Categories R&S

OSPF – Implement and troubleshoot route summarization – External summarization

Let’s now have a look at external summarization.

 

Theory:

This is specific to external routes redistributed in OSPF.

We need to implement this on the router that is doing the redistribution.

 

Requirements:

Create three new loopbacks on R5 (Lo 10,11,12) and R6 (Lo 10,11,12).

Make sure those new IP addresses are summarize in the most effective way.

 

Diagram:

Not-so-totally-stubby area

 

Configuration and verification:

So we create those three new loopbacks on R5 and R6:

R5(config)#int lo 10
R5(config-if)#ip add 172.20.1.1 255.255.255.248
R5(config-if)#int lo11
R5(config-if)#ip add 172.20.1.9 255.255.255.248
R5(config-if)#int lo 12
R5(config-if)#ip add 172.20.1.17 255.255.255.248

R6(config)#int lo 10
R6(config-if)#ip add 172.20.2.1 255.255.255.248
R6(config-if)#int lo11
R6(config-if)#ip add 172.20.2.9 255.255.255.248
R6(config-if)#int lo 12
R6(config-if)#ip add 172.20.2.17 255.255.255.248

 

Let’s see how the routes are seen on R3:

R3#sh ip route
O E2     172.20.1.0 [110/20] via 10.10.1.2, 00:00:25, Ethernet0/0
                    [110/20] via 10.10.1.1, 00:00:25, Ethernet0/0
O E2     172.20.1.8 [110/20] via 10.10.1.2, 00:00:25, Ethernet0/0
                    [110/20] via 10.10.1.1, 00:00:25, Ethernet0/0
O E2     172.20.1.16 [110/20] via 10.10.1.2, 00:00:25, Ethernet0/0
                     [110/20] via 10.10.1.1, 00:00:25, Ethernet0/0
O E2     172.20.2.0 [110/20] via 10.10.1.2, 00:00:17, Ethernet0/0
                    [110/20] via 10.10.1.1, 00:00:17, Ethernet0/0
O E2     172.20.2.8 [110/20] via 10.10.1.2, 00:00:17, Ethernet0/0
                    [110/20] via 10.10.1.1, 00:00:17, Ethernet0/0
O E2     172.20.2.16 [110/20] via 10.10.1.2, 00:00:17, Ethernet0/0
                     [110/20] via 10.10.1.1, 00:00:17, Ethernet0/0

We see two routes for each, one via R1 and the second one via R2.

 

Now we can simplify the routes seen with external summarization, the goal is to keep the number of routes in the routing table as low as possible.

We can start by the loopback from R5. Remember, external summarization take place on the device where the redistribution occurs:

R5(config)#router ospf 10
R5(config-router)#summary-address 172.20.1.0 255.255.255.224

 

On R3, we now have only one route to the three loopback of R5:

R3#sh ip route
O E2     172.20.1.0/27 [110/20] via 10.10.1.2, 00:00:05, Ethernet0/0
                       [110/20] via 10.10.1.1, 00:00:05, Ethernet0/0

 

In the OSPF external database, we have one /27 entry:

R3#sh ip ospf data external 172.20.1.0
            OSPF Router with ID (33.33.33.33) (Process ID 10)
                Type-5 AS External Link States
  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 82
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 172.20.1.0 (External Network Number )
  Advertising Router: 172.10.1.9
  LS Seq Number: 80000004
  Checksum: 0xC673
  Length: 36
  Network Mask: /27
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

 

We do the same for R6, but let’s try another subnet mask:

R6(config)#router ospf 10
R6(config-router)#summary-address 172.20.2.0 255.255.255.0

 

What do we see now on R3:

R3#sh ip route
O E2     172.20.2.0/24 [110/20] via 10.10.1.2, 00:00:08, Ethernet0/0
                       [110/20] via 10.10.1.1, 00:00:08, Ethernet0/0

R3#        sh ip ospf data ext 172.20.2.0
            OSPF Router with ID (33.33.33.33) (Process ID 10)
                Type-5 AS External Link States
  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 69
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 172.20.2.0 (External Network Number )
  Advertising Router: 172.10.1.17
  LS Seq Number: 80000003
  Checksum: 0x48CA
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

 

So that’s an easy implementation, one single command on the ASBR to group multiple route and reduce the size of the routing table.

 

Now on both R5 and R6 routing table a route pointing to null0 as been created:

R5#      sh ip route | i Null0
•	172.20.1.0/27 is a summary, 00:27:53, Null0

R6#       sh ip route | i Null0
•	172.20.2.0/24 is a summary, 00:08:26, Null0

This discard is automatically created to prevent routing loops to occur

 

 

We will now move on to OSPF filtering.

 

 

Thank you for reading.

 

OSPF – Implement and troubleshoot route summarization – External summarization