 - 10:11:56 GMT-3