Re: static mroute + MBGP + unicast route : which route is

Petr , you makes my life easier ,,

thanks for precise answer

On Mon, Aug 1, 2011 at 1:28 AM, Petr Lapukhov
<petr_at_internetworkexpert.com>wrote:

> Imran,
>
> To begin with, RPF check could be classified as either data-plane or
> control-plane. Data-plane RPF check is applied when router receives a
> multicast packet, to validate if the interface and upstream host match the
> RPF information. For data-plane multicast, the packet must be received from
> an active PIM neighbor, or RPF check would fail. Control-plane RPF check is
> performed when originating/receiving control-plane messages, such as sending
> PIM Join or receiving MSDP SA message. With PIM, RPF checks influences the
> actual multicast path selection, since it carves the route that PIM Join
> message would take. In both control and data-plane RPF check cases, the
> process is similar, and based on looking through all available RPF sources.
>
> The following is the list of possible RPF sources:
>
> 1) Unicast routes, static/dynamic (e.g. via OSPF)
> 2) Static mroutes, configured locally
> 3) Multicast extension routes, such as those learned via M-BGP
>
> The process of finding the RPF source information is different from unicast
> routing table lookup. It is not based on the longest-match rule, but rather
> the best source is selected based on the *best administrative distance*. The
> router selects best matching prefix from both unicast table (based on
> longest match) and static multicast routing table and compares their AD's,
> to select the best one. For mroute table, the order you configure static
> mroutes with is important - the FIRST matching route is selected, *not* the
> longest-matching one.
>
> By default, when you configure a static mroute, its distance is zero. Thus,
> for example, if you have a static default mroute "ip mroute 0.0.0.0 0.0.0.0"
> it will always be used over any longer-matching unicast prefix, since it
> matches everything and have the AD of zero. What if you want prefix
> 192.168.1.0/24 to be RPF checked again unicast table while the rest
> against the default mroute? You can configure something like this:
>
> ip mroute 192.168.1.0 255.255.255.0 null 0 255
> ip mroute 0.0.0.0 0.0.0.0 Serial 0/0/0.1
>
> Like mentioned before, the order of mroute statement is important here,
> since the FIRST matching static mroute is selected. What this configuration
> means, is that for sources in the range 192.168.1.0/24 the first matching
> static mroute has the AD of 255 and thus would be always *less* preferred as
> compared to unicast table. However, for all other sources, the default
> mroute with the distance of zero will be elected. If you put the static
> default mroute *ahead* of the specific mroute the trick will not work - the
> default mroute will always match anything.
>
> What if mroute and unicast route have the same admin distance? In this
> case, the static mroute wins, unless it is compared against directly
> attached route or *default route*. In the latter case, unicast direct or
> unicast default route would ace the mroute.
>
> Finally, what about M-BGP? M-BGP routes are treated the same way as static
> mroutes, but having distance of BGP process (110 or 20). However, when
> looking up for the best matching M-BGP prefix, a longest match is performed
> and selected for comparison, unlike linear ordering used for mroutes. Think
> of the following scenario: your router receives a unicast default route via
> OSPF and prefix 192.168.1.0/24 via M-iBGP session. A packet with the
> source address 192.168.1.100 arrives - what would be used for RPF check? It
> would be OSPF default route, because of OSPF's distance 110 and BGP's
> distance 200 for iBGP. You can solve this problem by lowering BGP's distance
> or increasing OSPF's distance or resorting to static mroute for the source
> prefix.
>
> For the last question - what happens if router has multiple equal-cost
> paths to the source. Firstly, only those routes that point to the PIM
> neighbors would be used. Secondly, the router will use the entry with the
> higher PIM neighbor IP address. This will effectively eliminate uncertainty
> in RPF decision. However, it is possible to use equal-cost multicast
> splitting, which is a separate IOS feature:
>
>
> http://www.cisco.com/en/US/docs/ios/12_4t/ip_mcast/configuration/guide/mctlsplt.html#wp1070778
>
> This feature allows splitting multicast trees among different RPF paths and
> accept packets from multiple RPF sources. However, for the classic
> multicast, there is only one RPF interface.
>
> HTH
>
> --
> Petr Lapukhov, petr_at_INE.com
> CCIE #16379 (R&S/Security/SP/Voice)
> CCDE #20100007
>
> Internetwork Expert, Inc.
> http://www.INE.com
> Toll Free: 877-224-8987
> Outside US: 775-826-4344
>
> 2011/7/31 imran ali <immrccie_at_gmail.com>
>
>> hi gurus
>>
>> 1) what is order of prefrence for RPF process if the router has all the
>> options
>>
>> * MBGP learned routes*
>>
>> * default static mroute point towards e0/0 (entered last) *
>> * /32 host Mroute of multicast server pointing towards e0/1*
>> * *
>> * unicast route pointing towards s0/0*
>> **
>> **
>> 2) how do i check the contents of MULTICAST TABLE which contains
>> MULTICAST
>> BGP ROUTES OR STATIC MROUTES
>>
>> 3) IF There are two routes pointing to server (equal cost laod balancing )
>> then both of them is present in unicast routing table , the on what basis
>> on
>> route will be discarded and other will be selected as RPF neighbor.
>>
>>
>>
>> 4) TRUE OR FALSE : THE ROUTES LEARNED BY MULTICAST BGP ARE ONLY FOR
>> INFLUNCING RFP DECISIONS .....U STILL NEED A UNICAST ROUTE FOR RECHABILITY
>> .
>> TO THE SOURCE .... MBGP ROTUES CANNOT INFLUENCE EIHTER MULTICAST OR
>> UNICAST
>> FLOW
>>
>> thanks
>>
>>
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Received on Mon Aug 01 2011 - 10:29:28 ART