4

I have a network of routers with two clients at both ends, as shown in the Figure below. All the routers use OSPF routing protocol over IP, and the three routers inside the "MPLS backbone" subnetwork use OSPF over MPLS over IP, and is supposed to load-balance the traffic from jperf-c to jperf-s. The clients are configured for normal ip routing (just a default gateway to the nearest router).

OSPF over MPLS network

I know that in practice, the load balancing is useless in this case. But this is for an exercise, not a real network. The load balancing works when I replace the MPLS by ATM switches. However with MPLS routers, all the TCP and UDP traffic goes through R3 and R5, even if there is congestion, and as a consequence R4 and R6 are never used. The weirdest thing is that when I traceroute R7 from R1, the packet go through R4 and R6.

How do I load-balance the traffic when risks of congestion occur? I have read that OSPF is supposed to do it by default, but in my case it does not?

All the links have a cost of 64 and my configuration for router 2, supposed to redirect to R2 and R4 by load-balancing, is:

config t
!
ip cef
!
interface Serial2/0
 ip address 192.168.0.2 255.255.255.252
 no shutdown
!
interface Serial2/1
 ip address 192.168.0.5 255.255.255.252
 no shutdown
 mpls label protocol ldp
 mpls ip
 mpls mtu 1512
 ip route-cache cef
!
interface Serial2/2
 ip address 192.168.0.9 255.255.255.252
 no shutdown
 mpls label protocol ldp
 mpls ip
 mpls mtu 1512
 ip route-cache cef
!
router ospf 1
 network 192.168.0.0 0.0.0.3 area 0
 network 192.168.0.4 0.0.0.3 area 0
 network 192.168.0.8 0.0.0.3 area 0
11

... The load balancing works when I replace the MPLS by ATM switches.

This doesn't sound like a parallel comparison, because routers don't care whether you're forwarding through an ATM PVC or an MPLS LSP, they will load-balance in the same way, assuming the routers have the same configuration. Perhaps you've done something unusual with your ATM VCs, but that isn't the focus of this question.

However with MPLS routers, all the TCP and UDP traffic goes through R3 and R5, even if there is congestion, and as a consequence R4 and R6 are never used. The weirest thing is that when I traceroute R7 from R1, the packet go through R4 and R6.

This actually is not weird, this is how routing works. Routing happens on a per-hop basis; routers have no forward-looking wisdom to predict congestion along subsequent hops of a particular path. Routing protocols don't carry information about load in their metric calculations (except for EIGRP if you play games with the K values, but that could result in path instability when you do that).

That said, you can use a different algorithm and force load-balancing in this case, use ip load-sharing per-packet on R2 (serial2/1 & serial 2/2) as well as R7 (serial 2/0 & 2/1):

interface Serial2/1
 ip address 192.168.0.5 255.255.255.252
 no shutdown
 mpls label protocol ldp
 mpls ip
 mpls mtu 1512
 ip route-cache cef
 ip load-sharing per-packet
!
interface Serial2/2
 ip address 192.168.0.9 255.255.255.252
 no shutdown
 mpls label protocol ldp
 mpls ip
 mpls mtu 1512
 ip route-cache cef
 ip load-sharing per-packet

Per-packet load-balancing forces the router to round-robin traffic between destinations. You should never use this is a production network without considering the consequences (most often, reordering packets... which makes VoIP / video, and sometimes TCP unhappy).

Per-packet load-balancing is great in the lab, and usually unwise in production, unless you're in a very specific situation and know what you're doing. Per-packet load-balancing also has very spotty platform support... some Cisco routers (such as the Catalyst 6500 Sup720) simply won't use per-packet load-balancing.

How do I do for the MPLS backbone to load-balance the traffic when risks of congestion occur? I have read that OSPF is supposed to do it by default, but in my case it does not?

As I mentioned above, you're misunderstanding how routing works... this is a huge topic, but I'll just summarize the most relevant info here...

How Cisco IOS load-balancing works by default

By default Cisco routers use per-source / per-destination hashes of IP addresses at the ingress Label Edge Router (LER) to decide which path to take. You can see this in action by performing show ip cef exact-route [src-ip] [dst-ip]... notice the label [FOO] TAG adj output below, which indicates the MPLS label allocated by LDP.

PE4#sh ip cef exact-route 192.168.4.1 192.168.0.5

192.168.4.1 -> 192.168.0.5 => label 154 TAG adj out of GigabitEthernet1/0/0, 
 addr 10.10.1.189

If I choose a different source or destination address, it's possible to see a different path chosen...

PE4#sh ip cef exact-route 192.168.4.1 192.168.0.12

192.168.4.1 -> 192.168.0.12 => label 152 TAG adj out of GigabitEthernet1/0/1, 
 addr 10.10.1.193

If you want to see the general load-balancing configuration on the router, use show ip cef [dst] internal (borrowing output from this blog)... also note the chosen load-balance algorithm (per-destination sharing).

R2#show ip cef 10.200.4.1 internal 
10.200.4.1/32, epoch 0, flags rib only nolabel, rib defined all labels, RIB[B], refcount 5, 
 per-destination sharing
  sources: RIB 
  feature space:
   IPRM: 0x00018000
  ifnums:
   FastEthernet1/1(5): 192.168.23.3
   FastEthernet2/0(6): 172.16.23.3
  path 6896722C, path list 68991DF0, share 1/1, type recursive, for IPv4
  recursive via 10.4.4.4[IPv4:Default], fib 6894F9B0, 1 terminal fib, v4:Default:10.4.4.4/32
    path 689672A4, path list 68991E3C, share 0/1, type attached nexthop, for IPv4
      MPLS short path extensions: MOI flags = 0x0 label 19
    nexthop 172.16.23.3 FastEthernet2/0 label 19, adjacency IP adj out of FastEthernet2/0, 
     addr 172.16.23.3 686FC0A0
    path 6896731C, path list 68991E3C, share 1/1, type attached nexthop, for IPv4
      MPLS short path extensions: MOI flags = 0x0 label 19
    nexthop 192.168.23.3 FastEthernet1/1 label 19, adjacency IP adj out of FastEthernet1/1, 
     addr 192.168.23.3 686FC360
  output chain:
    loadinfo 689C4DA0, per-session, 2 choices, flags 0003, 6 locks
    flags: Per-session, for-rx-IPv4
    16 hash buckets
      < 0 > label 19 TAG adj out of FastEthernet2/0, addr 172.16.23.3 686FBF40
      < 1 > label 19 TAG adj out of FastEthernet1/1, addr 192.168.23.3 686FC200
      < 2 > label 19 TAG adj out of FastEthernet2/0, addr 172.16.23.3 686FBF40
      < 3 > label 19 TAG adj out of FastEthernet1/1, addr 192.168.23.3 686FC200
      < 4 > label 19 TAG adj out of FastEthernet2/0, addr 172.16.23.3 686FBF40
      < 5 > label 19 TAG adj out of FastEthernet1/1, addr 192.168.23.3 686FC200
      < 6 > label 19 TAG adj out of FastEthernet2/0, addr 172.16.23.3 686FBF40
      < 7 > label 19 TAG adj out of FastEthernet1/1, addr 192.168.23.3 686FC200
      < 8 > label 19 TAG adj out of FastEthernet2/0, addr 172.16.23.3 686FBF40
      < 9 > label 19 TAG adj out of FastEthernet1/1, addr 192.168.23.3 686FC200
      <10 > label 19 TAG adj out of FastEthernet2/0, addr 172.16.23.3 686FBF40
      <11 > label 19 TAG adj out of FastEthernet1/1, addr 192.168.23.3 686FC200
      <12 > label 19 TAG adj out of FastEthernet2/0, addr 172.16.23.3 686FBF40
      <13 > label 19 TAG adj out of FastEthernet1/1, addr 192.168.23.3 686FC200
      <14 > label 19 TAG adj out of FastEthernet2/0, addr 172.16.23.3 686FBF40
      <15 > label 19 TAG adj out of FastEthernet1/1, addr 192.168.23.3 686FC200
    Subblocks:
     None

Cisco IOS with CEF Load-balancing, hashed on L4 ports...

In IOS 12.4, Cisco introduced another algorithm to load-balance traffic based on L4 ports... use ip cef load-sharing algorithm include-ports source destination at the global configuration level.

Load-balancing with MPLS TE

MPLS TE introduces yet another twist into load-balancing, including the potential for offline optimization of paths; typically you buy an application to perform offline load optimization among various MPLS TE tunnels in the network. This too, is a very deep subject... I mention it only because you're asking about load-balancing with MPLS "automatically".

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