couple of questions about OSPF

[daniel4579329]

hi im learning for ccna and i have a couple of questions about ospf 

1. who send the router lsa network lsa and summary lsa (i have an answer but i just want to be sure)

2.does  the longest prefix route being choose no matter the admin d. and the metric ?

3.when ospf flood and resend the lsa and what is the aging time on ospf 

4.can someone explain to me what is the 224.0.0.5 multicast used for because i saw that the DR use it to send to DRother 

and i also saw that DRother use it to send hello to each other 

thanks for all the help (:

[Martin L · Accepted solution]

Everyone sends router lsa, DR will be in charge of network lsa, and ABR will do summary lsa.  While 224.0.0.5 multicast address is used all over the OSPF network; nothing special about 224.0.0.5 address, However,  224.0.0.6 is reserved for DR/BDR communication via multicast. Anyone who belongs to DR/BDR segment (or network LSA) will use .6 to inform DR/BDR about its change.  Then, DR will inform others using .224.0.0.5 multicast.  

Flooding occurs every 30 minutes. Each LSA is identified by its Sequence Number, Aging time, checksum fields; originator router creates LSA with number 1 which is then incremented by 1 till reaches max number. then, it must start all over again with number 1.  In order to delete LSA for any reasons (like bad checksum or link is down), Max Age value is used.  Age time is used mostly as LSA loop-prevention mechanism as it also increments every flood time and at every hop (router).  If 2 LSAs have the same Sequence number, Aging time can be used to identify recent one (or oldest one).

Yes, the longest prefix route always wins! Then, AD and metric within the same protocol.

Edit: max Age value of 3600 is used  In order to delete LSA for any reasons. Max Age value is used.

Regards, ML
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[Martin L]

I made typo above; and edited just now : Max Age value of 3600 is used In order to delete LSA for any reasons. Not Max Sequence number.

[Richard Burts · Accepted solution]

I would like to clarify the use of longest match, AD, and Metric. AD and Metric are used when we have multiple candidate routes for insertion into the IP routing table and are trying to determine which candidate route should be used. If we have multiple candidate routes that come from different protocols (for example if we have a route for 192.168.1.0/24 from RIP and a route for that prefix from OSPF) then we use AD to compare the routes and choose which one to insert into the routing table (in this case AD for OSPF is better than for RIP and the OSPF route will be used). If we have multiple candidate routes that come from the same protocol (for example if we have a route for 192.168.1.0/24 in EIGRP from neighborA and a route for that network in EIGRP from neighborB then we would compare the metric of the 2 advertisements and choose the route with the better metric.

Longest Match is used when we are preparing to forward an IP packet and there is more than one entry in the routing table that could be used to forward the packet. If we are preparing to forward an IP packet whose destination address was 192.168.1.130 and we looked into the IP routing table and found 2 routes that would include that destination (perhaps 192.168.1.0/24 via one path and 192.168.1.128/25 via another path) then we would choose the longest match (most specific which would be 192.168.1.128/25).

So AD and Metric are used for inserting routes into the routing table and not for forwarding packets while longest match is used for forwarding packets and not for inserting routes into the routing table.

[daniel4579329]

Thanks for the reply

Can you explain to me which multicast

DROther to DR/BDR :
DR/BDR toDROther :
DR to BDR and vice versa :


and just to make sure, DR not sending to BDR and BDR not sending to DROther / DR right ?

[Richard Burts · Accepted solution]

You are welcome. Let me try to clarify the multicast addressing for communication between DR, BDR, and DROther. Let us think about a network segment that has 4 routers connected (routerA is the DR, routerB is the BDR, and routerC is DROther, and routerD is DROther). Let us assume that routerC (DROther) detects a change in its topology, so it will send an LSA to the DR and BDR using the multicast address for DR/BDR. Both routerA and routerB receive the LSA. routerA (as DR) will send an LSA using the multicast address for all OSPF routers. routerB had received the original LSA to DR/BDR and waits to see the LSA to all OSPF routers. routerB runs a timer for the LSA. If the timer runs out then routerB will assume that for some reason routerA is not sending the all routers LSA and routerB would have sent it. routers B, C, and D each receive the LSA to all OSPF routers, and now all routers update their Link State Data Base with the new information.

[daniel4579329]

But what about the multicast address 

(224.0.0.5 and 224.0.0.6 ) when do we use each of them

[Martin L]

we do not use those; those are reserved for OSPF who uses them accordingly;
you can use 239.0.0.0/8 range for private use within an organization.

[Richard Burts]

I thought that a general description would be sufficient. But that does not seem to be the case. So when I use the term the multicast address for DR/BDR it specifically refers to 224.0.0.6. And when I use there term the multicast address for all OSPF routers it specifically refers to 224.0.0.5. 

[Richard Burts]

I am glad that our explanations have been helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.