Re: Doubts regarding MPLS destination sharing and cef output

Hi,

show ip cef 6.6.6.0
6.6.6.0/24, version 32, epoch 0, per-destination sharing
0 packets, 0 bytes
 tag information from 66.66.66.66/32, shared
   local tag: 22
 via 66.66.66.66, 0 dependencies, recursive
   next hop 10.10.13.3, FastEthernet0/0 via 66.66.66.66/32
   valid adjacency
   tag rewrite with Fa0/1, 10.10.31.3, tags imposed: {21}
 Recursive load sharing using 66.66.66.66/32.

Here 'Recursive load sharing ...' indicates that iBGP next hop (66.66.66.66)
is load balanced and it requires further lookups during forwarding time.

It may show only one outbound interface ( I think the first one), but for
load sharing it still uses 2 interfaces.

Thanks,
Srinivas

On Wed, Aug 11, 2010 at 4:03 PM, Jomi <doctorfleming_at_gmail.com> wrote:

> Hi Daniel,
>
> Yeah, it seems a little weird. Have you got any news??
> Anyone could give an explanation about this behaviour??
>
>
> On Tue, Aug 3, 2010 at 4:00 PM, Daniel Rodriguez <
> daniel.rodriguez.lists_at_gmail.com> wrote:
>
> > Hi everybody,
> >
> > I m having a doubt regarding what I m seen on the show mpls / route
> > tables and what actually cef is showing up to me in a simple lab.
> >
> > I have a something like this topology:
> >
> > R6 = R5 = R2 = R3 = R4
> >
> > All connected using FastEthernet interfaces (two links between them).
> >
> > between R6 and R4 there is a iBGP neighbor relationship build up, and
> > a redistribute connected so a loopback addres (/24) gets into the BGP
> > table.
> >
> > From R4:
> >
> > show ip bgp 6.6.6.0
> > BGP routing table entry for 6.6.6.0/24, version 9
> > Paths: (1 available, best #1, table Default-IP-Routing-Table)
> > Not advertised to any peer
> > Local
> > 66.66.66.66 (metric 23) from 66.66.66.66 (66.66.66.66)
> > Origin incomplete, metric 0, localpref 100, valid, internal, best
> >
> > That looks good.
> >
> > show mpls for 6.6.6.0
> > Local Outgoing Prefix Bytes tag Outgoing Next Hop
> > tag tag or VC or Tunnel Id switched interface
> > 22 21 6.6.6.0/24 0 Fa0/1 10.10.31.3
> > 21 6.6.6.0/24 0 Fa0/0 10.10.13.3
> >
> > show mpls for 6.6.6.0 deta
> > Local Outgoing Prefix Bytes tag Outgoing Next Hop
> > tag tag or VC or Tunnel Id switched interface
> > 22 21 6.6.6.0/24 0 Fa0/1 10.10.31.3
> > MAC/Encaps=14/18, MRU=1500, Tag Stack{21}
> > C20115640020C202156400018847 00015000
> > No output feature configured
> > Per-destination load-sharing, slots: 0 2 4 6 8 10 12 14
> > 21 6.6.6.0/24 0 Fa0/0 10.10.13.3
> > MAC/Encaps=14/18, MRU=1500, Tag Stack{21}
> > C20115640010C202156400008847 00015000
> > No output feature configured
> > Per-destination load-sharing, slots: 1 3 5 7 9 11 13 15
> >
> > We have per-destination load-sharing with the reference to the slots
> > used. That looks fine to me.
> >
> > But here we come to the doubts I m having.
> >
> > The cef output only reference one of the interfaces as a tag rewrite
> > (in this case Fa0/1):
> >
> > show ip cef 6.6.6.0
> > 6.6.6.0/24, version 32, epoch 0, per-destination sharing
> > 0 packets, 0 bytes
> > tag information from 66.66.66.66/32, shared
> > local tag: 22
> > via 66.66.66.66, 0 dependencies, recursive
> > next hop 10.10.13.3, FastEthernet0/0 via 66.66.66.66/32
> > valid adjacency
> > tag rewrite with Fa0/1, 10.10.31.3, tags imposed: {21}
> > Recursive load sharing using 66.66.66.66/32.
> >
> > If I do recursive lookup on the 66.66.66.66 (destination router-id)
> > address I see that it actually have a reference to both outbound
> > interfaces (with tags imposed)
> >
> > show ip cef 66.66.66.66
> > 66.66.66.66/32, version 31, epoch 0, per-destination sharing
> > 0 packets, 0 bytes
> > tag information set, shared
> > local tag: 22
> > via 10.10.31.3, FastEthernet0/1, 0 dependencies
> > traffic share 1
> > next hop 10.10.31.3, FastEthernet0/1
> > valid adjacency
> > tag rewrite with Fa0/1, 10.10.31.3, tags imposed: {21}
> > via 10.10.13.3, FastEthernet0/0, 1 dependency
> > traffic share 1
> > next hop 10.10.13.3, FastEthernet0/0
> > valid adjacency
> > tag rewrite with Fa0/0, 10.10.13.3, tags imposed: {21}
> > 0 packets, 0 bytes switched through the prefix
> > tmstats: external 0 packets, 0 bytes
> > internal 0 packets, 0 bytes
> >
> > If I take a look at the internal information, it shows:
> >
> > show ip cef 6.6.6.0 inter
> > 6.6.6.0/24, version 32, epoch 0, per-destination sharing
> > 0 packets, 0 bytes
> > tag information from 66.66.66.66/32, shared
> > local tag: 22
> > via 66.66.66.66, 0 dependencies, recursive
> > next hop 10.10.13.3, FastEthernet0/0 via 66.66.66.66/32
> > valid adjacency
> > tag rewrite with Fa0/1, 10.10.31.3, tags imposed: {21}
> >
> > Recursive load sharing using 66.66.66.66/32
> > Load distribution: 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 (refcount 2)
> >
> > Hash OK Interface Address Packets Tags
> imposed
> > 1 Y FastEthernet0/1 10.10.31.3 0 {21}
> > 2 Y FastEthernet0/0 10.10.13.3 0 {21}
> > 3 Y FastEthernet0/1 10.10.31.3 0 {21}
> > 4 Y FastEthernet0/0 10.10.13.3 0 {21}
> > 5 Y FastEthernet0/1 10.10.31.3 0 {21}
> > 6 Y FastEthernet0/0 10.10.13.3 0 {21}
> > 7 Y FastEthernet0/1 10.10.31.3 0 {21}
> > 8 Y FastEthernet0/0 10.10.13.3 0 {21}
> > 9 Y FastEthernet0/1 10.10.31.3 0 {21}
> > 10 Y FastEthernet0/0 10.10.13.3 0 {21}
> > 11 Y FastEthernet0/1 10.10.31.3 0 {21}
> > 12 Y FastEthernet0/0 10.10.13.3 0 {21}
> > 13 Y FastEthernet0/1 10.10.31.3 0 {21}
> > 14 Y FastEthernet0/0 10.10.13.3 0 {21}
> > 15 Y FastEthernet0/1 10.10.31.3 0 {21}
> > 16 Y FastEthernet0/0 10.10.13.3 0 {21}
> >
> > I see that there is actually a reference to both interfaces with the
> > correct tags imposed.
> >
> > I understand that for the prefix 6.6.6.0, cef is actually doing a
> > recursive lookup on the destination router-id (66.66.66.66) and is
> > using that adjacency information with both outbound interfaces to send
> > packets:
> >
> > Router#show ip cef exact-route 44.44.44.44 6.6.6.2
> > 44.44.44.44 -> 6.6.6.2 : FastEthernet0/1 (next hop 10.10.31.3)
> > Router#show ip cef exact-route 44.44.44.44 6.6.6.3
> > 44.44.44.44 -> 6.6.6.3 : FastEthernet0/0 (next hop 10.10.13.3)
> >
> > But why in the show ip cef I only get one outbound rewrite interface:
> >
> > via 66.66.66.66, 0 dependencies, recursive
> > next hop 10.10.13.3, FastEthernet0/0 via 66.66.66.66/32
> > valid adjacency
> > tag rewrite with Fa0/1, 10.10.31.3, tags imposed: {21}
> >
> > Why only one?, why that one?, what s the criteria used?
> >
> > If any one can give me a light on this it would be very nice.
> >
> > Best Regards,
> >
> >
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Received on Wed Aug 11 2010 - 17:31:05 ART