OSPF Flashcards
BGP
Border Gateway Protocol
OSPF
Open Shortest Path First
Open Shortest Path First
- Internal gateway protocol
- Used on primarily smaller scale network
- The fastest route is preferred over shortest
- IP protocol
Border Gateway Protocol
- External gateway protocol (only EGP used)
- Mostly used on large scale networks such as the internet
- Best path is determined for the datagram
- TCP Protocol
EIGRP
Enhanced Interior Gateway Routing Protocol
Routing protocol
A set of messages, rules, and algorithms used by routers for the overall purpose of learning routes. This process includes the exchange and analysis of routing information. Each router chooses the best route to each subnet (path selection) and finally places those best routes in its IP routing table. Examples include RIP, EIGRP, OSPF, and BGP.
Routed protocol and routable protocol
Both terms refer to a protocol that defines a packet structure and logical addressing, allowing routers to forward or route the packets. Routers forward packets defined by routed and routable protocols. Examples include IP Version 4 (IPv4) and IP Version 6 (IPv6).
Routing Protocol Functions
- Learn routing information about IP subnets from neighboring routers.
- Advertise routing information about IP subnets to neighboring routers.
- If more than one possible route exists to reach one subnet, pick the best route based on a metric.
- Convergence
Convergence
all the routers collectively realize something has changed in topology, advertise the information about the changes to all the other routers, and all the routers then choose the currently best routes for each subnet.
IGP
(interior gateway protocols)A routing protocol that was designed and intended for use inside a single autonomous system (AS)
EGP
(exterior gateway protocols)A routing protocol that was designed and intended for use between different autonomous systems
Autonomous system (AS)
network under the administrative control of a single organization
Routing protocol algorithm
Three branches of RPA exist for IGP routing protocols:
-Distance vector (DV) (sometimes called Bellman-Ford after its creators)
-Advanced distance vector (sometimes called “balanced hybrid”)
-Link-state (LS)
Routing Information Protocol (RIP)
first popularly used IP distance vector protocol
VLSM
variable-length subnet masks.
Route Redistribution
router can take routes learned by OSPF and advertise them into EIGRP, and vice versa
Administrative Distance
-A number that denotes how believable an entire routing protocol is on a single router.
-When IOS must choose between routes learned using different routing protocols
LSA & LSDB
Link-State Advertisements
Link-State Advertisement databases
Dijkstra Shortest Path First (SPF)
link-state protocols type of math algorithm
Three phases of how OSPF routers accomplish exchanging LSAs and calculating routes
- Becoming neighbors
- Exchanging databases
- Adding the best routes
Becoming neighbors
A relationship between two routers that connect to the same data link, created so that the neighboring routers have a means to exchange their LSDBs.
Exchanging databases
The process of sending LSAs to neighbors so that all routers learn the same LSAs.
Adding the best routes
The process of each router independently running SPF, on their local copy of the LSDB, calculating the best routes, and adding those to the IPv4 routing table.
OSPF Hello messages
-The Hello message follows the IP packet header, with IP protocol type 89.
-packets are sent to multicast IP address 224.0.0.5, a multicast IP address intended for all OSPF-speaking routers.
-OSPF routers listen for packets sent to IP multicast address 224.0.0.5, in part hoping to receive Hello packets and learn about new neighbors.
RID
Router ID
-32-bit numbers that are part of the Hello Message.
-Most command output lists these as dotted-decimal numbers (DDN).
-By default, IOS chooses one of the router’s interface IPv4 addresses to use as its OSPF RID.
-OSPF RID can be directly configured
Link-State Update (LSU) packet
OSPF messages that actually send the LSAs between neighbors
Routers monitor each neighbor relationship using two related timers:
Hello Interval and the Dead Interval
Three maintenance tasks for maintaining Neighbors and the LSDB
-Maintain neighbor state by sending Hello messages based on the Hello Interval and listening for Hellos before the Dead Interval expires
-Flood any changed LSAs to each neighbor
-Reflood unchanged LSAs as their lifetime expires (default 30 mins)
OSPF Ethernet network type
defaults to use a network type of broadcast, which causes OSPF to elect one of the routers on the same subnet to act as the designated router (DR).
“All SPF Routers” multicast address
“All SPF DRs” multicast address
224.0.0.5
224.0.0.6
DROthers
routers that are neither a DR nor a BDR
Problems with a single area design include the following:
- A larger topology database requires more memory on each router.
- The SPF algorithm requires processing power that grows exponentially compared to the size of the topology database.
- A single interface status change anywhere in the internetwork (up to down, or down to up) forces every router to run SPF again.
OSPF Areas
- Put all interfaces connected to the same subnet inside the same area.
- An area should be contiguous.
- Some routers may be internal to an area, with all interfaces assigned to that single area.
- Some routers may be Area Border Routers (ABR) because some interfaces connect to the backbone area, and some connect to non-backbone areas.
- All non-backbone areas must have a path to reach the backbone area (area 0) by having at least one ABR connected to both the backbone area and the non-backbone area.
Area Border Router (ABR)
An OSPF router with interfaces connected to the backbone area and to at least one other area
Backbone router
A router connected to the backbone area (includes ABRs)
Internal router
A router in one area (not the backbone area)
Area
A set of routers and links that shares the same detailed LSDB information, but not with routers in other areas, for better efficiency
Backbone Area
A special OSPF area to which all other areas must connect— area 0
Intra-area route
A route to a subnet inside the same area as the router
Interarea route
A route to a subnet in an area of which the router is not a part
Summary LSA
- Does not include topology
- List subnet Id and mask in other areas
Arguing for the use of multiple areas in larger OSPF networks
-Routers require fewer CPU cycles to process the smaller per-area LSDB with the SPF algorithm, reducing CPU overhead and improving convergence time.
- The smaller per-area LSDB requires less memory.
- Changes in the network (for example, links failing and recovering) require SPF calculations only on routers in the area where the link changed state, reducing the number of routers that must rerun SPF.
-Less information must be advertised between areas, reducing the bandwidth required to send LSAs.
DR & BDR
Designated router and Backup designated router
(OSPFv2) LSA
-One router LSA for each router in the area
-One network LSA for each network that has a DR plus one neighbor of the DR
-One summary LSA for each subnet ID that exists in a different area
