Table of Contents

Chapter 1 IP Routing Overview.. 1-1

1.1 IP Routing and Routing Table. 1-1

1.1.1 Routing. 1-1

1.1.2 Routing Through a Routing Table. 1-1

1.2 Routing Protocol Overview. 1-3

1.2.1 Static Routing and Dynamic Routing. 1-3

1.2.2 Classification of Dynamic Routing Protocols. 1-3

1.2.3 Routing Protocols and Routing Priority. 1-4

1.2.4 Route backup. 1-5

1.2.5 Route Recursion. 1-5

1.2.6 Sharing of Routing Information. 1-6

1.3 Displaying and Maintaining a Routing Table. 1-6

 

Chapter 1  IP Routing Overview

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

 

 

1.1  IP Routing and Routing Table

1.1.1  Routing

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:

l           Direct routes: Routes discovered by data link protocols, also known as interface routes.

l           Static routes: Routes that are manually configured.

l           Dynamic routes: Routes that are discovered dynamically by routing protocols.

II. Contents of a routing table

A routing table includes the following key items:

l           Destination address: Destination IP address or destination network.

l           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.

l           Outbound interface: Specifies the interface through which the IP packets are to be forwarded.

l           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.

l           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:

l           Subnet routes: The destination is a subnet.

l           Host routes: The destination is a host.

Based on whether the destination is directly connected to a given router, routes can be divided into:

l           Direct routes: The destination is directly connected to the router.

l           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

1.2.1  Static Routing and Dynamic Routing

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.

1.2.2  Classification of Dynamic Routing Protocols

Dynamic routing protocols can be classified based on the following standards:

I. Operational scope

l           Interior gateway protocols (IGPs): Work within an autonomous system, including RIP, OSPF, and IS-IS.

l           Exterior gateway protocols (EGPs): Work between autonomous systems. The most popular one is BGP.

 

 

II. Routing algorithm

l           Distance-vector protocols: RIP and BGP. BGP is also considered a path-vector protocol.

l           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

l           Unicast routing protocols: RIP, OSPF, BGP, and IS-IS.

l           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

 

 

1.2.4  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.

1.2.5  Route Recursion

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.

1.2.6  Sharing of Routing Information

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