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Layer 3 unicast routing involves two basic activities:
determining optimal routing paths
packet switching
You can use routing algorithms to calculate the optimal path from the router to a destination. This calculation depends on
the algorithm selected, route metrics, and other considerations such as load balancing and alternate path discovery.
Routing Fundamentals
Routing protocols use a metric to evaluate the best path to the destination. A metric is a standard of measurement, such as
a path bandwidth, that routing algorithms use to determine the optimal path to a destination. To aid path determination, routing
algorithms initialize and maintain routing tables that contain route information such as the IP destination address, the address
of the next router, or the next hop. Destination and next-hop associations tell a router that an IP destination can be reached
optimally by sending the packet to a particular router that represents the next hop on the way to the final destination. When
a router receives an incoming packet, it checks the destination address and attempts to associate this address with the next
hop. See the Unicast RIB section for more information about the route table.
Routing tables can contain other information, such as the data about the desirability of a path. Routers compare metrics to
determine optimal routes, and these metrics differ depending on the design of the routing algorithm used. See the Routing Metrics section.
Routers communicate with one another and maintain their routing tables by transmitting a variety of messages. The routing
update message is one such message that consists of all or a portion of a routing table. By analyzing routing updates from
all other routers, a router can build a detailed picture of the network topology. A link-state advertisement, which is another
example of a message sent between routers, informs other routers of the link state of the sending router. You can also use
link information to enable routers to determine optimal routes to network destinations. For more information, see the Routing Algorithms section.
Packet Switching
In packet switching, a host determines that it must send a packet to another host. After acquiring a router address by some
means, the source host sends a packet that is addressed specifically to the router physical (Media Access Control [MAC]-layer)
address but with the IP (network layer) address of the destination host.
The router examines the destination IP address and tries to find the IP address in the routing table. If the router does not
know how to forward the packet, it drops the packet. If the router knows how to forward the packet, it changes the destination
MAC address to the MAC address of the next-hop router and transmits the packet.
The next hop might be the ultimate destination host or another router that executes the same switching decision process. As
the packet moves through the internetwork, its physical address changes, but its protocol address remains constant (see the
following figure).
Figure 1. Packet Header Updates Through a Network
Routing
Metrics
Routing algorithms use
many different metrics to determine the best route. Sophisticated routing
algorithms can base route selection on multiple metrics.
Path Length
The path length is the most common routing metric. Some routing protocols allow you to assign arbitrary costs to each network
link. In this case, the path length is the sum of the costs associated with each link traversed. Other routing protocols define
the hop count, which is a metric that specifies the number of passes through internetworking products, such as routers, that
a packet must take from a source to a destination.
Reliability
The reliability, in
the context of routing algorithms, is the dependability (in terms of the
bit-error rate) of each network link. Some network links might go down more
often than others. After a network fails, certain network links might be
repaired more easily or more quickly than other links. The reliability factors
that you can take into account when assigning the reliability rating are
arbitrary numeric values that you usually assign to network links.
Routing
Delay
The routing delay is
the length of time required to move a packet from a source to a destination
through the internetwork. The delay depends on many factors, including the
bandwidth of intermediate network links, the port queues at each router along
the way, the network congestion on all intermediate network links, and the
physical distance that the packet must travel. Because the routing delay is a
combination of several important variables, it is a common and useful metric.
Bandwidth
The bandwidth is the available traffic capacity of a link. For example, a 10-Gigabit Ethernet link is preferable to a 1-Gigabit
Ethernet link. Although the bandwidth is the maximum attainable throughput on a link, routes through links with greater bandwidth
do not necessarily provide better routes than routes through slower links. For example, if a faster link is busier, the actual
time required to send a packet to the destination could be greater.
Load
The load is the degree to which a network resource, such as a router, is busy. You can calculate the load in a variety of
ways, including CPU usage and packets processed per second. Monitoring these parameters on a continual basis can be resource
intensive.
Communication
Cost
The communication cost
is a measure of the operating cost to route over a link. The communication cost
is another important metric, especially if you do not care about performance as
much as operating expenditures. For example, the line delay for a private line
might be longer than a public line, but you can send packets over your private
line rather than through the public lines that cost money for usage time.
Router IDs
Each routing process
has an associated router ID. You can configure the router ID to any interface
in the system. If you do not configure the router ID,
Cisco NX-OS
selects the router ID based on the following criteria:
Cisco NX-OS
prefers loopback0 over any other interface. If loopback0 does not exist, then
Cisco NX-OS
prefers the first loopback interface over any other interface type.
If you have not
configured a loopback interface,
Cisco NX-OS
uses the first interface in the configuration file as the router ID. If you
configure any loopback interface after
Cisco NX-OS
selects the router ID, the loopback interface becomes the router ID. If the
loopback interface is not loopback0 and you configure loopback0 with an IP
address, the router ID changes to the IP address of loopback0.
If the interface
that the router ID is based on changes, that new IP address becomes the router
ID. If any other interface changes its IP address, there is no router ID
change.
Autonomous
Systems
An autonomous system (AS) is a network controlled by a single technical administration entity. Autonomous systems divide global
external networks into individual routing domains, where local routing policies are applied. This organization simplifies
routing domain administration and simplifies consistent policy configuration.
Each autonomous system can support multiple interior routing protocols that dynamically exchange routing information through
route redistribution. The Regional Internet Registries (RIR) assign a unique number to each public autonomous system that
directly connects to the Internet. This autonomous system number (AS number) identifies both the routing process and the autonomous
system.
The Border Gateway Protocol (BGP) supports 4-byte AS numbers that can be represented in asplain and asdot notations:
asplain—A decimal value notation where both 2-byte and 4-byte AS numbers are represented by their decimal value. For example,
65526 is a 2-byte AS number, and 234567 is a 4-byte AS number.
asdot—An AS dot notation where 2-byte AS numbers are represented by their decimal value and 4-byte AS numbers are represented
by a dot notation. For example, 2-byte AS number 65526 is represented as 65526, and 4-byte AS number 65546 is represented
as 1.10.
The BGP 4-byte AS number capability is used to propagate 4-byte-based AS path information across BGP speakers that do not
support 4-byte AS numbers.
Note
RFC 5396 is partially supported. The asplain and asdot notations are supported, but the asdot+ notation is not.
Private autonomous system numbers are used for internal routing domains but must be translated by the router for traffic that
is routed out to the Internet. You should not configure routing protocols to advertise private autonomous system numbers to
external networks. By default, Cisco NX-OS does not remove private autonomous system numbers from routing updates.
Note
The autonomous system number assignment for public and private networks is governed by the Internet Assigned Number Authority
(IANA). For information about autonomous system numbers, including the reserved number assignment, or to apply to register
an autonomous system number, see this URL:http://www.iana.org/
Convergence
A key aspect to
measure for any routing algorithm is how much time a router takes to react to
network topology changes. When a part of the network changes for any reason,
such as a link failure, the routing information in different routers might not
match. Some routers will have updated information about the changed topology,
while other routers will still have the old information. The convergence is the
amount of time before all routers in the network have updated, matching routing
information. The convergence time varies depending on the routing algorithm.
Fast convergence minimizes the chance of lost packets caused by inaccurate
routing information.
Load Balancing and
Equal Cost Multipath
Routing protocols can use load balancing or equal cost multipath (ECMP) to share traffic across multiple paths. When a router
learns multiple routes to a specific network, it installs the route with the lowest administrative distance in the routing
table. If the router receives and installs multiple paths with the same administrative distance and cost to a destination,
load balancing can occur. Load balancing distributes the traffic across all the paths, sharing the load. The number of paths
used is limited by the number of entries that the routing protocol puts in the routing table. For the number of ECMP paths
supported by each routing protocol, see the Cisco Nexus 9000 Series NX-OS Verified Scalability Guide.
Note
ECMP does not guarantee equal load-balancing across all links. It guarantees only that a particular flow will choose one particular
next hop at any point in time.
Route Redistribution Overview
If you have multiple routing protocols configured in your network, you can configure these protocols to share routing information
by configuring route redistribution in each protocol. For example, you can configure the Open Shortest Path First (OSPF) protocol
to advertise routes learned from the Border Gateway Protocol (BGP). You can also redistribute static routes into any dynamic
routing protocol. The router that is redistributing routes from another protocol sets a fixed route metric for those redistributed
routes, which prevents incompatible route metrics between the different routing protocols. For example, routes redistributed
from EIGRP into OSPF are assigned a fixed link cost metric that OSPF understands.
Note
You are required to use route maps when you configure the redistribution of routing information.
Route redistribution also uses an administrative distance (see the Administrative Distance section) to distinguish between routes learned from two different routing protocols. The preferred routing protocol is given
a lower administrative distance so that its routes are picked over routes from another protocol with a higher administrative
distance assigned.
Administrative
Distance
An administrative
distance is a rating of the trustworthiness of a routing information source. A
higher value indicates a lower trust rating. Typically, a route can be learned
through more than one protocol. Administrative distance is used to discriminate
between routes learned from more than one protocol. The route with the lowest
administrative distance is installed in the IP routing table.
Stub Routing
You can use stub routing in a hub-and-spoke network topology, where one or more end (stub) networks are connected to a remote
router (the spoke) that is connected to one or more distribution routers (the hub). The remote router is adjacent only to
one or more distribution routers. The only route for IP traffic to follow into the remote router is through a distribution
router. This type of configuration is commonly used in WAN topologies in which the distribution router is directly connected
to a WAN. The distribution router can be connected to many more remote routers. Often, the distribution router is connected
to 100 or more remote routers. In a hub-and-spoke topology, the remote router must forward all nonlocal traffic to a distribution
router, so it becomes unnecessary for the remote router to hold a complete routing table. Generally, the distribution router
sends only a default route to the remote router.
Only specified routes are propagated from the remote (stub) router. The stub router responds to all queries for summaries,
connected routes, redistributed static routes, external routes, and internal routes with the message “inaccessible.” A router
that is configured as a stub sends a special peer information packet to all neighboring routers to report its status as a
stub router.
Any neighbor that receives a packet that informs it of the stub status does not query the stub router for any routes, and
a router that has a stub peer does not query that peer. The stub router depends on the distribution router to send the proper
updates to all peers.
The following figure shows a simple hub-and-spoke configuration.
Figure 2. Simple
Hub-and-Spoke Network
Stub routing does not prevent routes from being advertised to the remote router. The figure Simple Hub-and-Spoke Network shows that the remote router can access the corporate network and the Internet through the distribution router only. A full
route table on the remote router, in this example, serves no functional purpose because the path to the corporate network
and the Internet is always through the distribution router. A larger route table reduces only the amount of memory required
by the remote router. The bandwidth and memory used can be lessened by summarizing and filtering routes in the distribution
router. In this network topology, the remote router does not need to receive routes that have been learned from other networks
because the remote router must send all nonlocal traffic, regardless of its destination, to the distribution router. To configure
a true stub network, you should configure the distribution router to send only a default route to the remote router.
OSPF supports stub areas, and the Enhanced Interior Gateway Routing Protocol (EIGRP) supports stub routers.
Note
The EIGRP stub routing feature should be used only on stub devices. A stub device is defined as a device connected to the
network core or distribution layer through which core transit traffic should not flow. The only route for IP traffic to follow
into the remote router is through a distribution router. A stub device should not have any EIGRP neighbors other than distribution
devices. Ignoring this restriction will cause undesirable behavior.
Routing
Algorithms
Routing algorithms
determine how a router gathers and reports reachability information, how it
deals with topology changes, and how it determines the optimal route to a
destination. Various types of routing algorithms exist, and each algorithm has
a different impact on network and router resources. Routing algorithms use a
variety of metrics that affect calculation of optimal routes. You can classify
routing algorithms by type, such as static or dynamic, and interior or
exterior.
Static Routes and Dynamic Routing Protocols
Static routes are route table entries that you manually configure. These static routes do not change unless you reconfigure
them. Static routes are simple to design and work well in environments where network traffic is relatively predictable and
where network design is relatively simple.
Because static routing systems cannot react to network changes, you should not use them for large, constantly changing networks.
Most routing protocols today use dynamic routing algorithms that adjust to changing network circumstances by analyzing incoming
routing update messages. If the message indicates that a network change has occurred, the routing software recalculates routes
and sends out new routing update messages. These messages permeate the network, triggering routers to rerun their algorithms
and change their routing tables accordingly.
You can supplement dynamic routing algorithms with static routes where appropriate. For example, you should configure each
subnetwork with a static route to the IP default gateway or router of last resort (a router to which all unrouteable packets
are sent).
Interior and Exterior Gateway Protocols
You can separate networks into unique routing domains or autonomous systems. An autonomous system is a portion of an internetwork
under common administrative authority that is regulated by a particular set of administrative guidelines. Routing protocols
that route between autonomous systems are called exterior gateway protocols or interdomain protocols. The Border Gateway Protocol
(BGP) is an example of an exterior gateway protocol. Routing protocols used within an autonomous system are called interior
gateway protocols or intradomain protocols. EIGRP and OSPF are examples of interior gateway protocols.
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 process 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) that contains information about each link and directly connected neighbor router.
Each LSA has a sequence number. When a router receives an 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 that it received to its neighbors because it wants to prevent an LSA update loop. Because the router floods
the LSAs immediately after it receives them, the 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 the router's 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
less likely to cause routing loops than distance vector algorithms.
However, link-state algorithms require more CPU power and memory
than distance vector algorithms and they can be more expensive to
implement and support. Link-state protocols are generally more
scalable than distance vector protocols.
OSPF is an example of a link-state protocol.
Layer 3
Virtualization
Cisco NX-OS supports multiple virtual routing and forwarding (VRF) instances and multiple Routing Information Bases (RIBs)
to support multiple address domains. Each VRF is associated with a RIB, and this information is collected by the Forwarding
Information Base (FIB). A VRF represents a Layer 3 addressing domain. Each Layer 3 interface (logical or physical) belongs
to one VRF. For more information, see Configuring Layer 3 Virtualization.
Cisco NX-OS can segment operating system and hardware resources into virtual device contexts (VDCs) that emulate virtual devices.
The Cisco Nexus 9000 Series switches currently do not support multiple VDCs. All switch resources are managed in the default
VDC.
Cisco NX-OS
Forwarding Architecture
The Cisco NX-OS forwarding architecture is responsible for processing all routing updates and populating the forwarding information
to all modules in the chassis.
Unicast RIB
The
Cisco NX-OS
forwarding architecture consists of multiple components, as shown in the
following figure.
Figure 3. Cisco NX-OS
Forwarding Architecture
The unicast RIB exists
on the active supervisor. It maintains the routing table with directly
connected routes, static routes, and routes learned from dynamic unicast
routing protocols. The unicast RIB also collects adjacency information from
sources such as the Address Resolution Protocol (ARP). The unicast RIB
determines the best next hop for a given route and populates the FIB by using
the services of the unicast FIB Distribution Module (FDM).
Each dynamic routing
protocol must update the unicast RIB for any route that has timed out. The
unicast RIB then deletes that route and recalculates the best next hop for that
route (if an alternate path is available).
Adjacency Manager
The adjacency manager exists on the active supervisor and maintains adjacency information for different protocols including
ARP, Neighbor Discovery Protocol (NDP), and static configuration. The most basic adjacency information is the Layer 3 to Layer
2 address mapping discovered by these protocols. Outgoing Layer 2 packets use the adjacency information to complete the Layer
2 header.
The adjacency manager can trigger ARP requests to find a particular Layer 3 to Layer 2 mapping. The new mapping becomes available
when the corresponding ARP reply is received and processed. For IPv6, the adjacency manager finds the Layer 3 to Layer 2 mapping
information from NDP. For more information, see IPv6 Addresses.
Unicast Forwarding
Distribution Module
The unicast Forwarding Distribution Module (FDM) exists on the active supervisor and distributes the forwarding path information
from the unicast RIB and other sources. The unicast RIB generates forwarding information that the unicast FIB programs into
the hardware forwarding tables on the standby supervisor and the modules. The unicast FDM also downloads the FIB information
to newly inserted modules.
The unicast FDM gathers adjacency information, rewrite information, and other platform-dependent information when updating
routes in the unicast FIB. The adjacency and rewrite information consists of interface, next hop, and Layer 3 to Layer 2 mapping
information. The interface and next-hop information is received in route updates from the unicast RIB. The Layer 3 to Layer
2 mapping is received from the adjacency manager.
FIB
The unicast FIB exists
on supervisors and switching modules and builds the information used for the
hardware forwarding engine. The unicast FIB receives route updates from the
unicast FDM and sends the information to be programmed in the hardware
forwarding engine. The unicast FIB controls the addition, deletion, and
modification of routes, paths, and adjacencies.
The unicast FIBs are
maintained on a per-VRF and per-address-family basis, that is, one for IPv4 and
one for IPv6 for each configured VRF. Based on route update messages, the
unicast FIB maintains a per-VRF prefix and next-hop adjacency information
database. The next-hop adjacency data structure contains the next-hop IP
address and the Layer 2 rewrite information. Multiple prefixes could share a
next-hop adjacency information structure.
Hardware
Forwarding
Cisco NX-OS
supports distributed packet forwarding. The ingress port takes relevant
information from the packet header and passes the information to the local
switching engine. The local switching engine does the Layer 3 lookup and uses
this information to rewrite the packet header. The ingress module forwards the
packet to the egress port. If the egress port is on a different module, the
packet is forwarded using the switch fabric to the egress module. The egress
module does not participate in the Layer 3 forwarding decision.
You also use the
show platform
fib or
show platform
forwarding
commands to display details on hardware forwarding.
Software
Forwarding
The software
forwarding path in
Cisco NX-OS
is used mainly to handle features that are not supported in the hardware or to
handle errors encountered during the hardware processing. Typically, packets
with IP options or packets that need fragmentation are passed to the CPU on the
active supervisor. All packets that should be switched in the software or
terminated go to the supervisor. The supervisor uses the information provided
by the unicast RIB and the adjacency manager to make the forwarding decisions.
The module is not involved in the software forwarding path.
This section provides
a brief introduction to the Layer 3 unicast features and protocols supported in
Cisco NX-OS.
IPv4 and IPv6
Layer 3 uses either the IPv4 or IPv6 protocol. IPv6 increases the number of network address bits from 32 bits (in IPv4) to
128 bits. For more information, see IPv4 Addresses or IPv6 Addresses.
IP Services
IP Services includes Dynamic Host Configuration Protocol (DHCP) and Domain Name System (DNS Client) clients. For more information,
see Configuring DNS.
OSPF
The Open Shortest Path First (OSPF) protocol is a link-state routing protocol used to exchange network reachability information
within an autonomous system. Each OSPF router advertises information about its active links to its neighbor routers. Link
information consists of the link type, the link metric, and the neighbor router that is connected to the link. The advertisements
that contain this link information are called link-state advertisements. For more information, see Configure OSPFv2.
EIGRP
The Enhanced Interior Gateway Routing Protocol (EIGRP) is a unicast routing protocol that has the characteristics of both
distance vector and link-state routing protocols. It is an improved version of IGRP, which is a Cisco proprietary routing
protocol. EIGRP relies on its neighbors to provide the routes. It constructs the network topology from the routes advertised
by its neighbors, similar to a link-state protocol, and uses this information to select loop-free paths to destinations. For
more information, see Configuring EIGRP.
IS-IS
The Intermediate System-to-Intermediate System (IS-IS) protocol is an intradomain Open System Interconnection (OSI) dynamic
routing protocol specified in the International Organization for Standardization (ISO) 10589. The IS-IS routing protocol is
a link-state protocol. IS-IS features are as follows:
The Border Gateway Protocol (BGP) is an inter-autonomous system routing protocol. A BGP router advertises network reachability
information to other BGP routers using Transmission Control Protocol (TCP) as its reliable transport mechanism. The network
reachability information includes the destination network prefix, a list of autonomous systems that needs to be traversed
to reach the destination, and the next-hop router. Reachability information contains additional path attributes such as preference
to a route, origin of the route, community and others. For more information, see Configuring BGP.
RIP
The Routing Information Protocol (RIP) is a distance-vector protocol that uses a hop count as its metric. RIP is widely used
for routing traffic in the global Internet and is an Interior Gateway Protocol (IGP), which means that it performs routing
within a single autonomous system. For more information, see Configuring RIP.
Static
Routing
Static routing allows you to enter a fixed route to a destination. This feature is useful for small networks where the topology
is simple. Static routing is also used with other routing protocols to control default routes and route distribution. For
more information, see Configuring Static Routing.
Layer 3
Virtualization
Virtualization allows you to share physical resources across separate management domains. Cisco NX-OS supports Layer 3 virtualization
with virtual routing and forwarding (VRF). VRF provides a separate address domain for configuring Layer 3 routing protocols.
For more information, see Configuring Layer 3 Virtualization.
Route Policy Manager
The Route Policy Manager provides a route filtering capability in Cisco NX-OS. It uses route maps to filter routes distributed
across various routing protocols and between different entities within a given routing protocol. Filtering is based on specific
match criteria, which is similar to packet filtering by access control lists. For more information, see Configuring Route Policy Manager.
Policy-Based
Routing
Policy-based routing uses the Route Policy Manager to create policy route filters. These policy route filters can forward
a packet to a specified next hop based on the source of the packet or other fields in the packet header. Policy routes can
be linked to extended IP access lists so that routing might be based on protocol types and port numbers. For more information,
see Configuring Policy-Based Routing.
First Hop Redundancy
Protocols
First hop redundancy protocols (FHRP), such as the Hot Standby Router Protocol (HSRP) and the Virtual Router Redundancy Protocol
(VRRP), allow you to provide redundant connections to your hosts. If an active first-hop router fails, the FHRP automatically
selects a standby router to take over. You do not need to update the hosts with new IP addresses because the address is virtual
and shared between each router in the FHRP group. For more information on HSRP, see Configuring HSRP. For more information on VRRP, see Configuring VRRP.
Object
Tracking
Object tracking allows you to track specific objects on the network, such as the interface line protocol state, IP routing,
and route reachability, and take action when the tracked object’s state changes. This feature allows you to increase the availability
of the network and shorten the recovery time if an object state goes down. For more information, see Configuring Object Tracking.
Related
Topics
Feature Name
Feature Information
Layer 3
features
Cisco NX-OS
9000 Series NX-OS Multicast Routing Configuration Guide
Cisco
Cisco NX-OS
9000 Series NX-OS High Availability and Redundancy Guide
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