From: Salau, Yemi (yemi.salau@siemens.com)
Date: Tue Sep 11 2007 - 10:19:22 ART
R5 installs 2 parallel routes because both routes has got same metrics,
not because the "Forward-Address" is reachable via 2 ABRs ... Well,
maybe there is a connection there, but there is no direct correlation I
believe.
The Forward-Address within the ospf-database doesn't have to be
reachable, it's the router-id of the ASBR bringing in the Type-7 into
the OSPF domain. Router-id ... Configurable using the router-id command,
and this doesn't have to be pingable, but inserted in the OSPF Header
router-id field.
Remember, in principle, there is usually 1 translator as you already
know, which is based on the router with highest router-id, but what that
does is simply convert the type field in the LSA headers from 7 to 5
while they flow into the backbone area. This translation is independent
or in other words has no direct impact on the metrics of the route,
hence, R5 still get 2 routes delivered with same metrics and installs
both in it's RIB.
Many Thanks
Yemi Salau
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Akhtar Rasool
Sent: Tuesday, September 11, 2007 1:31 PM
To: ccielab@groupstudy.com
Subject: LSA-Type 7 to LSA Type-5 Conversion
Dear all,
I can't understand when there are 2 ABRs(say R1 & R4) for an area which
is
NSSA( having a router R6), based on highest router-id one of these 2
ABRs
would be LSA-7 to 5 translator so far so is good.
When we check the routing table in the backbone area router(R5 having
links
to each ABR) there are 2 routes to the external route although ''show ip
ospf database external'' is showing LSA being only advertised from
R4(7-5
translator whose RID-4.4.4.4 is higher).
**** R5 is Backbone area router from which 2 links are going to 2
ABRs*************************
************************************************************************
*************************************
R5#show ip ospf data exter
OSPF Router with ID (5.5.5.5) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 327
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 160.1.60.0 (External Network Number )
Advertising Router: 4.4.4.4
LS Seq Number: 80000003
Checksum: 0x7B0D
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 20
Forward Address: 155.1.6.6
External Route Tag: 0
************************************************************************
***************************
R5#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS
level-2
ia - IS-IS inter area, * - candidate default, U - per-user static
route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
155.1.0.0/16 is variably subnetted, 3 subnets, 2 masks
*O IA 155.1.6.6/32 [110/75] via 155.1.0.4, 01:16:36, Serial0/0
[110/75] via 155.1.0.1, 01:16:36, Serial0/0
*C 155.1.0.0/24 is directly connected, Serial0/0
C 169.254.0.0/16 is directly connected, Ethernet1/1
160.1.0.0/24 is subnetted, 1 subnets
*O E1 160.1.60.0 [110/95] via 155.1.0.4, 01:16:27, Serial0/0
[110/95] via 155.1.0.1, 01:16:27, Serial0/0*
************************************************************************
***************************
What I have understood from this is since Forwarding address
reachability is
being learned from 2 paths(both ABRs are advertising) thats why R5 is
installing 2 parallel routes.
Any discussion on this would be highly appreciable.
Regards,
Akhtar
This archive was generated by hypermail 2.1.4 : Sat Oct 06 2007 - 12:01:10 ART