OSPF stub notes
Posted by Peter Kurdziel on June 12, 2009
Stub area’s block type 5 LSA’s making type 4 LSA’s unnecessary; these LSA’s are also blocked. ref: Routing TCP/IP vol I second editon, page 386.
Totally Stubby also block type 3 lsa’s.
NSSA – allows external routes to be advertised into OSPF while retaining the characteristics of a stub area.
ASBR’s are not allowed in stub area’s. They are allowed in NSSA areas.
stub – blocks 4,5
stub no-summ blocks 3,4,5
nssa – blocks 4,5 – originates type 7 LSA’s to advertise to external destinations. An ABR will block type 7 LSA’s ( will not advertise outside of the area) UNLESS the the P-bit is set to 1 = nssa translate type7 – YOU NEED TO ADD default-info originate for a default route.
nssa no-sum = blocks 3,4,5
With a stub area, no external routes can be propagated into it. You could use a stub area, for example, for a branch office that has no connection other than to head office. In that case, you might make it totally stubby, that is, give it only a default route.
An NSSA is a bit like a stub as far as the internal OSPF topology is concerned, but it is allowed to connect to the outside world. In other words, it is allowed to have an ASBR border router. Imagine you had a branch office that had an external link, say, to the Internet. You could run that as an NSSA, but not as a stub.
| allowed LSA’a | ||||||
| area type | 1&2 | 3 | 4 | 5 | 7 | |
| Backdone -Area 0 | Yes | Yes | Yes | Yes | No | |
| Non-Backdone, non-stub | Yes | Yes | Yes | Yes | No | |
| Stub | Yes | Yes | No | No | No | |
| Totally Stubby | Yes | No*& | No | No | No | |
| Not so stubby | Yes | Yes | No | No | Yes | |
| *single type3 per ABR, advertising a default route. | ||||||
More info on nssa translate type7
The following example causes OSPF to translate Type-7 LSAs from area 1 to Type-5 LSAs, but not place the Type-7 forwarding address into the Type-5 LSAs. OSPF places 0.0.0.0 as the forwarding address in the Type-5 LSAs.
nazir8121 said
Hi,
Excellent notes.
I dont understand the final lines ” area 1 nssa translate type7 suppress-fa ” . Could you please explain me the command with example .
Thanks
Nazir
Peter Kurdziel said
http://www.cisco.com/en/US/docs/ios/iproute/configuration/guide/irp_ospf_for_add_sup_ps6350_TSD_Products_Configuration_Guide_Chapter.html
The OSPF Forwarding Address Suppression in Translated Type-5 LSAs feature causes a not-so-stubby area (NSSA) area border router (ABR) to translate Type-7 link state advertisements (LSAs) to Type-5 LSAs, but use the address 0.0.0.0 for the forwarding address instead of that specified in the Type-7 LSA. This feature causes routers that are configured not to advertise forwarding addresses into the backbone to direct forwarded traffic to the translating NSSA ABRs.