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chrisblaine
Beginner

10.1.1.11 Activity - Select the Designated Router

I'm confused for the answers on Figure 3. I understand the bottom 3 answers but the first 2 make no sense. There are other IP addresses that are higher than their own networks yet they were chosen for the DR (priorities aren't in this activity). Also, if somebody can show me how to attach these pictures instead of posting them on the body paragraph, I'd greatly appreciate it (doesn't allow me to attach them with the "Insert/edit link"button.

 

Activity 10.1.1.11 F1.jpg

 

 

Activity 10.1.1.11 F2.jpg

 

 

Activity 10.1.1.11 F3.jpg

1 ACCEPTED SOLUTION

Accepted Solutions
kubn2
Beginner

Hi,

 

Sine all routers have default priority which is 1 then Designated router is chosen by highest router ID. 

Router ID is taken from: 1st router-id command, 2nd: highest loopback address, 3rd: highest interface address.

 

So for network 10.1.10.0/24 its router B why? Because his router ID is the highest in the 10.1.10.0/24 network. His Id is taken from serial interface which is 209.165.201.1 which is higher than for example loopback interface on router A (192.168.10.5).

 

For network 10.1.13.0 router D is designated because it have highest router ID in the 10.1.13.0 network which is from his loopback interface 192.168.10.3.

View solution in original post

2 REPLIES 2
kubn2
Beginner

Hi,

 

Sine all routers have default priority which is 1 then Designated router is chosen by highest router ID. 

Router ID is taken from: 1st router-id command, 2nd: highest loopback address, 3rd: highest interface address.

 

So for network 10.1.10.0/24 its router B why? Because his router ID is the highest in the 10.1.10.0/24 network. His Id is taken from serial interface which is 209.165.201.1 which is higher than for example loopback interface on router A (192.168.10.5).

 

For network 10.1.13.0 router D is designated because it have highest router ID in the 10.1.13.0 network which is from his loopback interface 192.168.10.3.

View solution in original post

Also just to add, I had the same confusion and I was applying the DR/BDR election criteria to the 10.1.10.0 link as a whole. The DR/BDR election criteria is applied to a single router and not to the link.

 

Router A has a loopback interface configured and applying the election criteria, this is preferred as the router-id over the physical interface. Router A ID is therefore 192.168.10.5 (even if the physical interface had a higher address the loopback would still win)

Router B then applies its own criteria. It has no configured loopback interface so the election process goes on to the third criteria and chooses the highest configured physical interface address which in its case is 209.165.201.1

 

Looking at all the other IDs, 209.165.201.1 comes out the highest router-id hence router B is the DR for the 10.1.10.0 link.

 

Just for fun and further exploration I labbed it up in Packet Tracer (labs attached below). I introduced a loopback (192.168.10.4) on RTB and this meant RTA was now the DR. I also played around changing RTB's loopback to 192.168.10.6 and RTB became the DR but using the loopback as the router-id instead (illustrating the fact that a loopback is preferred over a physical interface even if the physical interface has a higher address) :-)