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Last set of questions

Hi, some last questions (I hope):

bridging.ppt

p.47
Why is link L8 on B3 listed as "designated" ? Links L7 and L8 are on the same LAN, so, only one link can be designated, the other one should be backup, shouldn't it ?


ls.ppt

p.66/67
I don't understand what the forwarding address field in an AS-external-LSA represents and what is it used for.
- Is it the address of an AS boundary router in another AS ? But this information is useful only to the AS boundary router which originated the LSA, so why inform every one of it ?
- Is it the address of the router, in this AS, toward which traffic had to be send to reach the network described in link state id (in LSA header) ? But in this case, the forwarding address will always be the same as the advertising router (unless AS boundary router advertise routes to a linked AS through another AS boundary router).
In the example on p.73, the forwarding address is set to 0.0.0.0, what does this value means ?
- Is it to describe a default route ?
- Is a special value which means "advertising router" ?

p.76/77
Router 10.1.1.1 announce an infinite distance to AS boundary router 10.1.1.1 which is the same router. So 10.1.1.1 announce an infinite distance to itself ! Is it correct ?
Posted by René Giller on Tuesday 13 February 2007 at 18:38
Comments
ls.ppt

p.66/67
I don't understand what the forwarding address field in an AS-external-LSA represents and what is it used for.

A: Forwarding Address is the address to which packets for the advertised destination should be forwarded. If the forwarding address is 0.0.0.0, packets will be forwarded to the originating Autonomous System Boundary Router (this router advertised the destination; the advertisement was flooded throughout all Autonomous System).

- Is it the address of an AS boundary router in another AS ?

A: It can be (everything would still work), though I thing it makes more sense if the address is the IP address of an interface of the advertising router (remember similar posibilities with BGP).

But this information is useful only to the AS boundary router which originated the LSA,

A: not always true.

so why inform every one of it ?

A: if it is the address of an AS boundary router in another AS, this address still belongs to both "this AS" and another AS (interconnecting LAN for example), so OSPF in "this AS" will be able to compute how to reach that address.

- Is it the address of the router, in this AS, toward which traffic had to be send to reach the network described in link state id (in LSA header) ?

A: It can be (works). I think this is the most often and the most logical case.

But in this case, the forwarding address will always be the same as the advertising router (unless AS boundary router advertise routes to a linked AS through another AS boundary router).

A: Yes.

In the example on p.73, the forwarding address is set to 0.0.0.0, what does this value means ?
- Is it to describe a default route ?

A: no

- Is a special value which means "advertising router" ?

A: yes. If the forwarding address is 0.0.0.0, packets will be forwarded to the originating Autonomous System Boundary Router.

p.76/77
Router 10.1.1.1 announce an infinite distance to AS boundary router 10.1.1.1 which is the same router. So 10.1.1.1 announce an infinite distance to itself ! Is it correct ?

A: i am not sure for the right answer regarding the purpose of setting the metric value so high in this case, and will check this additionally... This advertisement seems to be an LSA type 4, advertising an A(S?)BR, and the value could be related to "distance vector method" used on levels higher then one area (slides, pg. 22) with aim to set high cost for routes going throughout another AS to some destination in this AS (so that internal routes will be prefered), but as I told this "explanation" is to be checked.

PS: if you want to learn more about OSPF, maybe check this page which explains OSPF in a "readable" way:
http://sysop.com.cn/document/routing_tcp_ip_v1/ch09lev1sec1.html
or look into RFCs.
Posted by Slavisa Sarafijanovic on Tuesday 27 February 2007 at 9:57
p.47
Why is link L8 on B3 listed as "designated" ? Links L7 and L8 are on the same LAN, so, only one link can be designated, the other one should be backup, shouldn't it ?

A: "Links L7 and L8 are on the same LAN" is completely wrong statement, so the quesiton doesn't make sense as it is (and the example is correct by the way).
Posted by Slavisa Sarafijanovic on Tuesday 27 February 2007 at 13:09