OSPD LSA type 3 show .255 in last octet

[Quang Do Xuan]

Hi guys,

I having a backbone area with 2 ABRs (R4 & R1) summarizing 155.1.146.0/23 and 155.1.146.0/24 respectively.

/24 is a subnet is in another area while /23 is created with "area range" command. When R5 received the LSAs it shows link ID as 155.1.146.0 for both. However when forwarding the LSAs to area 3, the /24 is showing 155.1.146.255.

Not a big deal but just curious why the /24 summary is showing 255 in the last octet. Is it because R5 needs to differentiate it self-generated LSAs?

LS age: 364
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 155.1.146.0 (summary Network Number)
Advertising Router: 150.1.5.5
LS Seq Number: 8000002F
Checksum: 0x1D0E
Length: 28
Network Mask: /23
MTID: 0 Metric: 20

LS age: 364
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 155.1.146.255 (summary Network Number)
Advertising Router: 150.1.5.5
LS Seq Number: 80000002
Checksum: 0x4C29
Length: 28
Network Mask: /24
MTID: 0 Metric: 1010

IOU5# sho ip ospf data | i 146|Sum
Summary Net Link States (Area 0)
155.1.146.0 150.1.1.1 663 0x8000002D 0x00F548
155.1.146.0 150.1.4.4 1452 0x800000D5 0x007818
Summary Net Link States (Area 3)
155.1.146.0 150.1.5.5 1446 0x8000002F 0x001D0E
155.1.146.255 150.1.5.5 1446 0x80000002 0x004C29
IOU5#

[Rolf Fischer · Accepted solution]

Hello,

Is it because R5 needs to differentiate it self-generated LSAs?

Exactly. You can find more information in RFC 2328 Appendix E:

"The Link State ID in AS-external-LSAs and summary-LSAs is usually set to the described network's IP address.
However, if necessary one or more of the network's host bits may be set in the Link State ID.
This allows the router to originate separate LSAs for networks having the same address, yet different masks.

(...)

The above algorithm can also be reworded as follows:  
When originating an AS-external-LSA, try to use the network number as the Link State ID.  
If that produces a conflict, examine the two networks in conflict.
One will be a subset of the other.
For the less specific network, use the network number as the Link State ID and for the more specific use the network's broadcast address instead (i.e., flip all the "host" bits to 1). (...)"

HTH
Rolf