Configuring multiple Static Mroutes for IPv4 in Cisco IOS
Last updated: 8/2003
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In certain cases it is necessary to configure multiple static routes.
One example is to support routing traffic across two interfaces.
each of which may go down. Depending on the distance configured for the 
static routes, one either wants to prefer one such interface (lower distance)
or load-split traffic across them (same distance on both interfaces).

For IPv4 unicast, the way to configure multiple equal cost static routes is
simple:

   ip route <address> <mask> <next-hop1> [ <distance1>]
   ip route <address> <mask> <next-hop2> [ <distance2>]

Unfortunately, if only IP multicast traffic is to be passed across these
routes, one need to use "ip mroute" instead of "ip route". If "ip route"
is used, both unicast and multicast traffic are passed across the route.

"ip mroute" unfortunately has the limitation that you can only configure
one "ip mroute" for each prefix (eg: <address>/<mask> combination), but
not two or more as shown above for the "ip route command". 

The purpose of this configuration note is to describe the limited workaround
supported for this problem today in Cisco IOS IPv4 multicast. Note that this
problem does not apply to IPv6 multicast and as such the workarounds presented
here are also not necessary:

  The following configuration is what IOS does not support:

   ip mroute <address> <mask> <next-hop1> [ <distance1>]
   ip mroute <address> <mask> <next-hop2> [ <distance2>]

  To get this working, choose an unused IPv4 unicast address, for example
  1.1.1.1 or something that is likewise easy identifyable as a <bogus> address
  to make troubleshooting easy. Then configure:

   ip  route <bogus> 255.255.255.255 <next-hop1> [ <distance1>]
   ip  route <bogus> 255.255.255.255 <next-hop2> [ <distance2>]
   ip mroute <address> <mask> <bogus>

  Effectively we use recursive route resolution to get around the problem
  that only one "ip mroute command" can be configured for each
  <address>/<mask> combination.  You will need one bogus address for
  each combination of next-hops that you want to route across of coures.

  Notes:

  o multipath routing not supported

    If <distance1> and <distance2> are different, only the route with the
    lower/lowest distance is being actively used. If the next-hop of that
    route become unreachable the route with the next-higher distance is
    being used.

    If <distance1> and <distance2> are equal, Cisco IOS will only use
    the first configured route entry when resolving this route
    recursively via "ip mroute". This also means that you can not use
    this workaround with recursive route resolution for equal cost
    multipath routing for in conjunction with "ip multicast multipath".
    You can use it without "ip mroute" though!

  o Number of static (m)routes needed
  
    In the most simple of cases (as shown above), the workaround solution
    requires one more configuration like, but if you actually have
    multiple sets of static mroutes to configure, the workaround also
    results in less configuration likes. Assume that you have the two possible
    <next-hop1> and <next-hop2> but a larger range of routes to pass across
    it, eg: <address1>/<mask1>,... <address20>/<mask20>. With recursive
    route resolution you effectively get 2 static routes and 20 static mroutes.
    Without route recursion, you would need to configure 40 static (m)routes.
    
  o Validation
    
    You may want to use "show ip rpf" to verify the correct configuration
    of your static routes/mroutes.

  o Background
    
    Recursive route resolution is quite standard when i(M)BGP is being used:
    The i(M)BGP route has a remote PE router as the next hop and route lookup
    recurses into the routing table to find an appropriate IGP routing table
    entry for this remote PE router address. This recursion in the Cisco
    IOS IPv4 unicast route lookup is not restricted to BGP->IGP recursion but 
    it works with all routes in the unicast routing table including static
    routes.  It is also not restricted to just one level of recursion. The
    Cisco IOS IPv4 multicast RPF lookup can also support recursion, although
    with some limitations not applicable to unicast: The first route examined
    in a multicast RPF lookup can be an MBGP route, static mroute, DVMRP route
    or a route from the unicast routing table (eg: IGPs, static routes and
    BGP).  If the result of this lookup renders a non-directly connected next
    hop, recursion will only look into the unicast routing table from there 
    on.  For example an mroute->mroute route recursion configuration is not
    supported.

  o Tunnels

    Please refer to the following configuration note for a description
    on how to apply this workaround to multiple GRE tunnel interfaces:

    ftp://ftpeng.cisco.com/ipmulticast/config-notes/gre-tunnel.txt
  
Should you have problems with this configuration, please send email
to multicast-support@cisco.com


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