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12 votes
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OSPF Election with a /30 link?

I have a few /30 subnets connecting OSPF routers over a broadcast capable link.. Should I also configure the interfaces involved in the link to ip ospf network point-to-point? Or does it matter? The ...
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12 votes
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Why does the MTU need to match on interior link state routing protocols like EIGRP and OSPF?

Pete said: I can't think of a situation where you would need this. I just wanted to know what the logic was that makes this an explicit check in these routing protocols. Short answer Routing ...
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11 votes
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What is the difference between broadcasting and flooding?

Broadcasting is a term that is used on a broadcast domain, which is bounded by layer-3 (routers). Broadcasts are sent to a special broadcast address, both for layer-2 and layer-3. A broadcast cannot ...
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10 votes

OSPF or iBGP for interior routing protocol?

iBGP requires a full mesh or use of mitigation like confederations or route reflectors, BGP doesn't converge with anything like the speed of OSPF, etc. Each OSPF router would have a full ...
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10 votes
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OSPF without Area 0

There is only one way to use OSPF without an Area 0, and that is to use OSPF with a single area. If you only have one OSPF area, you can number it any way you like, but if you have even two areas, you ...
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9 votes

Why must all areas connect to an Area 0 backbone in OSPF?

From "OSPF: Anatomy of a Routing Protocol" by John Moy, who wrote much of the OSPF specification. The exchange of routing information between areas is essentially Distance Vector. With Distance ...
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9 votes
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Do any routing protocols take link capacity into account?

Do there exist any routing protocols which take link capacities and loads into account? There are two ways to handle the issues you describe: MPLS TE w/ RSVP (complicated) EIGRP (easier, but isn't ...
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9 votes

OSPF or iBGP for interior routing protocol?

First off, I would dispute the assertion that some others have made that OSPF converges faster than BGP, but more on that later. To answer the OP question first. You want both. iBGP is designed to ...
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9 votes
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Why does MPLS need an IGP (like OSPF) in order to work?

Why does MPLS need an IGP (like OSPF) in order to work? In an MPLS network, OSPF is not used to route customer traffic. It is only used to provide routing for the internal provider network so that ...
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9 votes

OSPF and its use of Multicast

The benefit of multicast is that hosts not subscribing to the multicast group ignore frames they receive that are sent to the multicast group. With broadcast, they cannot ignore the frames. The OSPF ...
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8 votes
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Cisco IOS: OSPFv3 route cost

I believe you are looking for show ipv6 ospf rib. Almost all show ip * commands have a show ipv6 equivalent.
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8 votes

What is the difference between broadcasting and flooding?

I will explain here in the context of switches(Layer 2) FLOODING : When a switch receives a unicast frame(a frame with a specific mac address intended for a particular device), it looks for the frame'...
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8 votes
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OSPF Configuration: Router ID won't change to loopback address

Simply clearing the process may not be enough to re-start the RID selection process. That said, as Ron points out in the comments, it is by far best practice to manually specify the router ID in OSPF ...
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8 votes
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Configuring OSPF on two directly connected routers which are in different areas

In order to connect two areas, one router has to serve as an area border router (ABR). That router has interfaces in both areas. In other words, the boundary between two areas is a router, not the ...
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8 votes
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OSPF increase bandwidth with load-balancing

As these are Point-to-Point links, I would consider using the outage to configure each /30 interface with ip ospf network point-to-point. (New and existing links). This reduces the hello and dead ...
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  • 1,241
8 votes

OSPF increase bandwidth with load-balancing

There's two ways to do that. your proposed way, by adding a second link with its own /30 or /31, making sure that OSPF installs multiple equal cost routes in the routing table, and let CEF's ECMP (...
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7 votes

OPEN SHORTEST PATH FIRST(OSPF) ROUTER-ID

By default, the highest ip address in the router loopbacks is chosen as the router id. It's smarter to force the router to choose a router id that I want instead of letting it choose for me. OSPF ...
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  • 326
7 votes
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Why do we use OSPF Point-to-Point Networks for Loopbacks?

If we create a loopback and give classful or classless addresses, then by default the route to that loop back is advertised as the most specific route: /32 prefix and it will ignore any configured ...
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7 votes
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DR selection on OSPF

Let me clarify a misunderstanding: The DR is responsible for updating the LSDBs of the other routers on the network segment. It does not receive all the network traffic. All the non-DR routers ...
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7 votes
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When would we use more than 1 dynamic routing protocol?

The administrative distance isn't about "trustworthiness", it's about preference - a combination of cost, delay, bandwidth, resilience, loss rate. External routing and internal routing ...
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6 votes
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How do I setup cisco router not to use the default route to look for internal networks that are down?

In case that the connection to any of the internal routers/networks are not available I want the core router not to use the default route to look for any of the internal networks. Configure a null0 ...
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6 votes
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Why doesn't my OSPF config work?

There is a typo on router 2, the IP address of serial0/0/0 is incorrect: ip address 192.158.28.206 255.255.255.252 158 instead of 168...
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6 votes

Why do we use OSPF Point-to-Point Networks for Loopbacks?

By using point-to-point command you will be overriding the default categorization of OSPF. Per RFC 2328, OSPF only supports following types of links: Link type Description ...
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  • 352
6 votes
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OSPF route filtering on Vyatta device

Every OSPF router in an area has a full topology map so it can calculate the shortest path to every network. If you attempt to filter routes, you break the router's ability to calculate the path. ...
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6 votes
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OSPF backbone area design

The idea behind "L3 to the access layer" is to avoid blocked links that would occur when STP blocks L2 loops, and allow multiple paths for increased bandwidth. In your second drawing (assuming the ...
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6 votes
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2-way state in OSPF

Three. Router A sends a hello to router B. Router B lists A as Init Router B sends a hello to A, listing A as its neighbor. Router A lists B as 2-way Router A sends a hello to B listing B as its ...
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  • 63k
6 votes
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Whats's the function of a loopback interface in routing protocols?

The loopback interface is useful because it is an interface with an IP address which never goes down. OSPF, without a specifically defined Router ID, will pick a Router ID on its own. It chooses the ...
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6 votes
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When/Why to use OSPF?

Routing protocols, such as OSPF, are to share routes between routers. If your routing is done on a single router, or a pair of routers connected to the same networks (your layer-3 switches), it doesn'...
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6 votes
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What happens when a router receives an LSA with the same sequence nr?

Found the answer to my own question. When a router encounters two instances of an LSA, it must determine which is more recent. An LSA is identified by its LS type, ID, and LSA router. For two ...
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  • 181
6 votes
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OSPF configuration in MESH topology

The area design you have specified will not work. The interface and its adjacent interface need to be in the same area. You can't have one interface in area0 and the adjacent router interface in area1....
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