[Series of OSPF] Episode 4 detailed LSAs in OSPF Highlighted

Latest reply: Nov 28, 2018 19:16:48 301 4 1 1

In the last three episode(links has been list at the end of this episode), we introduced how the router establish adjacency relationship in multi-access network and build the whole LSDB of the local area. After the LSDB built up, router can calculate the route path based on LSDB.
As we mentioned before, OSPF is a link state routing protocol, it spreads LSAs rather than routing information. OSPF contains various types of LSA, every type of LSA generated by different routers, and carries different link state messages.
In this episode, we will introduce the common 7 types of LSA, from type 1 router-LSA to type 7.
Every type of LSA has a same LSA header

LSA header

LSA header options

LS age: indicate the period since the LSA is generated, the value keeps increasing regardless the LSA is transmitted on a link or restore in the LSDB.  Options:  E:indicate capability of the router to receive the AS-External-LSAs. value equals 1 mean router can receive AS-External-LSAs, value equals 0, router can't receive AS-External-LSAs. Combine the difference of the area, we could know that in stub, totally stub, NSSA or totally NSSA area, this field will set to 0.  N/P:in hello packets, this field considered as N bit, when N bit is set to 1, it means router support type 7 LSA, on the contrary, when N bit is set to 0, it means router can't support type 7 LSA. In NSSA LSA, N/P is considered as P bit, it indicates that ABR connets to a NSSA area should translate type 7 LSA to type 5 LSA.  MC:indicate capability of the router to support forwarding multicast traffics.  DC:indicate capability of the router to support the demand circuit property  link-state advertisement type:indicate the type of the LSA, this field value depends on different LSA types  link state ID: Works together with the LS type and LSA description in the LSA to describe an LSA in a routing domain  advertising router:indicate the RID of the router which generates the LSA  LS sequence number:indicates the sequence number of the LSA, it's used to determine which is the new one between two LSAs.  LS checksum: the checksum of all domains except LS age.  length: length of the entire LSA, including LSA header. 
After the LSA header field, the different LSAs followed.

router-LSA  type 1
router LSA
  • flags:flags field contains there bits, 
    • V: indicates virtual link when set to 1 
    • E: indicates ASBR when set to 1 
    • B: indicates ABR when set to 1 
in router-LSA, link state ID field in the LSA header indicates the RID of the router which generates the LSA.
link ID field, link data field and type field in the router-LSA correspond to each other as below.

link ID link data type
RID of the neighboring router IP address of the interface which connects to the network 1:connect to another router through P2P
interface IP address of the DR IP address of the interface which connects to the network 2:connect to a transport network
network segment or subnet number subnet mask 3:connect to a stub network
RID of the remote router index value of the interface 4:virtual link
  • TOS: indicates the number of TOSs connected to different links. 
  • metric: indicates the link cost. 
router-lsa-nssa_asbr_set
figure 1


router-lsa-stub_asbr_notset
figure 2

figure 1 and figure 2 both are route-lsa packets, figure 1 shows capture result in NSSA area, while figure 2 shows capture result in stub area. Can you find some different between them ? 

you may find that in figure 1, E bit in flags field has been set to 1, but in figure 2, E bit keeps 0. But the configurations are same except the type of the stub area. What makes the router AR1 become a ASBR ?
The reason for this difference is that AR1 translates the type 7 LSA which received in NSSA area to type 5 LSA, then spreads type 5 LSA in area 0. As we know, type 5 LSA generated by ASBR when importing external routes. This makes AR1 become a ASBR.


network-LSA  type 2
type 2 network lsa
  • link state ID in the LSA header: indicates the interface address of the DR 
  • network mask:mask of the network. 
  • attched router: lists all the RIDs of the routers in the same network. 

summary-LSA  type 3
type 3 summary lsa
  • link state ID in the LSA header: advertised network address. 
  • network mask: mask of the network address. 
  • metric: the cost from the ABR to the destination network. 
  • TOS: service type 
  • TOS metric: metric associated with the specified TOS value.

ASBR summary LSA   type 4 
type 4 asbr summary lsa
Like type 3 LSA, ASBR summary LSA also generated by ABR. It is same as type 3 LSA except values of a few fields. 
link state ID in type 3 LSA indicates advertised network segment, in type 4 LSA, link state ID indicates the RID of the ASBR.
in type 4 LSA, network mask is meanless, its value is set to 0.0.0.0 . 


AS-External-LSA  type 5
type 5 external lsa
  • link state ID in the LSA header: advertised network address. 
  • network mask: network mask. 
  • E: type of the external metric value: 
    • 0:indicates type 1 external routes. 
    • 1:indicates type 2 external routes. 
    • by default, when import external routes, E bit will be set to 1 
  • metric: cost of the route
  • forwarding address: when packets are sent to the imported route destinations, packets will be forwarded to this address. 
  • external route tag: indicates the tag added to the external route. OSPF does not use the field. The field, however, can be applied to manage external routes. 
  • TOS: service type 
  • TOS metric: TOS metric
NSSA External-LSA  type 7 
type 7 nssa external lsa
type 7 LSA is same as type 5 LSA except these two types of LSA can spreaded in different areas. Type 7 LSA only can exist in NSSA area, while the type 5 LSA can't. 
In type 7 LSA, P-bit is set to 1, while type 7 LSA spreaded to other area by ABR, P-bit will be set to 0, type 7 LSA translated to type 5 LSA.




Considering the different areas and different types of LSAs in the OSPF, we can summary the different types of LSAs that can exist in different areas as below:
Area  types of LSA
backbone 1,2,3,4,5
common 1,2,3,4,5
stub 1,2,3
totally stub 1,2
NSSA 1,2,3,7
totally NSSA 1,2,7



NSSA LSDB
LSDB on AR1 when area 2 configured as NSSA area

lsdb totally nssa
LSDB on AR1 when area 2 configured as NSSA area

lsdb-stub
LSDB on AR1 when area 2 configured as NSSA area

lsdb-totally stub
LSDB on AR1 when area 2 configured as NSSA area

As the picture above showing,
we can learn that when configuring area as stub area, totally stub area or totally NSSA area, ABR will advertise a type 3 default route summary-LSA into these areas. In NSSA area and totally NSSA area, ABR will advertise a type 7 default route NSSA summary-LSA into these areas.


In the next episode, we will introduce the SPF algorithm and how the OSPF generate the route path based on the LSDB 


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Created Nov 28, 2018 09:00:55 Helpful(0) Helpful(0)

thanks for your sharing
it indicates that ABR connets to a NSSA area should translate type 7 LSA to type 5 LSA. MC:indicate capability of the router to support forwarding multicast traffics. DC:indicate capability of the router to support the demand circuit property link-state advertisement type:indicate the type of the LSA
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Created Nov 28, 2018 19:16:05 Helpful(0) Helpful(0)

good examples and demo
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Created Nov 28, 2018 19:16:27 Helpful(0) Helpful(0)

thanks for sharing
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Created Nov 28, 2018 19:16:48 Helpful(0) Helpful(0)

i hope more topics will be covered in this series
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