"Routing without tears; Bridging without danger".
Because there's no crying in routing!
Showing posts with label ospf. Show all posts
Showing posts with label ospf. Show all posts
Sunday, December 5, 2010
Wednesday, October 13, 2010
Stubby and NOT SO STUBBY.
OSPF uses the concept of "area" to describe different chunks of the network. The center of the network is "area 0", the backbone area, and it's assumed that all areas connect to area 0.
Router interfaces, not entire routers are part of an area. So a router can span more than one area -- for example, a router could have one interface in area 0 and another interface in area 1 and a third in area 199. Other routers in area 1 would then see the router as a path back to area 0.
Areas can be defined as "stubby", meaning that there's only one path out from the area, and so external routes don't get advertised to the stubby area. A "not so stubby area" (nssa) can receive intra-area routes, but no external routes.
Router interfaces, not entire routers are part of an area. So a router can span more than one area -- for example, a router could have one interface in area 0 and another interface in area 1 and a third in area 199. Other routers in area 1 would then see the router as a path back to area 0.
Areas can be defined as "stubby", meaning that there's only one path out from the area, and so external routes don't get advertised to the stubby area. A "not so stubby area" (nssa) can receive intra-area routes, but no external routes.
Sunday, October 10, 2010
Who's the boss? (OSPF edition)
So, a router interface that's participating in OSPF floods the network with a HELO packet -- this is sent to a multicast address, so if you've got acl's in play make sure they allow the OSPF multicast. One of the things the HELO traffic does, besides just announcing "hey, I'm here" to any likely devices that might also be looking for neighbors, is share out the information used for election of the "Designated Router" (DR) and "Backup Designated Router" (BDR).
The DR and BDR serve as points of contact for exchange of routing information -- instead of each router updated every other OSPF neighbor with link state announcements, all the devices update the DR and BDR and they send out link state updates to all of the OSPF neighbors. This reduces the complexity of the exchange of routing information.
DR and BDR election is done by OSPF priority. If two interfaces on a given network segment have the same priority, the higher Router ID is used as a tie-breaker. For any given OSPF device, the "Router ID" is the highest IP address on the box (including loopback interfaces). Specific router interfaces may have an OSPF priority set explicitly as well to adjust whether or not they become DR; a priority of 0 means that the interface should never be the DR or BDR; this is described as the state "DROTHER"
The DR and BDR serve as points of contact for exchange of routing information -- instead of each router updated every other OSPF neighbor with link state announcements, all the devices update the DR and BDR and they send out link state updates to all of the OSPF neighbors. This reduces the complexity of the exchange of routing information.
DR and BDR election is done by OSPF priority. If two interfaces on a given network segment have the same priority, the higher Router ID is used as a tie-breaker. For any given OSPF device, the "Router ID" is the highest IP address on the box (including loopback interfaces). Specific router interfaces may have an OSPF priority set explicitly as well to adjust whether or not they become DR; a priority of 0 means that the interface should never be the DR or BDR; this is described as the state "DROTHER"
Sunday, September 26, 2010
OSPF types of routes
If you look at an OSPF routing table ("sh ip route ospf", for instance), there are a couple of different types of routes that might be displayed.
An External route is one that is learned from another routing protocol and redistributed into OSPF. Type E1 is an external route that includes the path metrics from the redistribution other routing protocol and the cost to get to the router that's doing the redistributing. Type E2 is an external route that only includes the cost from the redistributed routing protocol and does not pay attention to the cost to get to the border router that is doing the redistributing.
- "O" -- An OSPF route from within the same OSPF area.
- "IA" -- An OSPF route from a different OSPF area.
- "E1" -- An OSPF route that is a Type 1 external route.
- "E2" -- an OSPF route that is a Type 2 external route.
An External route is one that is learned from another routing protocol and redistributed into OSPF. Type E1 is an external route that includes the path metrics from the redistribution other routing protocol and the cost to get to the router that's doing the redistributing. Type E2 is an external route that only includes the cost from the redistributed routing protocol and does not pay attention to the cost to get to the border router that is doing the redistributing.
Wednesday, September 22, 2010
Open Most Complicated Routing Protocol First
OSPF! Open Shortest Path First. It's an interior gateway routing protocol - you'd use OSPF within a given AS, for instance. It has a whole bunch of things that can be tweaked. Most people (in my experience) use it as their interior routing protocol.
OSPF is based around the idea of a link state diagram -- that is, a diagram of the network connections between all the routers in a given OSPF instance. When making routing decisions, a given router will figure out what the best ("shortest") next-hop is for a given destination. If left untweaked, the route metric will be based on the number of hops and the bandwidth of each link (lower-bandwidth links are less preferred). You can adjust that and manually weight routes to encourage traffic to go over a path that it would otherwise not prefer.
An OSPF-enabled router uses broadcast packets to announce itself and to find out about other OSPF routers in the same collision domain. If compatible, the routers will establish a neighbor relationship, and exchange route information. Among neighbors, there's a designated router and a designated backup router. The designated router is a given router interface that is in charge of sending out link state announcements and letting the other router interfaces know what's up with the links. It's a specific interface, not a specific individual router -- a router that is a DR on interface 1 might not be the DR for a separate OSPF instance on interface 2.
OSPF is based around the idea of a link state diagram -- that is, a diagram of the network connections between all the routers in a given OSPF instance. When making routing decisions, a given router will figure out what the best ("shortest") next-hop is for a given destination. If left untweaked, the route metric will be based on the number of hops and the bandwidth of each link (lower-bandwidth links are less preferred). You can adjust that and manually weight routes to encourage traffic to go over a path that it would otherwise not prefer.
An OSPF-enabled router uses broadcast packets to announce itself and to find out about other OSPF routers in the same collision domain. If compatible, the routers will establish a neighbor relationship, and exchange route information. Among neighbors, there's a designated router and a designated backup router. The designated router is a given router interface that is in charge of sending out link state announcements and letting the other router interfaces know what's up with the links. It's a specific interface, not a specific individual router -- a router that is a DR on interface 1 might not be the DR for a separate OSPF instance on interface 2.
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About Me
- regis
- Regis has worked as a network engineer since 1994 for small companies and for large companies.
