From: Akhtar Rasool (akhtar.samo@gmail.com)
Date: Wed Sep 12 2007 - 11:05:56 ART
Yemi, thanks for the helpful explanation & reference. As far as FA=0.0.0.0 NSSA
ABR would be the one whom the traffic would be forwarded & yes as i said in
beginning since there are 2 entries in the routing table for the forwarding
address this is the main reason for 2 parallel external routes in the
routing table.
Thanks & Regards,
Akhtar
On 9/12/07, Salau, Yemi <yemi.salau@siemens.com> wrote:
>
>
> 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
>
>
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