Hot answers tagged

40

OSPF Backbone Why is area 0 the backbone area in OSPF? Why must all other areas connect to it? This is explained very well in RFC 3509, Section 1.21: 1.2 Motivation In OSPF domains the area topology is restricted so that there must be a backbone area (area 0) and all other areas must have either physical or virtual connections to the backbone....


26

Can anyone explain me what is the need of IBGP communication for the routes, when we have the IGP protocols (OSPF, RIP) for internal communication? Scalability1: Imagine that you're receiving 500,000 EBGP routes in more than one location2, and you need to influence the per route exit point in your AS. BGP can handle many more routes than IGP protocols. ...


21

Most of the references say "RIP is not scalable" hence can be used only in smaller networks. But none say "WHY?" What is that in RIP that actually preventing it from scaling to larger networks? And HOW OSPF overcomes the disadvantage of RIP? Summary RIPv1 floods routes frequently (every 30 seconds), which introduces large CPU loads as ...


19

Area 0 was chosen to be the backbone, the number 0 or 0.0.0.0 is just a number. Because OSPF is link state the LSDB must be identical within an area. This is to ensure that the SPF calculation is consistent and to prevent routing loops. If running single area OSPF any area can be used, there is no requirement to have area 0. When running multi area OSPF, ...


16

It's important to note that type 2 LSA's are only generated on segments where a DR/BDR has been elected - this includes BMA (Broadcast Multi-Access) and NBMA (Non-Broadcast Multi-Access) networks. The DR is what generates the type 2 LSA. This behavior can be bypassed by configuring your Ethernet interfaces that you're choosing to run OSPF on as point-to-...


16

This question is akin to asking, "Why can't I use a screwdriver to hammer nails?" You could use OSPF in certain inter-AS situations if you REALLY, REALLY wanted to/needed to. There would be many "gotchas" and caveats to keep in mind when utilizing OSPF as an EGP. Just like you could hammer nails with a screwdriver. However, you would risk breaking the ...


14

All routers within an OSPF area keep a link state database (note that this is completely separate from the main routing table), where they're aware of all other routers and their links within the area. Each router within the area builds a topology tree of the area, with shortest paths to all other links/routers with itself as the root. This last part is ...


12

If you're asking specifically what Areas do in OSPF, there are many resources out there which have a wealth of information. These are just a few selections: The Cisco OSPF Design Guide A Cisco Design Technote What Are OSPF Areas and Virtual Links? PacketLife.net blog entry on OSPF Area Types However, a good answer to "What is the advantage of OSPF Areas?",...


12

Your loopback 100 address is 172.168.100.0/24, but your network statement is 172.16.100.0/24, so Loopback 100 is not in the OSPF domain.


12

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 network type definitely matters, but you need to be sharp about handling this situation... there are some corner cases to consider. Let's compare the ...


12

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 protocols are some of the most fundamental building blocks on the internet; we need them to be very reliable in every possible case. It does no good to bring up an ...


11

OSPF is a link state protocol that uses multicast. It sends Link State Advertisements (LSAs) that are flooded. Every router builds a tree by running SPF where the router itself is the root of the tree. In OSPF we have transit networks and stub networks. Transit networks are networks that are used for transit to reach networks while stub networks are the ...


11

... The load balancing works when I replace the MPLS by ATM switches. This doesn't sound like a parallel comparison, because routers don't care whether you're forwarding through an ATM PVC or an MPLS LSP, they will load-balance in the same way, assuming the routers have the same configuration. Perhaps you've done something unusual with your ATM VCs, but ...


11

A Tale of Three Stubs... When using Cisco equipment, OSPF calls several things a "stub"... it gets a bit confusing.... Stub Host Stub Network Stub Area The meanings are very different for each term; even more confusing is the reality that you can potentially apply multiple stub terms to a single OSPF interface. I'll elaborate more below. ...


10

IGP usually is OSPF or ISIS which are link-state based, this gives us all the information of the network, everyone knows network from everyone's point-of-view, which allows for very interesting convergence options and traffic engineering options. BGP is essentially distance-vector, it knows very limited view on the network at whole. BGP handles very well ...


10

Nothing prevents you from using OSPF to connect two separate networks. When companies merge this happens quite often. Is it a good idea? Probably not. Why? Scalability: OSPF needs a lot more resources (by design) than BGP. Every router needs a full OSPF database (ignoring features like stubby areas) and the SPF calculations need lots of CPU time. Filtering: ...


10

Null 0 is a virtual interface that’s used to guarantee routes make it into the RIB; remember, routes must have a next-hop in order to make it into the RIB. When a packet arrives at a device that has a route to Null 0 and nowhere else, it will discard it similar to how an ACL would. Think of this as the virtual interface where packets go to die efficiently. ...


10

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 understanding of all the routes that are in the area in which it resides without needing a full mesh, and it converges very, very quickly. Using an IGP is recommended with ...


9

The short answer is because ZEUS (which is summarizing) is summarizing for Area 2, but it also shares Area 2 with WODAN. OSPF requires all routers in an area to have the same exact copy of the area's link-state database; OSPF ABRs run a separate SPF algorithm calculation for each area they participate in. Quoting RFC 2328 - Open Shortest Path First: ... ......


9

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 Vector, the larger number of redundant paths you have the worse your convergence properties get. OSPF requires all areas to attach directly to the backbone so it ...


9

It drops all the traffic that doesn't have a more specific route. It is used to prevent routing loops and to put routes in the routing table permanently so they can be announced with BGP.


9

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 so good out of the box) MPLS TE w/ RSVP: MPLS Traffic Engineering is not quite a routing protocol, but it adds the capability for OSPF or ISIS to calculate ...


9

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 run between loopback interfaces on routers and it doesn't (at least without tweaking and turning some knobs) change any attributes (which includes next-hop) of ...


9

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 cross a layer-3 device, and every host in a broadcast domain must be interrupted and inspect a broadcast. Flooding is used by a switch at layer-2 to send ...


9

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 multicast group is a link-local multicast that gets sent to every switch interface, even with IGMP snooping enabled on the switch. OSPF only uses multicast on ...


8

This will work but it would be a poor design choice (IMHO) unless you have a specific reason for doing so. See this discussion: Duplicate OSPF area IDs OSPF Best Practicies Recommended OSPF configuration best-practices (using the SSM example) OSPF area configuration best practices Update #1: For the sake of completeness, I mocked up this situation using ...


8

In a nutshell, the designated router is the one responsible for synchronizing the LS databases of all routers on the network and flooding LS updates to all other routers. If one of the spokes is the designated router, then all database synchronization and LS update traffic must flow down the spoke link to the DR and then back to the hub and then to the ...


8

Opaque LSA's are used to extend the capabilities of OSPF, and to allow for transmission of arbitrary data that OSPF does not necessarily need to care about. For example, if you were writing an application and you decided you wanted to use OSPF to transport the application's data but you did not want OSPF to use this data for its own purposes, ie route ...


8

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 prefix. Eg: interface Loopback0 ip address 2.2.2.2 255.255.255.0 Here, the loopback network address is 2.2.2.0/24. By default OSPF will advertise this route to ...


8

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


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