OSPF review

I'm having trouble with a concept in OSPF concerning DR election.

When I labbed OSPF studying for CCNA, I just set up a p2p link, established a neighbor relationship, confirmed it was working. Done.

Alot of text is dedicated to the DR election process, but it seems like it should be obvious: within an Area that is not Area 0, your ABR would always be your DR. However, the Cisco OCG book talks about having multiple routers in the same subnet going through the DR election process. Why? Redundant ABRs? I'm confused.

....if someone could set me straight here I'd appreciate it.

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instant000

ehnde wrote: »

I'm having trouble with a concept in OSPF concerning DR election.

When I labbed OSPF studying for CCNA, I just set up a p2p link, established a neighbor relationship, confirmed it was working. Done.

Alot of text is dedicated to the DR election process, but it seems like it should be obvious: within an Area that is not Area 0, your ABR would always be your DR. However, the Cisco OCG book talks about having multiple routers in the same subnet going through the DR election process. Why? Redundant ABRs? I'm confused.

....if someone could set me straight here I'd appreciate it.

I fail to make the quantum leap in understanding to assume that the ABR is always a DR.

From what I read, the DESIGNATED ROUTER actually refers to an INTERFACE, not a ROUTER.

the DR election occrus on a multi-access segment. This occurs per multi-access segment.

Consider this: you have an area that is a "transit area" that is, you need to configure the virtual-link on the ABR, so that you can "virtually" have area 0 contiguous.

In this case, you'd configure at least two ABR. By your definition, they'd have to both have a DR interface.

If you look at how the OSPF election process occurs, there is nothing saying that the ABR's have to have the DR role. It elects based upon priority and router ID on multiaccess media. It does not say that the OSPF router type: ABR, ASBR, etc. has anything to do with which interface is elected the DR.

What Are OSPF Areas and Virtual Links? [IP Routing] - Cisco Systems

OSPF Design Guide - Cisco Systems

In particular, look at the OSPF Design Guide.

ABR's already have to maintain link state database for every area they are connected to, if they also MANDATORILY had to be DR for whichever segment they are connected to, that adds even more overhead, and makes OSPF a lot less flexible than it actually is.

Two design considerations you can make specifically are:

1. restrict OSPF election to DR/BDR, by editing priority to 0 (this helps on router performance)

2. restrict number of areas an ABR participates in, thus avoiding extra load on the router

Please help me to understand this: Where does it say that ABR must have DR interface? (note again, i refer to interface, not router)

Thanks!

ehnde

instant000 wrote: »

Please help me to understand this: Where does it say that ABR must have DR interface? (note again, i refer to interface, not router)

After reading your entire post, I wanted to review the definition of a DR, but one is not provided. I can't thank you enough for the links. I was in need of multiple sources covering OSPF and had not come across the OSPF design guide page.

This makes more sense now thinking of a DR as a link instead of a router. With no definition provided I assumed "designated router" was talking about a specific router

instant000

ehnde wrote: »

After reading your entire post, I wanted to review the definition of a DR, but one is not provided. I can't thank you enough for the links. I was in need of multiple sources covering OSPF and had not come across the OSPF design guide page.

This makes more sense now thinking of a DR as a link instead of a router. With no definition provided I assumed "designated router" was talking about a specific router

Hah, to me, it is talking about a router function, moreso than a physical router.

that is, you have like ABR/ASBR, those are definite physical routers.

Whereas DR/BRD/DROTHER are controlled by an election process, and are a role that is established dependent upon an interface talking to a specific network, so to me, participating in the election on the network is dependent upon having an interface in that network.

To be 100% clear: I cannot now find something specifically saying that the DR is an interface, but I can say that any OSPF router can be DR/BDR/DROTHER, and that does not control whether or not it is an ABR.

EDIT: I did find something, but wouldn't you believe it was wikipedia? LOL. I'm currently scouring cisco.com for confirmation, but unable to find it, and I apologize for that.

OK, I found something pretty good, how does this work for you?

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094056.shtml#dr

Router1# show ip ospf interface ethernet 0
Ethernet0 is up, line protocol is up
Internet Address 10.10.10.1/24, Area 0
Process ID 1, Router ID 192.168.45.1, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 172.16.10.1, Interface address 10.10.10.2
Backup Designated router (ID) 192.168.45.1, Interface address 10.10.10.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:06
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 2, maximum is 2
Last flood scan time is 0 msec, maximum is 4 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 172.16.10.1 (Designated Router)
Suppress hello for 0 neighbor(s)

Priority
This is the OSPF priority that helps determine the DR and BDR on the network to which this interface is connected. Priority is an 8-bit field based on which DRs and BDRs are elected. The router with the highest priority becomes the DR. If the priorities are the same, the router with the highest router ID becomes the DR. By default, priorities are set to 1.
Use the ip ospf priority number value interface configuration command to set the OSPF router priority. A router with a priority of 0 never participates in the DR/BDR election process and does not become a DR/BDR.
Designated Router

This is the router ID of the DR for this broadcast network. In the example, it is 172.16.10.1.
Interface Address

This is the IP address of the DR interface on this broadcast network. In the example, the address is 10.10.10.2, which is Router 2.
Backup Designated Router

This is the router ID of the BDR for this broadcast network. In the example, it is 192.168.45.1.
Interface Address

This is the IP address of the BDR interface on this broadcast network. In the example, it is Router 1.

NOTE: The documentation specifically refers to "DR interface" and "BDR interface".

I hope this helps!

I know it helped me today!

EDIT4 or 5:

Note that you need TWO pieces of information to note a Designated Router, or a Backup Designated Router:
1. router ID
2. physical interface address

Because, to specify a DR or BDR, you are specifying a particular interface, on a particular router. (NOTE: NOT referring to a particular router ... consider an example where a particular router could have many interfaces, some interfaces could be DR, some could be DROTHER, and yet others could be BDR.)

The command output helps to confirm for me what it is referring to ... good learning experience today. I'm only in Chapter 5 (Appeared to be about IFS/SDM/CDP) of Todd Lammle, LOL. (But coming to this forum is also a boost, but I am wary to stay on my progress schedule, and dedicate "X" number of hours a day to studying.)

jwashington1981

The DR and BDR refer to actual routers, not to just the router interface.

Also, if you have only two routers connected together via a point-to-point link, no election takes place because it's not needed. When you have multiple routers on the same LAN, not using point-to-point links, then all routing updates are first sent to the DR before being sent to the other routers on the LAN.

tomaifauchai

ehnde wrote: »

Alot of text is dedicated to the DR election process, but it seems like it should be obvious: within an Area that is not Area 0, your ABR would always be your DR. However, the Cisco OCG book talks about having multiple routers in the same subnet going through the DR election process. Why? Redundant ABRs? I'm confused.

I don't remember reading anything where an ABR would be elected DR because it is an ABR

If you want it to be a DR you have 3 choices

-Play with loopback address on all routers (Worst choice)
-Set OSPF Priority to 0 on DROthers
-Hard code the Router-ID on the ABR so it has a higher one than the others

instant000

jwashington1981 wrote: »

The DR and BDR refer to actual routers, not to just the router interface.

Also, if you have only two routers connected together via a point-to-point link, no election takes place because it's not needed. When you have multiple routers on the same LAN, not using point-to-point links, then all routing updates are first sent to the DR before being sent to the other routers on the LAN.

I think the critical clarification we need to gather from this series of questions is that the DR election process occurs on multiaccess segments, and is definitely NOT a required role for any particular type of area router in OSPF.

I see it as a function, defined by a specific combination of router ID and interface.

A particular router could be DR using one interface, but it could very well be DROTHER using a different interface.

Since, to truly define DR or BDR, or DROTHER, you have to consider the interface, I feel it is not enough to say it is referring to a router, as a router could very well fulfill multiple roles: DR on this network, DROTHER on another network, or even BDR on another network, etc.

If you say that a router is DR, I feel that you have to at least say this:
This router is the DR for the network 192.168.35.0/30, or this router is the DR using interface eth0. Using one of these (or similar) combinations, you identify the router/interface combination.

And, the more I look at it, I feel silly. (At least, I can laugh at myself, arguing over a non-issue.)

Also, I've yet to find a credible source to define the DR or BDR as an interface, and even the RFC I can look up only say router, not interface. Maybe I made a mountain out of a molehill (looking up something on wikipedia a month ago, getting it absorbed into memory, then regurgitating it as fact later ... once I attempt to retrieve this information, I forgot where I got it from, and then pull it back up from memory as if it came from a good source. once I start searching for it, I wind up at a wikipedia article, LOL .... so sad, hahahahahaha)

Lesson learned: Don't trust your memory, unless it also includes sources.

CodeBlox

ehnde wrote: »

in the same subnet

Routers that participate in this DR election are in the same broadcast domain on the LAN. The relationship between DR and DROTHER is so that you won't have so many LSA exchanges from each router. LSA's are exchanged from the DR router to each DROTHER and the BDR router.

Forsaken_GA

ehnde wrote: »

Alot of text is dedicated to the DR election process, but it seems like it should be obvious: within an Area that is not Area 0, your ABR would always be your DR. However, the Cisco OCG book talks about having multiple routers in the same subnet going through the DR election process. Why? Redundant ABRs? I'm confused.

You're confusing the purpose of an ABR and a DR.

an ABR is a router that has interfaces in more than one area, simple as that. It also injects type 3 LSA's into a given area from all the other areas it's connected to (in normal operation). And yes, you can have redundant ABR's, and they present some unique challenges to make sure you overcome suboptimal routing.

The DR is confined to a network segment. The above poster is completely correct in that a router could be DR on one interface, BDR on another, and DROTHER on still a third.

The DR/BDR act as traffic cops on the network segment. All routing updates occur to them, all routing updates come from them. If you were to put 4 or more OSPF routers in the same multiaccess segment, you'd notice that the two that aren't DR/BDR never reach a full state of neighbor adjacency, they stop and stay at 2Way. Only the DR/BDR form FULL adjacencies to every OSPF router on the segment.

Do not confuse DR/BDR with any specific OSPF router type (ie, ABR, ASBR). The functions are completely separate.

Forsaken_GA

instant000 wrote: »

Also, I've yet to find a credible source to define the DR or BDR as an interface, and even the RFC I can look up only say router, not interface. Maybe I made a mountain out of a molehill (looking up something on wikipedia a month ago, getting it absorbed into memory, then regurgitating it as fact later ... once I attempt to retrieve this information, I forgot where I got it from, and then pull it back up from memory as if it came from a good source. once I start searching for it, I wind up at a wikipedia article, LOL .... so sad, hahahahahaha)

Lesson learned: Don't trust your memory, unless it also includes sources.

You're right to a degree, but it's more implied than anything. The proper definition is a multiaccess segment. Ie, a given subnet.

However, since IOS does not allow you to address interfaces in such a way as to make subnets overlap, conceptually, subnet/interface are interchangeable.

instant000

Thanks Forsaken. Chapter 6/14 is next, still reviewing Chapter 5/14 at the moment, though. Like to make sure everything is clear in my head before proceeding to the next chapter.

I haven't scheduled any test yet, so no pressure to go any faster than learning pace, LOL.

I feel this stuff can help me in my day-to-day job, so it cannot hurt to make sure I actually learn it.

EDIT: Didn't specify which book I was in: The Todd Lammle CCNA, 6th Edition.

After completion of the text, I'll schedule the exam and then just start labbing and reviewing for the test. I don't want to schedule it until I know everything really well (and quickly)...time management seems to be a big theme here.

jwashington1981

I'm going to be taking the exam tomorrow. I'm hoping I pass it this time around. I took the exam last Saturday, and I missed earning the CCNA by a matter of a few points so I am hoping I get it this time around.

instant000

jwashington1981 wrote: »

I'm going to be taking the exam tomorrow. I'm hoping I pass it this time around. I took the exam last Saturday, and I missed earning the CCNA by a matter of a few points so I am hoping I get it this time around.

Good luck!