Doubts regarding MPLS destination sharing and cef output

Hi everybody,

Im having a doubt regarding what Im 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 Im 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?, whats 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 Tue Aug 03 2010 - 16:00:38 ART