[c-nsp] load sharing Vs load balancing - MPLS VPN Topology consideration

[Oliver Boehmer (oboehmer)]

Hi David,

> 
> Thanks for the reply . Really usefull as usually . There is
> just one last thing i don't understand . I have made some
> testing in my lab here . As you can see below my PE router is
> now able to install 2 routes to the same destination in his
> Vrf routing table ..That is fine .
> 
> R2#sh ip bgp v v test 192.168.10.0
> BGP routing table entry for 2:551012:192.168.10.0/24, version 68
> Paths: (2 available, best #2, table test)
> Multipath: iBGP
> Flag: 0x820
>   Advertised to update-groups:
>      2
>   65000, imported path from 2:551011:192.168.10.0/24
>     10.152.65.3 (metric 2297856) from 10.152.65.3 (10.152.65.3)
>       Origin IGP, metric 0, localpref 100, valid, internal, multipath
>       Extended Community: RT:551:0
>       mpls labels in/out nolabel/21
>   65000, imported path from 2:551013:192.168.10.0/24
>     10.152.65.1 (metric 2297856) from 10.152.65.1 (10.152.65.1)
>       Origin IGP, metric 0, localpref 100, valid, internal,
>       multipath, best Extended Community: RT:551:0
>       mpls labels in/out nolabel/21
> R2#
> 
> 
> R2#sh ip route vrf test 192.168.10.0
> Routing entry for 192.168.10.0/24
>   Known via "bgp 10", distance 200, metric 0
>   Tag 65000, type internal
>   Last update from 10.152.65.3 00:00:06 ago
>   Routing Descriptor Blocks:
>     10.152.65.3 (Default-IP-Routing-Table), from 10.152.65.3,
>       00:00:06 ago Route metric is 0, traffic share count is 1
>       AS Hops 1
>       Route tag 65000
>   * 10.152.65.1 (Default-IP-Routing-Table), from 10.152.65.1,
>       00:03:23 ago Route metric is 0, traffic share count is 1
>       AS Hops 1
>       Route tag 65000
> 
> Now Cisco clearly state the following :
> "Even if the criteria are met and multiple paths are
> considered multipaths, a BGP speaking router will still
> designate one of the multipaths as the best path and
> advertise this best path to its neighbors."
> 
> Well this is exactly what is happening here . As you can see
> in my output the route to 192.168.10.0/24 is prefer via Next
> hop 10.152.65.1.
> So the question is if only ONE of the path is still
> designated as prefer , how is the router going to load-share
> between the two ?

Well, as you can see (by looking at "show ip route"), you have two paths
in the RIB, so traffic to 192.168.10.0/24 will be load-balanced. 
The fact that the PE only choses one path to send it to the CE doesn't
change this fact, does it? ;-) The CE will see one path with the
next-hop being the PE, so it forwards the packets to the PE where they
will be load-balanced just fine..

	oli

	oli