ospf part 8: Multi-Access OSPF

Hi Mate! OSPF again! yup Actually we are still on the portion of CCNA topics. There might be 4 to 5 CCNA topics before we proceed with CCNP OSPF topics. Hehehehe!

So we are now able to resolve the need of establishing areas on OSPF to avoid slow convergence and high CPU utilization. But we got another OSPF issue once again. See image below:


Let us play a scenario wherein a new network was added  to R3.

1.)So  R3 will create its own LSA then will have no choice but to forward it to the switch. The hello message was sent to multicast address however since it is forwarded to a switch, switch will broadcast it to all OSPF speaking neighbors.

2.) R1, R2 and R4 will received the hello message sent originally by R3 and will forward  and R1,R2 and R4 will save the LSA to their LSDB and will send a hello back message with the router-id of R3 as their neighbor however it will be forwarded first to a switch.

3.) Switch will broadcast the message to all OSPF speaking router and looping will be created. Let’s say for example , R2 send  a hello back message to R3 but since it is forwarded to the switch first,  it will broadcast to R3 ,R1 and R4  and so same scenario will happen to R1 and R4. R1 and R4’s hello back message will be broadcast by switch and looping occur.

To avoid this scenario, we created router hierarchy for multi-access OSPF.

Those are

A) DR- designated router – can only forward NETWORK LSA

B) Back-up DR- alternative to DR ONLY if DR fails

3.) DR other – non BDR or non DR routers

Network LSA- LSA sent to multicast address and can only be created by DR

Parameters for a router to become DR

1.) Highest priority –> we can manipulate the priority since we hard code it

2.) Highest router-id –> in the absence of priority configuration, the default priority will be 1 and if all of the routers share the same priority, routers with highest router-id will serve as the DR, and second to the highest will be the BDR. The losing router will serve as DR other

Note: Priority ranges from 1-225

If you want for a router to be considered as DR other without expecting that it will be the lowest priority on all routers, you can set the priority to zero

So let us continue the scenario:


We now set the router-id and the priority on the routers, since priority is on top of the hierarchy to be the DR, we can conclude that R3 will be the DR though it R4 has the highest router-id and R4 will serve as the BDR. Other routers since they got zero priority, they will server as DR other



1.) There dyou go! Let us play a scenario where in R1 has a new network to be advertised.

R1 will send hello to  multicast address: which is designated to be sent to DR and BDR.

2.) DR and BDR will update their LSDB with the router LSA that they received from R1

3.) DR which is R3 will create a network LSA to multicast address which is designated to be sent to DR other routers: R1 and R2

4.) After that, R1 and R2 which are DR other routers will save the LSA on its LSDB and will not anymore forward the network LSA that they received from R3.

In that way, looping will be avoided. The moment that  R1 and R2 received the network LSA, they are on loading state and will proceed with FULL since they already received acknowledgement from DR and BDR that they are updated on what new network added to R1.

You want more? we’ll try  to do configuration of these on our next blof















Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s