OSPF LSA type 4 – E bit for ASBR

As a reminder, LSA Type 4 are used by SPF to locate an ASBR. This is to know the path to the route redistributed by ASBR. All area are standard Area in this example.

Sample Lab topology

ASBR is generating LSA type 1 with E bit flag set to 1 in packet. I wanted to lab that so I make a packet between R3 and R5. Something funny (or not), the first time the OSPF DB description is generated, ASBR isn’t setting the E bit. But just atfer, in LS update, it adds this E bit.

On bellow, on the left, first packet generated to describe LSDB (DD), on the right, LS update (LSU) with E bit.

These flags are used to describe a specific router type :

  • Virtual-link endpoint
  • ASBR
  • ABR

LSA Type 4 are generated by ABR (not ASBR by itself). However, this is not the type 5 created by ASBR which trigger the generation of type 4 by ABR, this is the E bit in type 1 created by ASBR.

As a sample test to verify that, I created a route-map to filter networks comming from EIGRP networks (R4). So, the only LSA arriving to R2 is the router-LSA generated by ASBR R5 with E bit set. We can see the LSA T4 generated by ABR in OSPF topology table:

R2#sho ip ospf data router 5.5.5.5
OSPF Router with ID (2.2.2.2) (Process ID 1)
Router Link States (Area 1)
Routing Bit Set on this LSA
[…]
LS Type: Router Links
Link State ID: 5.5.5.5
Advertising Router: 5.5.5.5
[…]
AS Boundary Router

Author: Kévin SAS

I am Kévin SAS and live in France. I am an engineer in network and security. I worked for a lot of customers in these domains : Wireless, LAN, Datacenter, VoIP with several brands like Cisco, HPE, Aruba, Palo Alto amongst others. Previously working for a service provider. I currently work for an integrator while giving me lots of opportunity to learn. This blog stores some technicals notes I wanted to share.

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