From: Akhtar Rasool (akhtar.samo@gmail.com)
Date: Tue Sep 11 2007 - 12:28:07 ART
*>>>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.*
Basically forwarding address is the routable IP which needs to be present
in the routing table so as to avoid extra hops. Its not the router-id. For
details you can consult below mentioned link.
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009405a.shtml
This is the excerpt from RFC3101
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
Normally the next hop address of an installed AS external route
learned by an NSSA ASBR from an adjacent AS points at one of the
adjacent AS's gateway routers. If this address belongs to a network
connected to the NSSA ASBR via one of its NSSAs' active interfaces,
then the NSSA ASBR copies this next hop address into the forwarding
address field of the route's Type-7 LSA that is originated into this
NSSA, as is currently done with Type-5 LSAs.
For an NSSA with no such network the forwarding address
field may only be filled with an address from one of the its active
interfaces or 0.0.0.0. If the P-bit is set, the forwarding address
must be non-zero; otherwise it may be 0.0.0.0. If an NSSA requires
the P-bit be set and a non-zero forwarding address is unavailable,
then the route's Type-7 LSA is not originated into this NSSA.
When a router is forced to pick a forwarding address for a Type-7
LSA, preference should be given first to the router's internal
addresses (provided internal addressing is supported). If internal
addresses are not available, preference should be given to the
router's active OSPF stub network addresses. These choices avoid the
possible extra hop that may happen when a transit network's address
is used. When the interface whose IP address is the LSA's forwarding
address transitions to a Down state (see [OSPF] Section 9.3), the
router must select a new forwarding address for the LSA and then re-
originate it. If one is not available the LSA should be flushed.
****************************************************************************************************
As per ""show ip ospf database external"" the forward-address 155.1.6.6 is
routable thats why that external routes gets installed in the routing table.
The thing which i can't understand is, If only one router is Type7 to Type5
translator how R5 installs 2 parallel external routes. What I mean to say is
R-5 should only install only that route in the routing table whose LSA is
being geenrated by the translator.
Regards,
Akhtar
On 9/11/07, Salau, Yemi <yemi.salau@siemens.com> wrote:
>
> 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
>
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