Go to these sections for information you
are interested in:
l
IP Routing and Routing Table
l
Routing Protocol Overview
l
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.
|
Destination Network
|
Next hop
|
Interface
|
|
11.0.0.0
|
11.0.0.1
|
2
|
|
12.0.0.0
|
12.0.0.1
|
1
|
|
13.0.0.0
|
12.0.0.2
|
1
|
|
14.0.0.0
|
14.0.0.4
|
3
|
|
15.0.0.0
|
14.0.0.2
|
3
|
|
16.0.0.0
|
14.0.0.2
|
3
|
|
17.0.0.0
|
11.0.0.2
|
2
|
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:
|
Routing approach
|
Priority
|
|
DIRECT
|
0
|
|
OSPF
|
10
|
|
IS-IS
|
15
|
|
STATIC
|
60
|
|
RIP
|
100
|
|
OSPF ASE
|
150
|
|
OSPF NSSA
|
150
|
|
IBGP
|
255
|
|
EBGP
|
255
|
|
UNKNOWN
|
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.
I. Load balancing
In multi-route mode, a routing protocol can
be configured with multiple equal-cost routes to the same destination. These
routes have the same priority and will all be used to accomplish load balancing
if there is no route with a higher priority available.
A given routing protocol may find several
routes with the same metric to the same destination, and if this protocol has
the highest priority among all the active protocols, these routes will be considered
valid routes for load balancing.
In current implementations, routing
protocols supporting load balancing are static routing, RIP, OSPF, BGP and IS-IS.
II. Route backup
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 BGP routes (except
EBGP routes) and 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. Link-state routing protocols, such as
OSPF and IS-IS, do not need route recursion because they obtain nexthop
information through route calculation.
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
|
To do…
|
Use the command…
|
Remarks
|
|
Display brief information about the
active routes in the routing table
|
display ip routing-table [ verbose | | { begin | exclude | include } regular-expression
]
|
Available in any view
|
|
Display information about routes to the
specified destination
|
display ip routing-table ip-address [ mask-length | mask ] [ longer-match
] [ verbose ]
|
|
Display information about routes with destination
addresses in the specified range
|
display ip routing-table ip-address1 { mask-length | mask } ip-address2
{ mask-length | mask } [ verbose ]
|
|
Display information about routes
permitted by an IPv4 basic ACL
|
display ip routing-table acl acl-number [ verbose ]
|
|
Display routing
information permitted by an IPv4 prefix list
|
display
ip routing-table ip-prefix ip-prefix-name
[ verbose ]
|
Available in any view
|
|
Display
routes of a routing protocol
|
display
ip routing-table protocol protocol
[ inactive | verbose ]
|
|
Display
statistics about the network routing table
|
display
ip routing-table statistics
|
|
Clear statistics for the routing table
|
reset ip routing-table statistics
protocol { all | protocol }
|
Available in user view
|
|
Display the information of recursive
routes
|
display ip relay-route
|
Available in any view
|
|
Display IPv6 recursive route information
|
display ipv6 relay-route
|
|
Display brief IPv6 routing table
information
|
display ipv6 routing-table
|
|
Display verbose IPv6 routing table information
|
display ipv6 routing-table verbose
|
|
Display routing information for a
specified destination IPv6 address
|
display ipv6 routing-table ipv6-address prefix-length [ longer-match ]
[ verbose ]
|
|
Display routing information permitted by
an IPv6 ACL
|
display ipv6 routing-table acl acl6-number [ verbose ]
|
|
Display routing information permitted by
an IPv6 prefix list
|
display ipv6 routing-table ipv6-prefix ipv6-prefix-name [ verbose ]
|
|
Display IPv6 routing information of a
routing protocol
|
display ipv6 routing-table protocol protocol [ inactive | verbose
]
|
|
Display IPv6 routing statistics
|
display ipv6 routing-table statistics
|
|
Display IPv6 routing information for an
IPv6 address range
|
display ipv6 routing-table ipv6-address1 prefix-length1 ipv6-address2 prefix-length2
[ verbose ]
|
|
Clear specified IPv6 routing table
statistics
|
reset ipv6 routing-table statistics
protocol { all | protocol }
|
Available in user view
|
Go to these sections for information you
are interested in:
l
Introduction
to Graceful Restart
l
Basic
Concepts in Graceful Restart
l
Graceful
Restart Communication Procedure
l
Graceful
Restart Mechanism for Several Commonly Used Protocols
Throughout this
chapter, the term router and the router icon refers to a router in a generic
sense or a Layer 3 switch running routing protocols.
2.1 Introduction to Graceful Restart
Graceful Restart
ensures the continuity of packet forwarding when a routing protocol restarts.
The mechanism of Graceful Restart works as
follows: after the routing protocol on a Graceful Restart capable device has
restarted, the device will notify its neighbors to temporarily preserve its
adjacency with them and the routing information. The neighbors will help the
restarting device to update its routing information and to restore it to the
state prior to the restart in minimal time. The routing and forwarding remain
highly stable across the restart, the packet forwarding path remains the same,
and the whole system can forward IP packets continuously. Hence, it is called “Graceful
Restart”.
2.2 Basic
Concepts in Graceful Restart
A router with the Graceful Restart feature
enabled is called a Graceful Restart capable router. It can perform a Graceful
Restart when its routing protocol restarts. Routers that are not Graceful
Restart capable will follow the normal restart procedures after a routing
protocol restart.
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GR Restarter: Graceful restarting router, the
router whose routing protocol has restarted due to administrator instructions
or network failure. It must be Graceful Restart capable.
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GR Helper: The neighbor of the GR Restarter,
which helps the GR Restarter to retain the routing information. It must be
Graceful Restart capable.
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GR Session: A Graceful Restart session, which is
the negotiation between the GR Restarter and the GR Helper. A GR session
includes restart notification and communications across restart. Through this
session, GR Restarter and GR Helper can know the GR capability of each other.
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GR Time: The time taken for the GR Restarter and
the GR Helper to establish a session between them. Upon detection of the down
state of a neighbor, the GR Helper will preserve the topology and routing
information sent from the GR Restarter for a period as specified by the GR
Time.
Configure a device as GR Restarter in a
network. This device and its GR Helper must support GR or be GR capable. Thus,
when GR Restarter restarts, its GR Helper can know its restart process.
In some cases, GR
Restarter and GR Helper can replace with each other.
The communication procedure between the GR
Restarter and the GR Helper works as follows:
1)
A GR session is established between the GR
Restarter and the GR Helper.
Figure 2-1 A GR session is established
between the GR Restarter and the GR Helper
As illustrated in Figure 2-1, Router A
works as GR Restarter, Router B, Router C and Router D are the GR Helpers of
Router A. A GR session is established between the GR Restarter and the GR
Helper.
2)
GR Restarter restarting
Figure 2-2 Restarting process for the GR
Restarter
As illustrated in Figure 2-2. The GR
Helper detects that the GR Restarter has restarted its routing protocol and
assumes that it will recover within the GR Time. Before the GR Time expires,
the GR Helper will neither terminate the session with the GR Restarter nor
delete the topology or routing information of the latter.
3)
GR Restarter signaling to GR Helper
Figure 2-3 The GR Restarter signals to
the GR Helper(s) after restart
As illustrated in Figure 2-3, after
the GR Restarter has recovered, it will signal to all its neighbors and will
reestablish GR Session.
4)
The GR Restarter obtaining topology and routing
information from the GR Helper
Figure 2-4 The GR Restarter obtains
topology and routing information from the GR Helper
As illustrated in Figure 2-4, the GR
Restarter obtains the necessary topology and routing information from all its
neighbors through the GR sessions between them and calculates its own routing
table based on this information.
The switch supports Graceful Restart based
on Boarder Gateway Protocol (BGP), Open Shortest Path First (OSPF), and
Intermediate System to Intermediate System (IS-IS).
For the implementation and configuration
procedure of the Graceful Restart mechanism of the above protocols, refer to BGP
Configuration, OSPF Configuration, and IS-IS Configuration in
IPv4 Routing Configuration.
