From: NET HE (he_net@hotmail.com)
Date: Fri Sep 14 2007 - 00:02:09 ART
I guess advertising router and forward address are used together for ospf
algorithm.
Firstly, since router-id is always with LSAs, ospf algorithm is very easy to
compute the metric to the router with that specific router-id. This metric
is decisive to select next-hop to reach that router. If there are 2
next-hops with equal metric, 2 routes will be placed in routing table. And
for metric-type 2 routes, the metric value in routing table doesn't tell the
truth.
But how to use advertising router and forward address, I guess as below
If forward address is not 0.0.0.0, ospf chooses forward address to compute
metric. If forward address is 0.0.0.0, ospf uses advertising router to
compute metric.
Above guesses can be verified by manipulating path costs.
Best Regards,
Net (Xin) He
>From: "Salau, Yemi" <yemi.salau@siemens.com>
>Reply-To: "Salau, Yemi" <yemi.salau@siemens.com>
>To: "Akhtar Rasool" <akhtar.samo@gmail.com>
>CC: <ccielab@groupstudy.com>
>Subject: RE: LSA-Type 7 to LSA Type-5 Conversion
>Date: Wed, 12 Sep 2007 13:24:54 +0100
>MIME-Version: 1.0
>Received: from lists.groupstudy.com ([207.44.210.9]) by
>bay0-mc11-f16.bay0.hotmail.com with Microsoft SMTPSVC(6.0.3790.2668); Wed,
>12 Sep 2007 05:55:51 -0700
>Received: (from sympa@localhost)by lists.groupstudy.com
>(8.12.11.20060308/8.11.6) id l8CCtTOc027636;Wed, 12 Sep 2007 08:55:29 -0400
>Received: from groupstudy.com (www.groupstudy.com [209.51.144.7])by
>lists.groupstudy.com (8.12.11.20060308/8.11.6) with ESMTP id
>l8CCP1nd026783for <ccielab@lists.groupstudy.com>; Wed, 12 Sep 2007 08:25:01
>-0400
>Received: from groupstudy.com (groupstudy.com [127.0.0.1])by groupstudy.com
>(8.12.11.20060308/8.12.11) with ESMTP id l8CCP41Q021927GroupStudy Mailer;
>Wed, 12 Sep 2007 08:25:04 -0400
>Received: (from listserver@localhost)by groupstudy.com
>(8.12.11.20060308/8.12.11/Submit) id l8CCP4DE021925for ccielabxhiddenx;
>Wed, 12 Sep 2007 08:25:04 -0400
>Received: from GBMDDMH212ASVR.siemens.co.uk (gbmddmh212asvr.siemens.co.uk
>[212.58.233.141]) by groupstudy.com (8.12.11.20060308/8.12.11) with ESMTP
>id l8CCP2sn021876 GroupStudy Mailer; Wed, 12 Sep 2007 08:25:02 -0400
>Received: from GBMDDMH213ASVR (GBMDDMH213ASVR [192.168.200.74] (may be
>forged)) by GBMDDMH212ASVR.siemens.co.uk (Switch-3.1.5/Switch-3.1.2) with
>ESMTP id Z8CC0W2J300003B0C for <ccielab@groupstudy.com>; Wed, 12 Sep 2007
>13:32:02 +0100
>Received: from GBMDDMH202AMSX.gb001.siemens.net
>([137.223.62.125]unverified) by GBMDDMH213ASVR with InterScan Messaging
>Security Suite; Wed, 12 Sep 2007 13:22:53 +0100
>X-Message-Delivery: Vj0zLjQuMDt1cz0wO2k9MDtsPTA7YT0w
>X-Message-Info:
>R0VWDD4MWRnx++F3/Dfss+Qv7cn1Ii0g6OyOxOsWx9N/X6oVrYUfPa8xvA6+3ooGSS6iyrusXeez2IASRmzMY3Uh0bygkYIa
>Content-class: urn:content-classes:message
>Thread-Topic: LSA-Type 7 to LSA Type-5 Conversion
>Thread-Index: Acf0+6RGB4d5GN+JTVynp9VAlbxIawAOkujQ
>References: <c3f571c80709110530j7493a836yb604b031153ddea@mail.gmail.com>
><787488417E7F8C45BB546E0958B136DA054B525D@GBMDDMH202AMSX.gb001.siemens.net>
> <c3f571c80709110828m714fc0e8v909201368a214f47@mail.gmail.com>
><c3f571c80709112213l1d97276s47493ab1a536fd6f@mail.gmail.com>
>X-MIME-Autoconverted: from quoted-printable to 8bit by groupstudy.com id
>l8CCP2sn021876
>X-ASK-Info: Whitelist match [from yemi\.salau@siemens\.com] (2007/09/12
>08:25:04)
>X-Loop: ccielab@groupstudy.com
>X-Sequence: 1145
>Errors-to: ccielab-owner@groupstudy.com
>Precedence: bulk
>X-no-archive: yes
>List-Id: <ccielab.groupstudy.com>
>List-Help: <mailto:sympa@groupstudy.com?subject=help>
>List-Subscribe: <mailto:sympa@groupstudy.com?subject=subscribe%20ccielab>
>List-Unsubscribe:
><mailto:sympa@groupstudy.com?subject=unsubscribe%20ccielab>
>List-Post: <mailto:ccielab@groupstudy.com>
>List-Owner: <mailto:ccielab-request@groupstudy.com>
>Return-Path: ccielab-owner@groupstudy.com
>X-OriginalArrivalTime: 12 Sep 2007 12:55:51.0960 (UTC)
>FILETIME=[3FBDE180:01C7F53C]
>
>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
>
>
>_______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
><http://www.groupstudy.com/list/CCIELab.html>
>
>_______________________________________________________________________
>Subscription information may be found at:
>http://www.groupstudy.com/list/CCIELab.html
This archive was generated by hypermail 2.1.4 : Sat Oct 06 2007 - 12:01:11 ART