OSPF Route Summarization

• Route summarization allows only consolidated routes to propagate into the backbone area (area 0)
  • Without it every specific-link LSA is propagated into the OSPF backbone and beyond.
• Type 3 Summary LSAs and Type 5 External LSAs are not summarized by default.

Two types of summarization:

• Inter-area route summarization
  • Occurs on ABRs and applies to routes from within each area.
  • Does not apply to external routes redistributed into OSPF
  • For efficient effect, network numbers within the area should be contiguous to create the least number of summarized addresses.
• External route summarization
  • Occurs on ASBRs
  • Applies to external routes redistributed into OSPF.
  • It is again important to ensure that summarized external addresses are contiguous to avoid problems with overlapping subnet ranges.
    • OSPF is a classless routing protocol, which means subnet mask information is carried along the route advertisement. RIPv1, however, could cause problems when OSPF is redistributed into that domain. VLSM information that is redistributed into RIPv1 is lost and static routes have to be configured in the RIPv1 domain.

Configure OSPF Route Summarization on an ABR

• Use the router configuration command:

area area-id range address-mask [advertise | not-advertise] [cost cost]

• Following is a description of the parameters:
  • area-id
    • Identifies the area whose networks is to be summarized

• • address
    • The summary address of the range of network addresses
  • mask
    • The subnet mask for the summary route
  • advertise
    • Optional parameter which tells the router to advertise the range and generate a type 3 summary LSA
  • not-advertise
    • Optional setting that suppresses type 3 summary LSA and hide component networks from other networks
  • cost
    • Optional setting that sets the metric or cost for this summary route. This cost is used by OSPF to determine the shortest path.
    • The value is anywhere between 0 to 16777215

• Example:

R1(config)#router ospf 100
R1(config-router)#network 172.16.32.1 0.0.0.0 area 1
R1(config-router)#network 172.16.96.1 0.0.0.0 area 0
R1(config-router)#area 0 range 172.16.32.0 255.255.224.0
R1(config-router)#area 1 range 172.16.96.0 255.255.224.0

• area 0 range 172.16.32.0 255.255.224.0 - this command summarizes the network 172.16.32.0 from area 0 into area 1. The networks 172.16.32.0 – 172.16.63.0 is summarized into 172.16.32.0/19 by the ABR R1.
• area 1 range 172.16.96.0 255.255.224.0 – this command summarizes the network 172.16.96.0 from area 1 into area 0. The networks 172.16.96.0 – 172.16.127.0 is summarized into 172.16.96.0/19 by the ABR R1.

Configuring OSPF Route Summarization on an ASBR

• Use the router configuration command:

summary-address ip-address mask [not-advertise] [tag tag]

• Following is the description of the parameters:

• • ip-address
    • The summary address designated for a range of address
  • mask
    • The subnet mask used for the summary route
  • not-advertise
    • Optional setting used to suppress routes that match the address/mask pair.
  • tag tag
    • Optional value that can be used as a “match” value to control redistribution via route maps.
• Example

R1(config)#router ospf 100
R1(config-router)#network 172.16.64.1 0.0.0.0 area 1
R1(config-router)#summary-address 172.16.32.0 255.255.224.0

• In this configuration, instead of flooding 32 external type 5 LSAs (subnets 172.16.32.0 – 172.16.63.0) into OSPF, only 1 summarized LSA type 5 LSA is flooded.

OSPF Default Route

• Default Routes injected into a normal area can be originated by any OSPF router.
  • However, by default, the OSPF router does not generate a default route into the OSPF domain.
  • default-information originate: used by OSPF router to generate a default route.
• Two ways to advertise a default route into a normal area:
  1. Advertise 0.0.0.0 into the OSPF domain, when the advertising router already has a default route.
  2. Advertise 0.0.0.0 into the OSPF regardless of whether the advertising router already has a default route.
     • This second method can be accomplished by adding the keyword always to the default-information originate command.

default-information orginate Router Command

default-information originate [always] [metric metric-value] [metric-type type-value] [route-map map-name]

• The parameters are:
  • always - (Optional) Specifies that OSPF always advertises the default route regardless of whether the router has a default route in the routing table.
  • metric metric-value – (Optional) Metric for generating a default route. If value is omitted and the default-metric router configuration is not configured, the default metric value is 1. *Note that Cisco documentations indicate that the default metric value is 10. But testing shows that it is 1.

• Example

R1 Configuration:

R1(config)#router ospf 100
R1(config-router)#network 10.1.1.1 0.0.0.0 area 0
R1(config-router)#default-information originate metric 10
!
R1(config)#ip route 0.0.0.0 0.0.0.0 198.1.1.2

R2 Configuration:

R2(config)#router ospf 100
R2(config-router)#network 10.2.1.1 0.0.0.0 area 0
R2(config-router)#default-information originate metric 100
!
R2(config)#ip route 0.0.0.0 0.0.0.0 198.2.1.2

• The optional metric parameter is used on R1 to prefer the default route to ISP A.
• The default route generated has a metric-type of E2 by default.
  • Because of this, the metric remains the same as it travels through the area. As a result, all routers prefer ISP A over ISP B.
• The default-information originate command causes the router to send a default route to all its OSPF neighbors.
  • Notice in the configuration of R1 and R2, the network command does not include the connection to the ISP routers. Therefore, a default route is not passed to the ISP routers.

Resources

1. OSPF Design Guide – OSPF and Route Summarization
2. Chris Bryant – Route Summarization and the OSPF Null Interface
3. How OSPF Injects a Default Route into a Normal Area

This entry is not an authoritative guide. These are merely notes and rehash of the primary text materials and resources that I use. For a thorough guide of the BSCI course, consider purchasing Building Scalable Cisco Internetworks (BSCI) (Authorized Self-Study Guide) (3rd Edition) by Diane Teare and Catherine Paquet, as well as following the links on the resources section of this entry.