IP Routing
The Internet is a large collection of hosts that communicate with each other and use routers as intermediate packet switches.
Routers forward a packet through the interconnected system of networks and routers until the packet reaches a router that is attached to the same network as the destination host. The router delivers the packet to the specified host on its local network.
Routing Tables
A router makes forwarding decisions based on the information that is contained in the router's routing table. Routers announce and receive route information to and from other routers. They build routing tables based on the collected information on all the best paths to all the destinations they know how to reach.
Figure 2-8 shows an example of a small network composed of four networks and three routers. The hosts that are attached to each network are not displayed, because each router makes its forwarding decisions based on the network number and not the address of each individual host. A router uses ARP to find the physical address that corresponds to the Internet address for any host or router on networks directly connected to it.
The routing tables are shown in the following tables for routers Boston, NY, and LA. The routing tables contain one entry for each route. The columns of the routing table include:
- The destination IP network address
- The IP address of the next-hop router
- An administrative distance that is used to select the least-cost route if multiple routes to the destination network have the same metric.
- A metric that is used to select the least-cost route if more than one route exists for the destination network.
Setting the Administrative Distance for a Route
The administrative distance is an integer that is associated with each route known to a router. The distance represents how reliable the source of the route is considered to be. A lower value is preferred over a higher value. An administrative distance of 255 indicates no confidence in the source; routes with this distance are not installed in the routing table.
Table 2-4 shows the default distance for each type of source from which a route can be learned.
If the IP routing table contains several routes to the same prefix—for example, an OSPF route and a RIP route—the route with the lowest administrative distance is used for forwarding.
To set the administrative distance for BGP routes, see Setting the Administrative Distance for a Route in ERX Routing Protocols Configuration Guide, Vol. 2, Chapter 1, Configuring BGP Routing.
To set the administrative distance for RIP, IS-IS, and OSPF, use the following distance commands in Router Configuration mode.
distance
- Use to set an administrative distance for RIP or OSPF routes in the range 0-255.
- For RIP routes, the default value is 120.
- For OSPF routes, the default value is 110.
- Example
host1(config)#router rip
host1(config-router)#distance 100
distance ip
host1(config)#router isis
host1(config-router)#distance 80 ip
Routing Operations
Routers keep track of next-hop information that enables a data packet to reach its destination through the network. A router that does not have a direct physical connection to the destination checks its routing table and forwards packets to another next-hop router that is closer to that destination. This process continues until the packet reaches its final destination.
Identifying a Router Within an Autonomous System
The router ID is commonly one of the router's defined IP addresses. Although the router ID is, by convention, formatted as an IP address, it is not required to be a configured address of the router. If you do not use the ip router-id command to assign a router ID, the system uses one of its configured IP addresses as the router ID.
ip router-id
- Use to assign a router ID—a unique identifier that IP routing protocols use to identify the router within an autonomous system.
- Example
host1(config)#ip router-id 192.32.15.23
Establishing a Static Route
You can set a destination to receive and send traffic by a specific route through the network.
ip route
host1(config)#ip route 192.56.15.23 255.255.255.0 192.66.0.1
Setting Up Default Routes
A router examines its routing table to find a path for each packet. If the router cannot locate a route, it must discard the packet. You can set up a default route using the special address: 0.0.0.0. If the router cannot locate a path to a destination network and a default route is defined, the system forwards the packet to the default router. For example:
host1(config)#ip route 0.0.0.0 0.0.0.0 192.56.21.3
Default routes are typically used to reduce the size of the routing table. Routing is simplified because the router can test for a few local networks or use the default route. However, a disadvantage of default routes is the possible creation of multiple paths and routing loops.
Setting Up an Unnumbered Interface
An unnumbered interface does not have an IP address assigned to it. Unnumbered interfaces are often used in point-to-point connections where an IP address is not required.
ip unnumbered
- Use to set up an unnumbered interface.
- This command enables IP processing on an interface without assigning an explicit IP address to the interface.
- You supply an interface location, which is the type and number of another interface on which the router has an assigned IP address. This interface cannot be another unnumbered interface.
- Example
host1(config-if)#ip unnumbered fastEthernet 0/0
Adding a Host Route to a Peer on a PPP Interface
You can enable the ability to create host access routes on a PPP interface. This feature is useful in B-RAS applications.
ip access-routes
host1(config-if)#ip access-routes
Enabling Source Address Validation
Source address validation verifies that a packet has been sent from a valid source address. When a packet arrives on an interface, the system performs a routing table lookup using the source address. The result from the routing table lookup is an interface to which packets destined for that address are routed. This interface must match the interface on which the packet arrived. If it does not match, the system drops the packet.
ip sa-validate
host1(config-if)#ip sa-validate
Shutting Down an IP Interface
The system lets you disable an interface at the IP level without removing it.
ip shutdown
host1(config-if)#ip shutdown
Removing the IP Configuration
You can remove the IP configuration from an interface or subinterface.
no ip interface
- Use to remove the IP configuration from an interface or subinterface and disable IP processing on the interface.
- Example
host1(config-if)#no ip interface
Clearing IP Routes
The system enables you to clear the specified routing entries from the routing table. You must specify the IP address prefix and the mask of the IP address prefix to clear specific routes. You can later reinstall the routes you have cleared.
clear ip routes
- Use to clear specified IP routes according to an IP prefix or a VPN routing and forwarding (VRF) table.
- Use an asterisk (*) to clear all dynamic routes from the routing table.
- Example
host1#clear ip routes *
ip refresh-route
- Use to enable the owning protocols (BGP, IS-IS, OSPF) to reinstall routes removed from the IP routing table by the clear ip routes command.
- Example
host1#ip refresh-route
Clearing IP Interfaces
The system enables you to clear the counters on the specified IP interface(s).
clear ip interface
host1#clear ip interface pos 2/0
Setting a Baseline
The system enables you to set a baseline for statistics on an IP interface.
baseline ip interface
host1#baseline ip interface pos 2/0
Disabling Forwarding of Packets
The system allows you to disable forwarding of packets on an SRP Ethernet interface.
ip disable-forwarding
- Use to disable forwarding of packets on the SRP Ethernet interface.
- The purpose of this command is to maintain system performance by maximizing the CPU time available for routing protocols. Although you can allow data forwarding on the SRP Ethernet interface, system performance will be affected.
- You see an error message if you try to set this command for interfaces other than the SRP Ethernet interface.
- Example
host1(config-if)#ip disable-forwarding
Enabling Forwarding of Source-Routed Packets
IP packets are normally routed according to the destination address they contain based on the routing table at each hop through a path. The originator or source of the source-routed packets specifies the path (the series of hops) that the packets must traverse; the source makes the routing decisions. The source can specify either of the following types of source routing:
- Strict-source routing specifies every hop that the packet must traverse. The specified path consists of adjacent hops. The source generates an ICMP error if the exact path cannot be followed. For example, for a path going from source router A to router B to router C to router D, router A would specify a strict-source route as B, C, D.
- Loose-source routing specifies a set of hops that the packet must traverse, but not necessarily every hop in the path. That is, the specified hops do not have to be adjacent. For example, for a path going from source router A to router B to router C to router D, router A would specify a loose-source route as B, D or C, D, or B, C, D.
ip source-route
host1#ip source-route
Forcing an Interface to Appear Up
The system enables you to force an IP interface to appear as if it is up, regardless of the state of the lower layers.
ip alwaysup
- Use to force an IP interface to appear as up regardless of the state of lower layers.
- This command reduces route topology changes when the network attached to this link is single-homed.
- Example
host1(config-if)#ip alwaysup
Specifying a Debounce Time
You can set a debounce time that requires an IP interface to be in a given state—for example, up or down—for the specified time before the state is reported. This feature prevents a link that briefly goes up or down from causing an unnecessary topology change, for example by causing an interface down condition.
ip debounce-time
- Use to set the interval for which an interface must maintain a given state before the state change is reported.
- Example
host1(config)#ip debounce-time 5000
Adding a Description
The system enables you to add a text description or an alias to an IP interface or subinterface. Adding a description helps you identify the interface and keep track of interface connections.
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ip description
- Use to assign a text description or an alias to an IP interface or subinterface.
- The description or alias can be a maximum of 256 characters.
- Use the show ip interface command to display the text description.
- Examples
host1(config-if)#ip description canada01 ip interface
host1(config-subif)#ip description montreal011 ip subinterface
Enabling Link Status Traps
The system allows you to enable link status traps on an interface.
snmp trap ip link-status
host1(config-if)#snmp trap ip link-status
Configuring the Speed
The system enables you to set the speed of an IP interface.
ip speed
- Use to set the speed of the interface in bits per second.
- By default, the speed is determined from a lower-layer interface.
- Example
host1(config-if)#ip speed 1000
Configuring Equal-Cost Multipath Load Sharing
Equal-cost multipath (ECMP) sets are formed when the system finds routing table entries for the same destination with equal cost. You can add routing table entries manually (as static routes), or they are formed as routers discover their neighbors and exchange routing tables (via OSPF, BGP, and other routing protocols). The system then balances traffic across these sets of equal-cost paths using one of the following ECMP modes:
- Hashed - uses hashing of source and destination addresses to determine which of the available paths in the ECMP set to use
- Round-robin - distributes packets equally among the available paths in the ECMP set
ip multipath round-robin
- Use to specify round-robin as the mode for ECMP load sharing on an interface.
- ECMP uses the round-robin mode when you have configured all interfaces in the set to round-robin. Otherwise, ECMP defaults to hashed mode because round-robin mode could cause reordering of packets. You must explicitly ensure that the possible reordering is acceptable on all the member interfaces by setting them to round-robin mode.
- Use the no version to set the ECMP mode to the default, hashed.
- Example
host1(config)#virtual-router router_0
host1:router_0(config)#interface serial 4/0:1/22.22
host1:router_0(config-subif)#ip multipath round-robin
host1:router_0(config-subif)#exit
host1:router_0(config)#exit
host1:router_0#show ip interface serial 4/0:1/22.22
serial4/0:1/22.22 is up, line protocol is up
Network Protocols: IP
Internet address is 190.121.1.1/255.255.0.0
Broadcast address is 255.255.255.255
Operational MTU = 1600 Administrative MTU = 0
Operational speed = 64000 Administrative speed = 0
Router advertisement = disabled
Administrative debounce-time = disabled
Operational debounce-time = disabled
Access routing = disabled
Multipath mode = round-robin
In Received Packets 0, Bytes 0
In Policed Packets 0, Bytes 0
In Error Packets 0
In Invalid Source Address Packets 0
Out Forwarded Packets 0, Bytes 0
Out Scheduler Drops Packets 0, Bytes 0
maximum-paths
- Use to control the maximum number of parallel routes that the routing protocol (BGP, IS-IS, OSPF, or RIP) can support.
- The maximum number of routes can range from 1-6 for BGP and from 1-16 for IS-IS, OSPF, or RIP.
- Example
host1(config-router)#maximum-paths 2
Setting a TTL Value
You can use the ip ttl command to set the TTL (time-to-live) field in the IP header for all IP operations. The TTL specifies a hop count. This configured TTL value can be overridden by other commands that specify a TTL.
ip ttl
host1(config)#ip ttl 255