Linux, FreeBSD, Juniper, Cisco / Network security articles and troubleshooting guides

OSPF not-so-stubby area is described in RFC 3101 ([url]https://tools.ietf.org/html/rfc3101[/url]) and it’s main characteristics are that it does not accept OSPF external LSAs (Type 5) as a stubby area, but it allows injection of AS external prefixes, as a normal area. It makes use of Type 7 LSAs that get translated into Type 5 External LSAs by the ABR into the rest of the OSPF domain. Section 1.3 of the RFC describes NSSA areas better than I can ever try :).

A default route in an not-so-stubby OSPF area can result either from a Type 3 LSA or from a Type 7 LSA.

[h2]JunOS: How to inject a default route into an NSSA area with a Type - 3 Summary LSA[/h2]
To inject a default route from a Juniper device which is an ABR into a not so stubby area, Junos needs to actually have “no-summaries” configured. This will prevent other Type 3 Summaries to go into the NSSA area with the exception of a “0.0.0.0” Summary. From the ABR:

[code]
@hub-vSRX1# show protocols ospf
area 0.0.0.1 {
nssa {
default-lsa {
default-metric 1;
metric-type 1;
}
no-summaries;
}
interface st0.0 {
interface-type p2p;
}
interface st0.1 {
interface-type p2p;
}
}[/code]
and the result in OSPF database:
[code]
@spoke-vSRX2# run show ospf database

OSPF database, Area 0.0.0.1
Type ID Adv Rtr Seq Age Opt Cksum Len
Router 172.16.255.1 172.16.255.1 0x80000097 1256 0x20 0x7cfe 72
Router *172.16.255.2 172.16.255.2 0x800000a0 1256 0x20 0x6516 84
Summary 0.0.0.0 172.16.255.1 0x80000002 100 0x20 0x1768 28
NSSA *8.0.0.0 172.16.255.2 0x80000038 811 0x28 0xf17a 36[/code]

See the default LSA is a Summary LSA.
[h2]JunOS: How to inject a default route into an NSSA area with a Type - 7 NSSA LSA[/h2]
On the ABR:
[code]
admin@hub-vSRX1# set protocols ospf area 1 nssa default-lsa type-7

[edit]
admin@hub-vSRX1# commit
commit complete[/code]

And the result:
[code]
[edit]
admin@spoke-vSRX2# run show ospf database

OSPF database, Area 0.0.0.1
Type ID Adv Rtr Seq Age Opt Cksum Len
Router 172.16.255.1 172.16.255.1 0x80000097 1311 0x20 0x7cfe 72
Router *172.16.255.2 172.16.255.2 0x800000a0 1311 0x20 0x6516 84
Summary 0.0.0.0 172.16.255.1 0x80000002 3600 0x20 0x1768 28
NSSA 0.0.0.0 172.16.255.1 0x80000001 2 0x20 0xf083 36
NSSA *8.0.0.0 172.16.255.2 0x80000038 866 0x28 0xf17a 36[/code]
The default Summary LSA gets aged out (age 3600) and a new default NSSA LSA is generated by the ABR.


To note here:
“no-summaries” disables Type 3 Summary LSAs the NSSA with the exception of default one. This turns the area practically into a NSSA totally stub area.
“Type-3” is default type for the default LSA into a NSSA area when "no-summaries" is set. If "no-summaries" is left out, it is injected as a Type-7.
The ABR needs an area 0 adjacency in order to inject a default route into the NSSA area.

Juniper official documentation: [url]https://www.juniper.net/documentation/en_US/junos14.1/topics/topic-map/ospf-stub-and-not-so-stubby-areas.html[/url].
Please comment.