BSCI - NF73

[Januk]

Would somebody be able to explain the answer for me?

I just cannot see:

The router will prefer the next-hop of 172.26.11 for packets destined for the 192.168.1.0 network.

I could understand it if  ACL65 was ACL20.

Is this an error?

[vhira1]

HI

I think you are right. I thought the same and I figured out that It was a typo as there are no any other ACL in output.

I hope they will correct this.

Good luck!!!

[joshuad31]

Actually this question is quite difficult.  The answer that is provided is the correct answer.

Local preference is used only within an autonomous system between IBGP speakers to determine the best path to leave the autonomous system to reach an outside network.  The local preference is set to 100 by default; higher values are preferred over lower ones.  The set local-preference command within a route map changes the default local preference value.  With this command, all IBGP routes that are advertised have the local preference set to the value specified.  If an EBGP neighbor receives a local preference value, the EBGP neighbor ignores it.

This router is located in autonomous system 65001 and the 172.26.1.1 network is an external peer in autonomous system 65555.  The route map is informing the router to see routes indicated by an "access-list 20" command advertised from network 172.26.1.1  as having a higher local preference of 200.

The problem with this output seems to be that the access list statement number does not correspond with the same number referenced in the route map.  Access lists contain an implicit deny at the end of their output.  The access list combined with the route map would be permitting all networks included in an "access-list 20" command to receive a local preference of 200.  Since there is no networks given for "access-list 20" the implicit deny would then deny all other networks from receiving a local preference of 200.

Route maps have an implicit permit at the end of their output so they would therefore permit all other networks from neighbor 172.26.1.1 but would not apply any attributes to those networks.

Given these facts the following options can be eliminated because the route map statement is not affecting the advertisement of routes to external neighbors:

The router will advertise the 192.168.1.0 network only to 172.30.1.1
The router will advertise the 192.168.1.0 network only to 172.26.1.1

This statement is also incorrect:

The router will prefer the next hop of 172.26.1.1 for packets except those destined for the 192.168.1.0 network

The remaining statements are for two networks, one of which is an internal IBGP peer and the other is an external EBGP peer.  Network 172.16.1.1 is in AS 65001 same as this router.  Network 172.26.1.1 is in AS 65555 which makes it an external peer.

As previously stated local preference is used to determine the best path to leave the autonomous system to reach an outside network which leaves only one correct answer.