Cisco – OSPF NSSA load balancing

ciscoospfrouting

I have a OSPF network partitioned onto 2 areas as shown in the Figure below.

Area 4 is configured as a NSSA with no-summary which receives only default route. Routers R3, R4, R5 are redistributing the same IP address 100.0.0.1 (locally configured Loopback interface). My problem is that R2 installs 3 routes (OSPF NSSA external type 2) to the destinations and R1 installs only one (OSPF external type 2).

R2 routing table:

R2#show ip route 
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/2] via 10.0.12.1, 00:10:38, FastEthernet0/1
     2.0.0.0/32 is subnetted, 1 subnets
C       2.2.2.2 is directly connected, Loopback0
     100.0.0.0/32 is subnetted, 1 subnets
O N2    100.0.0.1 [110/20] via 10.0.99.5, 00:03:45, FastEthernet0/0
                  [110/20] via 10.0.99.4, 00:04:14, FastEthernet0/0
                  [110/20] via 10.0.99.3, 00:04:33, FastEthernet0/0
     3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/2] via 10.0.99.3, 00:11:17, FastEthernet0/0
     4.0.0.0/32 is subnetted, 1 subnets
O       4.4.4.4 [110/2] via 10.0.99.4, 00:11:17, FastEthernet0/0
     5.0.0.0/32 is subnetted, 1 subnets
O       5.5.5.5 [110/2] via 10.0.99.5, 00:11:18, FastEthernet0/0
     10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
C       10.0.12.0/30 is directly connected, FastEthernet0/1
C       10.0.27.0/30 is directly connected, FastEthernet1/0
O       10.0.16.0/30 [110/2] via 10.0.12.1, 00:10:40, FastEthernet0/1
O       10.0.67.0/30 [110/2] via 10.0.27.2, 00:11:28, FastEthernet1/0
C       10.0.99.0/24 is directly connected, FastEthernet0/0

R1 routing table:

R1#show ip route 
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     1.0.0.0/32 is subnetted, 1 subnets
C       1.1.1.1 is directly connected, Loopback0
     2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/2] via 10.0.12.2, 00:12:06, FastEthernet0/1
     100.0.0.0/32 is subnetted, 1 subnets
O E2    100.0.0.1 [110/20] via 10.0.99.5, 00:05:02, FastEthernet0/0
     3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/2] via 10.0.99.3, 00:12:06, FastEthernet0/0
     4.0.0.0/32 is subnetted, 1 subnets
O       4.4.4.4 [110/2] via 10.0.99.4, 00:12:06, FastEthernet0/0
     5.0.0.0/32 is subnetted, 1 subnets
O       5.5.5.5 [110/2] via 10.0.99.5, 00:12:07, FastEthernet0/0
     10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
C       10.0.12.0/30 is directly connected, FastEthernet0/1
O       10.0.27.0/30 [110/2] via 10.0.12.2, 00:12:07, FastEthernet0/1
C       10.0.16.0/30 is directly connected, FastEthernet1/0
O       10.0.67.0/30 [110/2] via 10.0.16.2, 00:12:07, FastEthernet1/0
C       10.0.99.0/24 is directly connected, FastEthernet0/0

I understand that one of the routers performs Type7/5 translation (in that case R2 as it has higher router-id) therefore it installs 3 all the routes.

My question is, is it possible to load balance the traffic so that all the 3 routes are installed on R1 as well? If not, would it be possible to remove the route to 100.0.0.1 from R1 so that the traffic goes via R2 only (PBR is not an option in my case).

Router configurations:

*********         R1        *************
!
interface Loopback0
 ip address 1.1.1.1 255.255.255.255
 ip ospf 1 area 0
!         
interface FastEthernet0/0
 ip address 10.0.99.1 255.255.255.0
 ip ospf 1 area 4
!         
interface FastEthernet0/1
 ip address 10.0.12.1 255.255.255.252
 ip ospf 1 area 0
!         
interface FastEthernet1/0
 ip address 10.0.16.1 255.255.255.252
 ip ospf 1 area 0     
!         
router ospf 1
 router-id 1.1.1.1
 log-adjacency-changes
 area 4 nssa no-summary
!
*********         R2        *************
!
interface Loopback0
 ip address 2.2.2.2 255.255.255.255
 ip ospf 1 area 0
!
interface FastEthernet0/0
 ip address 10.0.99.2 255.255.255.0
 ip ospf 1 area 4
!
interface FastEthernet0/1
 ip address 10.0.12.2 255.255.255.252
 ip ospf 1 area 0
!         
interface FastEthernet1/0
 ip address 10.0.27.1 255.255.255.252
 ip ospf 1 area 0
!
!
router ospf 1
 router-id 2.2.2.2
 log-adjacency-changes
 area 4 nssa no-summary
!
*********         R3        *************
!
interface Loopback0
 ip address 3.3.3.3 255.255.255.255
 ip ospf 1 area 4
!
interface Loopback1
 ip address 100.0.0.1 255.255.255.255
!
interface FastEthernet0/0
 ip address 10.0.99.3 255.255.255.0
 ip ospf 1 area 4
!
router ospf 1
 router-id 3.3.3.3
 log-adjacency-changes
 area 4 nssa no-summary
 redistribute connected subnets
!
*********         R4        *************
!
interface Loopback0
 ip address 4.4.4.4 255.255.255.255
 ip ospf 1 area 4
!
interface Loopback1
 ip address 100.0.0.1 255.255.255.255
!
interface FastEthernet0/0
 ip address 10.0.99.4 255.255.255.0
 ip ospf 1 area 4
!
router ospf 1
 router-id 4.4.4.4
 log-adjacency-changes
 area 4 nssa no-summary
 redistribute connected subnets
!
*********         R5        *************
!
interface Loopback0
 ip address 5.5.5.5 255.255.255.255
 ip ospf 1 area 4
!
interface Loopback1
 ip address 100.0.0.1 255.255.255.255
!
interface FastEthernet0/0
 ip address 10.0.99.5 255.255.255.0
 ip ospf 1 area 4
!
router ospf 1
 router-id 5.5.5.5
 log-adjacency-changes
 area 4 nssa no-summary
 redistribute connected subnets

show ip ospf database:

R1# show ip ospf database 

            OSPF Router with ID (1.1.1.1) (Process ID 1)

        Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         35          0x80000003 0x00A5F4 3
2.2.2.2         2.2.2.2         34          0x80000003 0x00E88E 3
10.0.67.1       10.0.67.1       39          0x80000003 0x00FAB3 2
10.0.67.2       10.0.67.2       40          0x80000003 0x00F79D 2

        Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
10.0.12.2       2.2.2.2         37          0x80000001 0x00AC64
10.0.16.2       10.0.67.1       41          0x80000001 0x00EE91
10.0.27.2       10.0.67.2       40          0x80000001 0x00ABC3
10.0.67.2       10.0.67.2       44          0x80000001 0x00748C

        Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
3.3.3.3         1.1.1.1         25          0x80000001 0x00F436
3.3.3.3         2.2.2.2         35          0x80000001 0x00D650
4.4.4.4         1.1.1.1         39          0x80000001 0x00C660
4.4.4.4         2.2.2.2         39          0x80000001 0x00A87A
5.5.5.5         1.1.1.1         39          0x80000001 0x00988A
5.5.5.5         2.2.2.2         39          0x80000001 0x007AA4
10.0.99.0       1.1.1.1         80          0x80000001 0x00AD1D
10.0.99.0       2.2.2.2         83          0x80000001 0x008F37

        Router Link States (Area 4)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         42          0x80000002 0x00CB74 1
2.2.2.2         2.2.2.2         43          0x80000002 0x008DA9 1
3.3.3.3         3.3.3.3         43          0x80000002 0x00D041 2
4.4.4.4         4.4.4.4         52          0x80000002 0x00E420 2
5.5.5.5         5.5.5.5         51          0x80000002 0x00F8FE 2

        Net Link States (Area 4)

Link ID         ADV Router      Age         Seq#       Checksum
10.0.99.5       5.5.5.5         43          0x80000002 0x00ACB7

        Summary Net Link States (Area 4)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         1.1.1.1         86          0x80000001 0x001B17
0.0.0.0         2.2.2.2         89          0x80000001 0x00FC31

        Type-7 AS External Link States (Area 4)

Link ID         ADV Router      Age         Seq#       Checksum Tag
100.0.0.1       3.3.3.3         86          0x80000001 0x006CAC 0
100.0.0.1       4.4.4.4         97          0x80000001 0x008090 0
100.0.0.1       5.5.5.5         94          0x80000001 0x009474 0

        Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
100.0.0.1       2.2.2.2         29          0x80000001 0x0051D1 0

R2#show ip ospf database 

            OSPF Router with ID (2.2.2.2) (Process ID 1)

        Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         72          0x80000003 0x00A5F4 3
2.2.2.2         2.2.2.2         69          0x80000003 0x00E88E 3
10.0.67.1       10.0.67.1       76          0x80000003 0x00FAB3 2
10.0.67.2       10.0.67.2       75          0x80000003 0x00F79D 2

        Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
10.0.12.2       2.2.2.2         72          0x80000001 0x00AC64
10.0.16.2       10.0.67.1       77          0x80000001 0x00EE91
10.0.27.2       10.0.67.2       75          0x80000001 0x00ABC3
10.0.67.2       10.0.67.2       80          0x80000001 0x00748C

        Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
3.3.3.3         1.1.1.1         62          0x80000001 0x00F436
3.3.3.3         2.2.2.2         70          0x80000001 0x00D650
4.4.4.4         1.1.1.1         75          0x80000001 0x00C660
4.4.4.4         2.2.2.2         73          0x80000001 0x00A87A
5.5.5.5         1.1.1.1         75          0x80000001 0x00988A
5.5.5.5         2.2.2.2         73          0x80000001 0x007AA4
10.0.99.0       1.1.1.1         117         0x80000001 0x00AD1D
10.0.99.0       2.2.2.2         115         0x80000001 0x008F37

        Router Link States (Area 4)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         79          0x80000002 0x00CB74 1
2.2.2.2         2.2.2.2         76          0x80000002 0x008DA9 1
3.3.3.3         3.3.3.3         78          0x80000002 0x00D041 2
4.4.4.4         4.4.4.4         87          0x80000002 0x00E420 2
5.5.5.5         5.5.5.5         87          0x80000002 0x00F8FE 2

        Net Link States (Area 4)

Link ID         ADV Router      Age         Seq#       Checksum
10.0.99.5       5.5.5.5         77          0x80000002 0x00ACB7

        Summary Net Link States (Area 4)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         1.1.1.1         122         0x80000001 0x001B17
0.0.0.0         2.2.2.2         121         0x80000001 0x00FC31

        Type-7 AS External Link States (Area 4)

Link ID         ADV Router      Age         Seq#       Checksum Tag
100.0.0.1       3.3.3.3         120         0x80000001 0x006CAC 0
100.0.0.1       4.4.4.4         131         0x80000001 0x008090 0
100.0.0.1       5.5.5.5         128         0x80000001 0x009474 0

        Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
100.0.0.1       2.2.2.2         63          0x80000001 0x0051D1 0

UPDATE

So I found partial solution to the problem. In case the load sharing is not possible I wanted to route the traffic through one of the routers (R1 or R2) only. This is possible if we suppress forwarding address on R2. This forces other routers in Area 0 to use R2 as a gateway to the 100.0.0.1 IP address.

Following command has to be added on R2.

router ospf 1
 area 4 nssa translate type7 suppress-fa

Best Answer

I've noticed one problem in your configuration. Your ASBRs (R3-5) and ABRs (R1-2) are configured with area 4 nssa no-summary According to configuration guide of NSSA area http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/6208-nssa.html you shoud issue no-summary only on ABRs.

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".

Cisco – How to Summarize LSA Type 7/NSSA ASBR External routes

I would suggest creating a static route to Null 0 on the external router:

ip route 192.168.0.0 255.255.248.0 Null 0

Static route to Null interface is seen as a connected interface on the local router.

Then advertise the summary address in whatever routing protocol is originating the routes with a network statement in the routing process.

When you redistribute from one routing process to the other the summary route should be injected into the OSPF process.

You could also filter out the subnets with a prefix list or route map to be sure the smaller subnets don't get injected.

Hopefully this helps.

Best Answer

Related Solutions

Cisco – load-balancing on congestion, OSPF over MPLS

How Cisco IOS load-balancing works by default

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

Load-balancing with MPLS TE

Cisco – How to Summarize LSA Type 7/NSSA ASBR External routes

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