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DV - slide 15 - Bellman ford
Inpact of initial conditions:

for p^3(5), I find 2, but in the slides it's 3.

Where is the mistake?

p^3(5) = min j!=5 [A(5, j)+p^2(1), A(5,2)+p^2(2), A(5,3)+p^2(3), A(5,4)+p^2(4)]
= min {inf+0, 3+1, 1+2, 1+1} = 2
Posted by Raphaël Tagliani at 9:50
Comments (1)
Exam tomorrow: 12:30, inm202 and inm203
The exam tomorrow takes place at 12:30.

we have two rooms at our disposal: inm202 and inm203
so please use both of them.

reminder: any electronic equipment is NOT allowed. so print the slides before coming.
Posted by Manuel Flury at 19:25
I'd like to know what's the difference between the answer in 2.3.3 and 2.3.4, when what you send are DNS' queries and HTTP's queries.
Posted by Javier Sanchez Tamargo at 11:53
Comments (1)
Exos 2.3
Good Evening!

A few questions regarding ex. 2.3:

1) 2.3.2: Is it Ok for each router to indicate only the /28 subnet(s) which isn't (aren't) on-link as well as the proxy subnet for R1 and R2 or do we *have* to indicate a default gateway?

2) 2.3.8: I'm not sure how the bridges will react with the loop. I guessed they will automatically use only one of the possible routes (hopefully the shortest) and therefore the ping packets will only be seen at one observation point each time... is this correct?

Thanks alot,

Christophe Gudin.
Posted by Christophe Gudin at 0:01
Comments (6)
next-hop path attribute bgp question

One thing about next-hop path attribute is not clear to me. In slides 35 and 80 (solutions for 35) edge router changes next-hop attribute to self ( Is this necessary for the whole scheme to work?

External router's (R5) address ( is also reachable by all internal routers from AS x as this router has an IP address on a network shared by AS z and AS x. If this is true, no change in next-hop attribute is required.

Also, RFC 1771 (page 22, last paragraph) says:

"When a BGP speaker advertises the route to a BGP speaker located in
its own autonomous system, the advertising speaker shall not modify
the NEXT_HOP attribute associated with the route. When a BGP speaker
receives the route via an internal link, it may forward packets to
the NEXT_HOP address if the address contained in the attribute is on
a common subnet with the local and remote BGP speakers."

Thank you for helping,
Posted by Aleksandar Dragojevic at 23:42
Comments (1)
exo 2.4.3

In question 2.4.3 when the links btw A1 and B1 fails, it is said that when A1 and B1 detects the failure, A1 and B1 will delete the entries learned over this link from their RIB-in tables, and update related entries in their forwarding tables. Up to now I have no problems. But after, it is said that they will also advertise the changes (I-BGP) to other BGP routers within their domains. What do they actually advertize? The routes they have now are routes learnt by I-BGP from A4 and B4 respectively, so they shouldn't be advertising them. Am I missing sthg?

Thank you very much for your help.


Posted by Ghita Mezzour at 21:06
Comments (1)
I-BGP messages and adj-RIB-out

There's something I don't get. In the BGP slides, on page 20 we are told "Only routes learned from E-BGP are put in Adj-RIB-out" and then in the example on page 26 we have "Since there is no change in Loc-RIB there is no change in Adj-RIB-out". I guess this means if the I-BGP message had lead to a change in Loc it would have been forwarded to Adj-RIB-out.... Isn't this in contradiction?

Thanks for the help,

Christophe Gudin.
Posted by Christophe Gudin at 17:58
Comments (1)
dv slides 67 question

In dv slides, slide 67 there are some things that are not clear to me:

For initial conditions 0 0 0 0 0 in step 3: why does cost for 5 increase to 3 instead of staying 2? Cost to 4 is 1 and cost of 4 is also 1 in previous round.

For initial conditions 0 6 1 1 0 in step 2: why cost of 3 increases to 2? 3's neighbors are 2 and 5. Cost to 2 is 6 and cost of total path would be 7. Cost to 5 is one and cost of 5 is already 2 in previous round, so total cost of path is 3.

Thank you,
Aleksandar Dragojevic
Posted by Aleksandar Dragojevic at 0:33
Comments (1)
Office Hours Before Test 2
Could you put office hours before test 2 so that we can check with you if our answers to question 2.3 are correct? Thanks in advance
Posted by Claire Belmont at 0:05
Comments (3)
exercice 2.3
Is it possible to have the correction of the point 2, 3 ,4 and 5 of the exercice 2.3? They are missing in the available correction.

Posted by Jean-Philippe Clivaz at 12:07
Comments (3)
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