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Beginner

Effect of this hidden command "ip multicast longest-match"

Guys,

Can someone explain the default RPF selection rules implemented on Cisco router, i would assume this differs from platform to platform? My special

concern here is routes announced within MBGP AFI1/SAFI2 in relation to unicast routes, and what effect will above command have on the RPF selection rules.

Many Thanks

Guan

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2 REPLIES 2
Beginner

Re: Effect of this hidden command "ip multicast longest-match"

Hi,

By default, when the router searches its multicast table (irrespective of how it is populated either PIM/DVMRP/MBGP/staticmroute), the router does distance preferred lookup. Which means suppose a multicast packet from source 10.20.0.3/32 has arrived. The router learns this from two sources. i.e. from OSPF with subnet mask 32 and second from static mroute with subnet mask 24. By default method of distance preferred, the router chooses the route with staitc mroute because it is the one with lower AD. Longest  match will happen only when you have multiple entries with same AD. Suppose you have two static mroute entries one with /32 and other with /24. Then the router chooses the static mroute with /32. But by default this will not happen.

To force the router to choose the route with the longest match irrespective of AD, then "ip multicast longest-match" command will be used. If this command is used, then in the above case the router chooses the route with OSPF over static mroute as OSPF learned as host route.


You can find how the router is doing lookup by doing "show ip rpf x.x.x.x"

GSR-12404-P1#show ip rpf 10.20.0.3
RPF information for ? (10.20.0.3)
  RPF interface: POS2/1
  RPF neighbor: ? (10.10.1.6)
  RPF route/mask: 10.20.0.3/32
  RPF type: unicast (ospf 10)
  RPF recursion count: 0
  Doing longest-match lookups across tables

GSR-12404-P1#show ip rpf 10.20.0.3
RPF information for ? (10.20.0.3)
  RPF interface: POS2/0
  RPF neighbor: ? (10.10.1.2)
  RPF route/mask: 10.20.0.0/24
  RPF type: static
  RPF recursion count: 0
  Doing distance-preferred lookups across tables

Below are the AD of multiast routes:

Unicast - Derived from the distance of unicast route

e-MBGP - 20

i-MBGP - 200

DVMRP- 0

Static - 0

PS: I think the above selection rules is for all router platform. I have tested the above setup in GSR and 7604 and working the same way.

HTH

thanks

Arun

Please rate if it helps.

Beginner

Re: Effect of this hidden command "ip multicast longest-mat

Ok thanks a lot, i think i am hitting a bug on 4924-10G running 53SG software, will try to open a TAC for that.

The box, when doing longest match, can no more find it's upstream interface although sh ip rpf shows correct result.

Cheers

Guan

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