Specifications

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Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, Release 5.0(3)U2(2)
OL-25782-02
Chapter 1 Overview
Routing Algorithms
Distance Vector Protocols
Distance vector protocols use distance vector algorithms (also known as Bellman-Ford algorithms) that
call for each router to send all or some portion of its routing table to its neighbors. Distance vector
algorithms define routes by distance (for example, the number of hops to the destination) and direction
(for example, the next-hop router). These routes are then broadcast to the directly connected neighbor
routers. Each router uses these updates to verify and update the routing tables.
To prevent routing loops, most distance vector algorithms use split horizon with poison reverse which
means that the routes learned from an interface are set as unreachable and advertised back along the
interface that they were learned on during the next periodic update. This feature prevents the router from
seeing its own route updates coming back.
Distance vector algorithms send updates at fixed intervals but can also send updates in response to
changes in route metric values. These triggered updates can speed up the route convergence time. The
Routing Information Protocol (RIP) is a distance vector protocol.
Link-State Protocols
The link-state protocols, also known as shortest path first (SPF), share information with neighboring
routers. Each router builds a link-state advertisement (LSA), which contains information about each link
and directly connected neighbor router.
Each LSA has a sequence number. When a router receives and LSA and updates its link-state database,
the LSA is flooded to all adjacent neighbors. If a router receives two LSAs with the same sequence
number (from the same router), the router does not flood the last LSA received to its neighbors to prevent
an LSA update loop. Because the router floods the LSAs immediately after they receive them,
convergence time for link-state protocols is minimized.
Discovering neighbors and establishing adjacency is an important part of a link state protocol. Neighbors
are discovered using special Hello packets that also serve as keepalive notifications to each neighbor
router. Adjacency is the establishment of a common set of operating parameters for the link-state
protocol between neighbor routers.
The LSAs received by a router are added to its link-state database. Each entry consists of the following
parameters:
• Router ID (for the router that originated the LSA)
• Neighbor ID
• Link cost
• Sequence number of the LSA
• Age of the LSA entry
The router runs the SPF algorithm on the link-state database, building the shortest path tree for that
router. This SPF tree is used to populate the routing table.
In link-state algorithms, each router builds a picture of the entire network in its routing tables. The
link-state algorithms send small updates everywhere, while distance vector algorithms send larger
updates only to neighboring routers.
Because they converge more quickly, link-state algorithms are somewhat less prone to routing loops than
distance vector algorithms. However, link-state algorithms require more CPU power and memory than
distance vector algorithms. Link-state algorithms can be more expensive to implement and support.
Link-state protocols are generally more scalable than distance vector protocols.