From: Salau, Yemi (yemi.salau@siemens.com)
Date: Wed Sep 12 2007 - 09:24:54 ART
You actually have a point, but, what of cases where forward-address is
zero values, will it still be reachable, as in must I have a default
entry in the RIB then? I would say NO to that! But that doesn't mean
you're not right, having read the docs, I totally agree with you on that
one .... BUT, check this out:
RFC 2328 , section 16.4 (Calculating AS external routes), states:
"If the forwarding address is non-zero, look up the forwarding address
in the routing table. The matching routing table entry must specify an
intra-area or inter-area path; if no such path exists, do nothing with
the LSA and consider the next in the list."
I labbed this up, and tried changing the metric of the forward address
ip from one of the ABR, so the Backbone Router installs only single
entry in its RIB. And then didn't load-balance obviously due to the
single entry in its RIB.
But by default, there was 2 entries for the forward-address network,
hence 2 entries for the external network on the Backbone router. Let's
just say, the number of entries for the external network (within the
backbone router) is dependent on the number of entries for the forward
address in the routing table. If you find a way to reduce the number of
entries for the FA, then you'll effectively reduce the number of entries
of the external route.
1 Router still translates type-7 to type-5 externals, but with the
concept of FA introduced to avoid extra hops, if none zeros, they are
looked up in the RIB and that's the reason why your R5 will have 2
entries for the external routes.
Hope that helps ...
Many Thanks
Yemi Salau
________________________________
From: Akhtar Rasool [mailto:akhtar.samo@gmail.com]
Sent: Wednesday, September 12, 2007 6:13 AM
To: Salau, Yemi
Cc: ccielab@groupstudy.com
Subject: Fwd: LSA-Type 7 to LSA Type-5 Conversion
>>>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_note09186a008009
405a.shtml
<http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800
9405a.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 <http://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 <http://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
<http://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
<mailto: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
<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 <http://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 <http://160.1.60.0/> (External
Network Number )
Advertising Router: 4.4.4.4 <http://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 <http://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 <http://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 <http://155.1.0.4/>
, 01:16:36, Serial0/0
[110/75] via 155.1.0.1
<http://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 <http://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 <http://160.1.60.0/> [110/95] via
155.1.0.4 <http://155.1.0.4/> , 01:16:27, Serial0/0
[110/95] via 155.1.0.1
<http://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 - 12:01:11 ART