After some thought, I labbed this up and now have the same doubts
again... Here's a type 5 LSA:
Berta#sh ip ospf database external 1.1.1.0
OSPF Router with ID (192.168.24.4) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 20
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 1.1.1.0 (External Network Number )
Advertising Router: 192.168.13.1
LS Seq Number: 80000001
Checksum: 0x5A52
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
>>>>>>>>Forward Address: 192.168.13.3 <<<<<<<<<<<<<<<<<<<<<<<<<<
External Route Tag: 0
Notice the forward address above, 192.168.13.3. On the same router:
Berta#sh ip ospf database summary 192.168.13.0
OSPF Router with ID (192.168.24.4) (Process ID 1)
Summary Net Link States (Area 2)
Routing Bit Set on this LSA
LS age: 1660
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 192.168.13.0 (summary Network Number)
Advertising Router: 192.168.24.2
LS Seq Number: 80000001
Checksum: 0x9BA5
Length: 28
Network Mask: /24
TOS: 0 Metric: 2
So this Type 3 LSA has all the reach-ability information needed to
forward this packet to the ASBR without a Type4 LSA. Speaking of the
Type 4 LSA, here it is:
Berta#sh ip ospf database asbr-summary
OSPF Router with ID (192.168.24.4) (Process ID 1)
Summary ASB Link States (Area 2)
Routing Bit Set on this LSA
LS age: 1902
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(AS Boundary Router)
Link State ID: 192.168.13.1 (AS Boundary Router address)
Advertising Router: 192.168.24.2
LS Seq Number: 80000001
Checksum: 0x79C6
Length: 28
Network Mask: /0
TOS: 0 Metric: 1
This type 4 LSA doesn't have any information that we don't already have
with the type 3. So now I'm back to where Manish began... When/why do we
need the type 4?
Thanks!
Doug
-------- Original Message --------
Subject: [OSL | CCIE_RS] OSPF LSA Type 3 and Type 4 Confusion ?
From: MANISH <[email protected]>
Date: Tue, August 14, 2012 9:40 am
To: [email protected]
Hi All,
can some one please explain me the difference in LSA Type 3 and LSA Type
4
as per my understanding
LSA Type 3 is used for inter area routes ........meaning all the Type
1/2
inside an area when crosses in to other area will be advertised using
type
3
LSA Type 4 is the IP address of ASBR.
my confusion is if we already know about ip add of ASBR through Type 3
since every thing gets flooded in to other areas why do we need type 4
explicitly to define ip add of ASBR.
I read many blogs and other sources but still could not undersdtand.
take a look at this blog explanation, will fa0/0 of R6 not be known to
other areas using type 3 lsa ?
http://blog.ipexpert.com/2009/11/04/ospf-type-4-lsa-the-forward-address-part-1/
Thank You.
Manish
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_______________________________________________
For more information regarding industry leading CCIE Lab training, please visit
www.ipexpert.com
Are you a CCNP or CCIE and looking for a job? Check out
www.PlatinumPlacement.com
http://onlinestudylist.com/mailman/listinfo/ccie_rs
