OSPF - Link State ID: (External Network Number )

[whistleblower14]

Hi all,

I`m currently preparing for a certification and in the course of this I noticed that in one of my OSPF databases there are differences in the OSPF database for "Type-5 AS External Link States"! Can someone explain to me on what basis the link state ID is generated and why for some of the networks either the network-ip and for other the broadcast-ip is the Link-State ID? The Subnetmask for all of the networks is a /24...

Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag
10.1.4.255 172.16.1.248 1670 0x800006EF 0x0037F1 0
10.1.9.0 172.16.1.248 1191 0x800015ED 0x00D640 0
10.1.11.0 172.16.1.248 799 0x8000269A 0x003423 0
10.1.13.0 172.16.1.248 919 0x8000268A 0x003E27 0
10.1.14.255 172.16.1.248 211 0x800026D2 0x00A279 0

thanks in advance for every reply and clarification!

[ngkin2010]

Hi,

The LSID for Type-5 LSA should be the destination network, where the destination network is advertised by a ASBR.

If you see 10.1.4.255 in the OSPF Type-5 LSA database, please have a look on the advertising router 172.16.1.248

Link ID ADV Router Age Seq# Checksum Tag
10.1.4.255 172.16.1.248 1670 0x800006EF 0x0037F1 0

Ref: https://tools.ietf.org/html/rfc2328#page-118

[whistleblower14]

yes, it`s correct that the ASBR ist advertising this external Route...

LS Type: AS External Link
Link State ID: 10.1.4.255 (External Network Number )
Advertising Router: 172.16.1.248
LS Seq Number: 80000700
Checksum: 0x1404
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 1
Forward Address: 172.16.1.248
External Route Tag: 0

the problem for me is, that the advertising device is a Meraki MX and I´ve no option to check a database or other OSPF relevant informations there! What I`d also need to know is, what I should check/see on the ASBR? I`m not sure if the ASBR is changing anything because the IP Prefix is the Network of 10.1.4.0/24 in the ASBR`s routing table and the same as most of the other Prefxies as well... I don`t get what the differences are

[Giuseppe Larosa]

Hello @whistleblower14 ,

the Link state id should be 10.1.4.0 and not 10.1.4.255 given the /24 prefix length.

For some reasons the ASBR device Meraki MX is not behaving correctly.

Hope to help

Giuseppe

[whistleblower14]

Hi Giuseppe,

thanks for your reply, this is also the guess I had, so I´ve opened a support-case and the Engineer replied me the following...

According to section 12.1.4 of RFC 2328, the Link State may ID additionally have one or more of the destination network's "host" bits set. For example, when originating an AS-external-LSA for the network 10.0.0.0 with mask of 255.0.0.0, the Link State ID can be set to anything in the range 10.0.0.0 through 10.255.255.255 inclusive.

unfortunatly I don't have a notion yet why this behavior only applies to some prefixes...

[Giuseppe Larosa]

Hello @whistleblower14 ,

from a routing point of view until the prefix length is correct there is no impact because the FEC entry is calculated making a bitwise AND of the LINK ID and the subnet mask so the end result  should be prefix 10.1.4.0/24 seen as O E2 or O E1 in IP routing table.

Check on neighboring device if you have the correct entry 10.1.4.0/24 in routing table then  you should be fine.

Hope to help

Giuseppe

[paul driver]

Hello
The link ID would mainly represent the neighbors router id however it depends on the connected link type:
Loopback interface 1.1.1.1/24
Stub link  - LINK ID = 1.1.1.0

Broadcast interface 1.1.1.0/24
Transit link LINK ID =the elected designated router ip address that's residing on this transit link of 1.1.1.0/24

POINT-TO-POINT Interface 1.1.1.0/24
P2P LINK ID - Neighbors router ID (which can be manually set)