Go to these sections for information you
are interested in:
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IP Routing and Routing
Table
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Routing Protocol
Overview
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Displaying and
Maintaining a Routing Table
The term
“router” in this document refers to a Layer 3 switch running
routing protocols.
1.1 IP Routing and Routing Table
Routing in the Internet is achieved through
routers. Upon receiving a packet, a router finds an optimal route based on the
destination address and forwards the packet to the next router in the path
until the packet reaches the last router, which forwards the packet to the
intended destination host.
1.1.2 Routing Through a Routing Table
I. Routing table
Routing tables play a key role in routing.
Each router maintains a routing table, and each entry in the table specifies
which physical interface a packet destined for a certain destination should go
out to reach the next hop (the next router) or the directly connected
destination.
Routes in a routing table can be divided
into three categories by origin:
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Direct routes: Routes discovered by data link
protocols, also known as interface routes.
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Static routes: Routes that are manually
configured.
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Dynamic routes: Routes that are discovered dynamically
by routing protocols.
II. Contents of a routing table
A routing table includes the following key
items:
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Destination address: Destination IP address or
destination network.
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Network mask: Specifies, in company with the
destination address, the address of the destination network. A logical AND
operation between the destination address and the network mask yields the
address of the destination network. For example, if the destination address is
129.102.8.10 and the mask 255.255.0.0, the address of the destination network
is 129.102.0.0. A network mask is made of a certain number of consecutive 1s.
It can be expressed in dotted decimal format or by the number of the 1s.
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Outbound interface: Specifies the interface
through which the IP packets are to be forwarded.
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IP address of the next hop: Specifies the
address of the next router on the path. If only the outbound interface is
configured, its address will be the IP address of the next hop.
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Priority for the route. Routes to the same
destination but having different nexthops may have different priorities and be
found by various routing protocols or manually configured. The optimal route is
the one with the highest priority (with the smallest metric).
Routes can be divided into two categories
by destination:
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Subnet routes: The destination is a subnet.
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Host routes: The destination is a host.
Based on whether the destination is
directly connected to a given router, routes can be divided into:
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Direct routes: The destination is directly
connected to the router.
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Indirect routes: The destination is not directly
connected to the router.
To prevent the routing table from getting
too large, you can configure a default route. All packets without matching
entry in the routing table will be forwarded through the default route.
In Figure 1-1, the IP address on each cloud
represents the address of the network. Router G resides in three networks and therefore
has three IP addresses for its three physical interfaces. Its routing table is
shown on the right of the network topology.
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Destination Network
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Next hop
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Interface
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11.0.0.0
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11.0.0.1
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2
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12.0.0.0
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12.0.0.1
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1
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13.0.0.0
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12.0.0.2
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1
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14.0.0.0
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14.0.0.4
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3
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15.0.0.0
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14.0.0.2
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3
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16.0.0.0
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14.0.0.2
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3
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17.0.0.0
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11.0.0.2
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2
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Figure 1-1 A sample routing table
1.2 Routing Protocol Overview
Static routing is easy to configure and
requires less system resources. It works well in small, stable networks with
simple topologies. Its major drawback is that you must perform routing
configuration again whenever the network topology changes; it cannot adjust to
network changes by itself.
Dynamic routing is based on dynamic routing
protocols, which can detect network topology changes and recalculate the routes
accordingly. Therefore, dynamic routing is suitable for large networks. Its
disadvantages are that it is complicated to configure, and that it not only
imposes higher requirements on the system, but also eats away a certain amount
of network resources.
Dynamic routing protocols can be classified
based on the following standards:
I. Operational scope
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Interior gateway protocols (IGPs): Work within
an autonomous system, including RIP, OSPF, and IS-IS.
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Exterior gateway protocols (EGPs): Work between
autonomous systems. The most popular one is BGP.
An autonomous
system refers to a group of routers that share the same routing policy and work
under the same administration.
II. Routing algorithm
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Distance-vector protocols: RIP and BGP. BGP is
also considered a path-vector protocol.
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Link-state protocols: OSPF and IS-IS.
The main differences between the above two
types of routing algorithms lie in the way routes are discovered and
calculated.
III. Type of the destination
address
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Unicast routing protocols: RIP, OSPF, BGP, and
IS-IS.
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Multicast routing protocols: PIM-SM and PIM-DM.
This chapter focuses on unicast routing
protocols. For information on multicast routing protocols, refer to the Multicast
Protocol Configuration.
IV. Version of IP protocol
IPv4 routing protocols: RIP, OSPFv2, BGP4
and IS-IS.
IPv6 routing protocols: RIPng, OSPFv3, IPv6
BGP, and IPv6 IS-IS.
1.2.3 Routing Protocols and Routing Priority
Different routing
protocols may find different routes to the same destination. However, not all
of those routes are optimal. In fact, at a particular moment, only one protocol
can uniquely determine the current optimal routing to the destination. For the
purpose of route selection, each routing protocol (including static routes) is
assigned a priority. The route found by the routing protocol with the highest
priority is preferred.
The following table lists some routing
protocols and the default priorities for routes found by them:
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Routing approach
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Priority
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DIRECT
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0
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OSPF
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10
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IS-IS
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15
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STATIC
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60
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RIP
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100
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OSPF ASE
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150
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OSPF NSSA
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150
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IBGP
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255
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EBGP
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255
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UNKNOWN
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256
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The smaller the priority value, the higher the
priority.
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The priority for a direct route is always 0,
which you cannot change. Any other type of routes can have their priorities
manually configured.
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Each static route can be configured with a
different priority.
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IPv4 and IPv6 routes have their own respective
routing tables.
Route backup can help improve network
reliability. With route backup, you can configure multiple routes to the same
destination, expecting the one with the highest priority to be the main route
and all the rest backup routes.
Under normal circumstances, packets are forwarded through the main
route. When the main route goes down, the route with the highest priority among
the backup routes is selected to forward packets. When the main route recovers,
the route selection process is performed again and the main route is selected
again to forward packets.
The nexthops of some static routes
configured with nexthops may not be directly connected. To forward the packets,
the outgoing interface to reach the nexthop must be available. Route recursion
is used to find the outgoing interface based on the nexthop information of the
route.
As different routing protocols use different routing algorithms to
calculate routes, they may find different routes. In a large network with
multiple routing protocols, it is required for routing protocols to share their
routing information. Each routing protocol has its own route redistribution
mechanism. For detailed information, refer to the description about route
redistribution in each routing protocol.
1.3 Displaying and Maintaining a Routing
Table
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To do…
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Use the command…
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Remarks
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Display brief information about the
active routes in the routing table
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display ip routing-table [ verbose | | { begin | exclude | include } regular-expression
]
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Available in any view
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Display information about routes to the
specified destination
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display ip routing-table ip-address [ mask-length | mask ] [ longer-match
] [ verbose ]
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Display information about routes with
destination addresses in the specified range
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display ip routing-table ip-address1 { mask-length | mask } ip-address2
{ mask-length | mask } [ verbose ]
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Display information about routes
permitted by an IPv4 basic ACL
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display ip routing-table acl acl-number [ verbose ]
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Display
routing information permitted by an IPv4 prefix list
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display
ip routing-table ip-prefix ip-prefix-name
[ verbose ]
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Available in any view
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Display
routes of a routing protocol
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display
ip routing-table protocol protocol
[ inactive | verbose ]
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Display
statistics about the network routing table
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display
ip routing-table statistics
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Clear statistics for the routing table
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reset ip routing-table statistics
protocol { all | protocol }
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Available in user view
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Display
the information of recursive routes
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display
ip relay-route
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Available
in any view
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Display IPv6
recursive route information
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display
ipv6 relay-route
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Display
brief IPv6 routing table information
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display
ipv6 routing-table
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Display
verbose IPv6 routing table information
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display
ipv6 routing-table verbose
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Display routing information for a
specified destination IPv6 address
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display ipv6 routing-table ipv6-address prefix-length [ longer-match ]
[ verbose ]
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Display routing information permitted by
an IPv6 ACL
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display ipv6 routing-table acl acl6-number [ verbose ]
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Display routing information permitted by
an IPv6 prefix list
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display ipv6 routing-table ipv6-prefix ipv6-prefix-name [ verbose ]
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Display IPv6 routing information of a
routing protocol
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display ipv6 routing-table protocol protocol [ inactive | verbose
]
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Display IPv6 routing statistics
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display ipv6 routing-table statistics
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Display IPv6 routing information for an
IPv6 address range
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display ipv6 routing-table ipv6-address1 prefix-length1 ipv6-address2
prefix-length2 [ verbose ]
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Clear specified IPv6 routing table
statistics
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reset ipv6 routing-table statistics
protocol { all | protocol }
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Available in user view
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