Categories R&S

OSPF – Implement and troubleshoot route summarization – Internal summarization

Let’s move to route summarization.

First internal summarization.

 

Theory:

OSPF can summarize LSA type 3 on an ABR.

 

Requirements:

Use internal summarization so that R3 prefer to go through R1 when trying to reach R6 ip 172.10.1.17

 

Diagram:

Not-so-totally-stubby area

 

Configuration and verification:

For testing purpose we will create some new loopback interfaces on R4 (Lo5 IP add=172.10.1.1/29), R5 (Lo5 IP add=172.10.1.9/29) and R6 (Lo5 IP add=172.10.1.17/29), add them to OSPF area 2 using point-to-point network.

R4#sh run int lo5
interface Loopback5
 ip address 172.10.1.1 255.255.255.248
 ip ospf network point-to-point
 ip ospf 10 area 2

 

Let’s check the routing table on R1, R2 and R3 :

R1#sh ip route
      172.10.0.0/29 is subnetted, 3 subnets
O        172.10.1.0 [110/2] via 10.10.14.2, 00:03:38, Ethernet1/0
O        172.10.1.8 [110/1002] via 10.10.14.2, 00:03:33, Ethernet1/0
O        172.10.1.16 [110/1002] via 10.10.14.2, 00:03:33, Ethernet1/0

R2#sh ip route
      172.10.0.0/29 is subnetted, 3 subnets
O        172.10.1.0 [110/2] via 10.10.24.2, 00:00:16, Ethernet0/2
O        172.10.1.8 [110/1002] via 10.10.24.2, 00:00:16, Ethernet0/2
O        172.10.1.16 [110/1002] via 10.10.24.2, 00:00:16, Ethernet0/2

R3#sh ip route
      172.10.0.0/29 is subnetted, 3 subnets
O IA     172.10.1.0 [110/3] via 10.10.1.2, 00:00:02, Ethernet0/0
                    [110/3] via 10.10.1.1, 00:00:02, Ethernet0/0
O IA     172.10.1.8 [110/1003] via 10.10.1.2, 00:00:02, Ethernet0/0
                    [110/1003] via 10.10.1.1, 00:00:02, Ethernet0/0
O IA     172.10.1.16 [110/1003] via 10.10.1.2, 00:00:02, Ethernet0/0
                     [110/1003] via 10.10.1.1, 00:00:02, Ethernet0/0

 

Now on R1 and R2 we will use internal summarization, this will stop them to send specific LSA for the three routes and group them into a single route.

R1(config)#router ospf 10
R1(config-router)#area 2 range 172.10.1.0 255.255.255.192

R2(config)#router ospf 10
R2(config-router)#area 2 range 172.10.1.0 255.255.255.192

 

In R3 routing table, we see a summary route:

R3#sh ip route
      172.10.0.0/26 is subnetted, 1 subnets
O IA     172.10.1.0 [110/3] via 10.10.1.2, 00:00:02, Ethernet0/0
                    [110/3] via 10.10.1.1, 00:00:02, Ethernet0/0

R3#        sh ip route 172.10.1.17
Routing entry for 172.10.1.0/26
  Known via "ospf 10", distance 110, metric 3, type inter area
  Last update from 10.10.1.2 on Ethernet0/0, 00:00:08 ago
  Routing Descriptor Blocks:
    10.10.1.2, from 11.11.11.11, 00:00:08 ago, via Ethernet0/0
      Route metric is 3, traffic share count is 1
  * 10.10.1.1, from 11.11.11.11, 00:00:08 ago, via Ethernet0/0
      Route metric is 3, traffic share count is 1

 

In the OSPF database, we can see that R1 and R2 advertise the route with a /26 subnet mask.

R3#          sh ip ospf data sum 172.10.1.0
            OSPF Router with ID (33.33.33.33) (Process ID 10)
                Summary Net Link States (Area 0)
  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 66
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 172.10.1.0 (summary Network Number)
  Advertising Router: 11.11.11.11
  LS Seq Number: 80000001
  Checksum: 0x8E08
  Length: 28
  Network Mask: /26
        MTID: 0         Metric: 2

  LS age: 1038
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 172.10.1.0 (summary Network Number)
  Advertising Router: 22.22.22.22
  LS Seq Number: 80000001
  Checksum: 0x4327
  Length: 28
  Network Mask: /26
        MTID: 0         Metric: 2

 

Traceroute is showing traffic going to R1 and R2 to reach R6 loopback:

R3#traceroute 172.10.1.17 probe 5
Type escape sequence to abort.
Tracing the route to 172.10.1.17
VRF info: (vrf in name/id, vrf out name/id)
  1 10.10.1.1 1 msec
    10.10.1.2 1 msec
    10.10.1.1 5 msec
    10.10.1.2 4 msec
    10.10.1.1 1 msec
  2 10.10.24.2 3 msec
    10.10.14.2 1 msec
    10.10.24.2 0 msec
    10.10.14.2 1 msec
    10.10.24.2 0 msec
  3 10.10.2.6 1 msec *  1 msec *  1 msec

 

Now, we can use internal summarization to influence the traffic path.

Because the longest match is preferred we can change the summarization on R1 so it gets preferred over R2.

R1(config)#router ospf 10
R1(config-router)#no area 2 range 172.10.1.0 255.255.255.192
R1(config-router)#area 2 range 172.10.1.0 255.255.255.224
R1#clear ip ospf proc
Reset ALL OSPF processes? [no]: yes

 

Let’s check on R3:

R3#sh ip route
      172.10.0.0/16 is variably subnetted, 2 subnets, 2 masks
O IA     172.10.1.0/26 [110/3] via 10.10.1.2, 00:00:53, Ethernet0/0
O IA     172.10.1.0/27 [110/3] via 10.10.1.1, 00:00:12, Ethernet0/0

R3#        sh ip route 172.10.1.17
Routing entry for 172.10.1.0/27
  Known via "ospf 10", distance 110, metric 3, type inter area
  Last update from 10.10.1.1 on Ethernet0/0, 00:00:06 ago
  Routing Descriptor Blocks:
  * 10.10.1.1, from 11.11.11.11, 00:00:06 ago, via Ethernet0/0
      Route metric is 3, traffic share count is 1

 

Only one route to 172.10.1.17 is now seen, traceroute show traffic is only going via R1:

R3#traceroute 172.10.1.17 probe 5

Type escape sequence to abort.
Tracing the route to 172.10.1.17
VRF info: (vrf in name/id, vrf out name/id)
  1 10.10.1.1 5 msec 0 msec 5 msec 0 msec 5 msec
  2 10.10.14.2 1 msec 0 msec 1 msec 1 msec 0 msec
  3 10.10.2.6 0 msec *  1 msec *  1 msec

 

In the next post, we will have a look on external summarization.

 

 

Thank you for reading.

 

OSPF – Implement and troubleshoot route summarization – Internal summarization