some OSPF questions....

two questions i have on OSPF...

Q1)
in my lab im mucking round with OSPF, and i noticed that after a show ip ospf nei command that there were two BDRs:

routerB#sh ip ospf nei

Neighbor ID Pri State Dead Time Address Interface
192.168.1.1 0 FULL/ - 00:00:34 192.168.1.1 Serial0
12.3.1.1 1 FULL/BDR 00:00:39 172.16.1.2 FastEthernet0
10.3.0.1 1 FULL/BDR

im guess that this is is because they have the same priority but i also thought that the IP address would be used as a tie breaker, like it is with the DR election.... so i guess my question is, is it possible to have more than one BDR?

Q2)
I know that to find the cost you use the 100000000/Bandwidth command. But im just a bit confused on how it actually works... like with EIGRP to get the cost you use the slowest bandwidth in the algorithm. So i have this config:

routerB#sh ip route

Gateway of last resort is not set

100.0.0.0/30 is subnetted, 1 subnets
O 100.1.1.0 [110/260] via 172.16.1.2, 01:06:20, FastEthernet0
172.16.0.0/30 is subnetted, 2 subnets
C 172.16.1.0 is directly connected, FastEthernet0
C 172.16.2.0 is directly connected, Ethernet0
10.0.0.0/32 is subnetted, 3 subnets
O 10.3.0.1 [110/196] via 172.16.2.2, 01:06:20, Ethernet0
O 10.2.0.1 [110/196] via 172.16.2.2, 01:06:20, Ethernet0
O 10.1.0.1 [110/196] via 172.16.2.2, 01:06:20, Ethernet0
O 192.168.0.0/24 [110/1171] via 192.168.1.1, 01:06:20, Serial0
12.0.0.0/32 is subnetted, 4 subnets
O 12.0.1.1 [110/131] via 172.16.1.2, 01:06:20, FastEthernet0
O 12.1.1.1 [110/131] via 172.16.1.2, 01:06:20, FastEthernet0
O 12.2.1.1 [110/131] via 172.16.1.2, 01:06:24, FastEthernet0
O 12.3.1.1 [110/131] via 172.16.1.2, 01:06:24, FastEthernet0
192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.0 is directly connected, Serial0

so take the 10.0.0.0 network, it has a cost of 196 via 172.16.2.2. So my understanding is that the cost to 172.16.2.2 is 196. Then the route from 172.16.2.2 to 10.0.0.0 would also have its own cost which would be in its routing table (it happens to be directly connected to 172.16.2.2 in this case) But say that 10.0.0.0 was another few hops away from 172.16.2.2, then it would have its cost to the next hop router in its routing table right?

cheers!

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Comments

Paul Boz

Can you post the output of a "show ip ospf neighbor detail" command?

networker050184

For each multi-access network a DR and BDR are elected that is why you have more than one.

Netstudent

So my understanding is that the cost to 172.16.2.2 is 196

This is the total accumulated cost of each outgoing link at each hop all the way to the destination subnet (10.1.0.0) The cost to 172.16.2.2 depends on the bandwidth of the link between the local router and the next hop. It has opened the shortest path first.

As far as having 2 BDR's....this happened to me once when I was setting up OSPF over frame-relay. Give one of the two routers a slightly higher priority to make it the DR. I think you may have to clear OSPF to initiate a re-election. Just some ideas...

rakem

Netstudent wrote:

So my understanding is that the cost to 172.16.2.2 is 196

This is the total accumulated cost of each outgoing link at each hop all the way to the destination subnet (10.1.0.0)

so if there a a few hops to the destination it will add all the costs together and show the total in the routing table?

Netstudent

yes it is the total accumulated cost. Hence the SPF in the protocol. the shortest path is the lowest cost path.

rakem

networker050184 wrote:

For each multi-access network a DR and BDR are elected that is why you have more than one.

Ok.... but you can't have more than one DR can you?

Paul Boz

rakem wrote:

networker050184 wrote:

For each multi-access network a DR and BDR are elected that is why you have more than one.

Ok.... but you can't have more than one DR can you?

No, only one DR. The DR is responsible for controlling Link State Announcements. If you had multiple DR's imagine the pandemonium of topology changes. One DR may tell you one thing while another tells you something completely different, effectively causing massive routing inconsistencies. DR's and BDR's maintain the same databases and listen to the same LSA traffic, but the DR is the only router that actually acts on the data. That is, unless the DR fails, in which case the BDR takes over.

The best way to remember it is to realize that while several routers maintain the same data that a DR does (in the case of multiple BDR's), the DR is the only one that's actually capable of using the data. The BDR's are like vise presidents. They're privy to the same information that the president is, but until the head honcho is assassinated it's "just in case."

ljp

rakem wrote:

Q1)
in my lab im mucking round with OSPF, and i noticed that after a show ip ospf nei command that there were two BDRs:

routerB#sh ip ospf nei

Neighbor ID Pri State Dead Time Address Interface
192.168.1.1 0 FULL/ - 00:00:34 192.168.1.1 Serial0
12.3.1.1 1 FULL/BDR 00:00:39 172.16.1.2 FastEthernet0
10.3.0.1 1 FULL/BDR

im guess that this is is because they have the same priority but i also thought that the IP address would be used as a tie breaker, like it is with the DR election.... so i guess my question is, is it possible to have more than one BDR?

Each OSPF area can, and usually does, have multiple DRs and BDRs. Your router was likely connected to at least 2 Broadcast Multi Access networks