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CSS Administration Guide (Software Version 6.10)
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Index
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Preface
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Booting, Logging In, and Getting Started
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Configuring CSS Basics
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Managing the CSS Software
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Specifying the CSS Boot Configuration
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Configuring Interfaces and Circuits
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Configuring CSS Network Protocols
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Configuring Open Shortest Path First (OSPF)
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Using the CSS Logging Features
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Configuring Flow and Port Mapping Parameters
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Configuring User Profiles
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Configuring CSS Remote Access Methods
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Configuring Simple Network Management Protocol (SNMP)
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Configuring Remote Monitoring (RMON)
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Upgrading Your CSS Software
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Using the Offline Diagnostic Monitor Menu
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Troubleshooting the Boot Process
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Table Of Contents
Configuring CSS Network Protocols
Configuring the Domain Name Service
Specifying a Primary DNS Server
Specifying a Secondary DNS Server
Specifying UDP Traffic on the DNS Server Port
Configuring the Address Resolution Protocol
Configuring Routing Information Protocol
Configuring Equal-Cost RIP Routes
Configuring the Internet Protocol
Disabling an Implicit Service for the Static Route Next Hop
Configuring an IP Source Route
Configuring the IP Record Route
Configuring Box-to-Box Redundancy
Configuring IP Equal-Cost Multipath
Forwarding IP Subnet Broadcast Addressed Frames
Configuring IP Unconditional Bridging
Configuring IP Opportunistic Layer 3 Forwarding
Showing IP Configuration Information
Showing IP Global Configuration Parameters
Showing IP Interface Information
Showing IP Routing Information
Showing a Summary of IP Global Statistics
Configuring the Cisco Discovery Protocol
Setting the CDP Transmission Rate
Configuring Spanning-Tree Bridging for the CSS
Configuring Spanning-Tree Bridge Aging-Time
Configuring Spanning-Tree Bridge Forward-Time
Configuring Spanning-Tree Bridge Hello-Time
Configuring Spanning-Tree Bridge Max-Age
Configuring Spanning-Tree Bridge Priority
Disabling Bridge Spanning-Tree
Configuring the DHCP Relay Agent
Adding a DHCP Destination on a Circuit
Enabling and Disabling DHCP on the Circuit
Defining the Hops Field Value for Forwarding DHCP Messages
Displaying the DHCP Relay Configuration
Configuring CSS Network Protocols
This chapter describes how to configure Domain Name Service (DNS), Address Resolution Protocol (ARP), Routing Information Protocol (RIP), Internet Protocol (IP) routing, spanning-tree bridging, and Dynamic Host Configuration Protocol (DHCP). Information in this chapter applies to all CSS models, except where noted.
This chapter includes the following major sections:
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Configuring the Domain Name Service
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Configuring the Domain Name Service
Use the dns command to enter commands that control Domain Name Service (DNS), the facility that translates host names such as myhost.mydomain.com to IP
addresses such as 192.168.11.1.This section includes the following topics:
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Use the show running-config global command to display DNS configurations (refer to Chapter 3, Managing the CSS Software).
Specifying a Primary DNS Server
Use the dns primary command to specify the primary DNS server. Enter the IP address in dotted-decimal notation (for example, 192.168.11.1) of the DNS server you want to specify as the primary DNS server.
For example:
(config)# dns primary 192.168.11.1To remove the primary DNS server, enter:
(config)# no dns primaryUsing DNS Resolve
Use the dns resolve command to resolve a host name by querying the DNS server. Enter the host name you want to resolve in mnemonic host-name format (for example, myhost.mydomain.com).
For example:
(config)# dns resolve fred.arrowpoint.comSpecifying a Secondary DNS Server
When a primary DNS server fails, the CSS uses the secondary DNS server to resolve host names to IP addresses. Use the dns secondary command to specify a secondary DNS server. Enter the IP address of the secondary DNS server in dotted-decimal notation (for example, 192.168.11.1).
(config)# dns secondary 192.168.3.6You can specify a maximum of two secondary servers. To specify each additional server, repeat the dns secondary command. The order in which you enter the IP addresses is the order in which they are used when the primary DNS server fails.
To remove a secondary DNS server, specify the no version of the command followed by the IP address of the DNS server you wish to remove. For example:
(config)# no dns secondary 192.168.3.6Specifying a DNS Suffix
Use the dns suffix command to specify the default suffix to use when querying the DNS facility. Enter the default suffix as an unquoted text string with no spaces and a maximum of 64 characters.
For example:
(config)# dns suffix arrowpoint.comTo remove the default DNS suffix, enter:
(config)# no dns suffixSpecifying UDP Traffic on the DNS Server Port
For DNS UDP traffic on port 53, use the dnsflow command to determine whether the CSS sets up flows using flow control blocks (FCBs) for DNS requests and responses. This command provides the following options:
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For example:
(config)# dnsflow disableConfiguring the Address Resolution Protocol
Use the arp command to statically configure the IP to Media Access Control (MAC) translations necessary for the CSS to send data to network nodes. You can configure static ARP mapping for any of the CSS Ethernet interface ports.
This section includes the following topics:
Configuring ARP
Use the arp command to define a static ARP mapping. The syntax for this global configuration mode command is:
arp ip_or_host mac_address interface {vlan}
The variables and options are as follows:
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For example:
(config)# arp 192.168.11.1 00-60-97-d5-26-ab e2To remove a static mapping address, use the no arp command. For example:
(config)# no arp 192.168.11.1
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Configuring ARP Timeout
Use the arp timeout command to set the time, in seconds, to hold an ARP resolution result. When you change the timeout value, this value affects only new ARP entries. All previous ARP entries retain the old timeout value. To remove all entries with the old timeout value, enter the clear arp cache command.
The timeout value is the number of seconds the CSS holds an ARP resolution result. To set a timeout value, enter an integer from 60 to 86400 (24 hours) seconds. The default is 14400 seconds (4 hours). If you do not want the ARP entries to time out, enter none or 86401.
For example:
(config)# arp timeout 120To restore the default timeout value of 14400 seconds, enter:
(config)# no arp timeoutConfiguring ARP Wait
Use the arp wait command to set the time, in seconds, to wait for an ARP resolution. The wait time is the number of seconds the CSS waits for an ARP resolution in response to an ARP request to the network. Enter an integer from 5 to 30 seconds. The default is 5.
For example:
(config)# arp wait 15To restore the default wait time of 5 seconds, enter:
(config)# no arp waitUpdating ARP Parameters
Use the update arp command to update the file containing hosts reachable through ARP. This command is available only in SuperUser mode. For example:
# update arp fileClearing ARP Parameters
The CSS enables you to clear ARP parameters for the ARP file or ARP cache. To clear the file that contains known hosts reachable through ARP, use the clear arp file command. For example:
# clear arp fileUse the clear arp cache command to delete dynamic entries from the ARP cache. To specify an address for the single ARP entry you want to remove from the ARP cache, use the clear arp cache ip_or_host command. Enter the address in either dotted-decimal IP notation (for example, 192.168.11.1) or mnemonic host-name format (for example, myhost.mydomain.com).
For example:
# clear arp cache 192.168.11.1Showing ARP Information
Use the show arp command to display ARP information. To show static ARP mapping when you use the show arp command, the IP route must exist in the routing table.
The syntax for this global configuration mode command is:
show arp {config|file|ip_or_host}
The syntax and options for the command are as follows:
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For example, to display the complete ARP resolution table, enter:
# show arpTable 6-1 describes the fields in the show arp command output.
To display the global ARP configuration, enter:
# show arp configTable 6-2 describes the fields in the show arp config command output.
To display the host IP addresses entered at initialization or boot time through ARP, enter:
# show arp fileTo display the resolution for a host IP address, enter:
# show arp 192.50.1.6To display the host IP addresses entered at initialization or boot time through ARP, enter:
# show arp fileConfiguring Routing Information Protocol
The CSS enables you to configure global Routing Information Protocol (RIP) attributes used to advertise routes on the CSS. By default, RIP advertises RIP routes and local routes for interfaces running RIP. The rip command advertises other routes.
The timers used by RIP in the CSS include the following default values. These RIP timer values are not user-configurable in the CSS.
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This section includes the following topics:
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Configuring RIP Advertise
Use the rip advertise command to advertise a route through RIP on the CSS. The syntax for this command is:
rip advertise ip_address subnet_mask {metric}
The variables for this command are as follows:
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For example:
(config)# rip advertise 192.168.1.0/24 9
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To stop advertising a route through RIP on the CSS, enter:
(config)# no rip advertise 192.168.1.0/24Configuring RIP Redistribute
Use the rip redistribute command to advertise routes from other protocols through RIP. By default, RIP advertises RIP routes and local routes for interfaces running RIP. This command instructs RIP to advertise other routes, such as firewall routes, OSPF routes, and so on.
The syntax for this command is
rip redistribute [firewall|local|ospf|static] {metric}
The options and variables for this command are as follows:
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For example:
(config)# rip redistribute static 3To stop advertising routes from other protocols through RIP, use either the local, static, or firewall option.
The following commands stop advertising static routes:
(config)# no rip redistribute firewall(config)# no rip redistribute local(config)# no rip redistribute static(config)# no rip redistribute ospfConfiguring Equal-Cost RIP Routes
Use the rip equal-cost command to set the maximum number of routes that RIP can insert into the routing table. Enter a number from 1 to 15. The default is 1. For example:
(config)# rip equal-cost 4To reset the number of routes to the default value of 1, enter:
(config)# no rip equal-costShowing RIP Configurations
Use the show rip command to show a RIP configuration for one IP address or all IP addresses configured in the CSS. This command provides the following options and variables:
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Table 6-3 describes the fields in the show rip command output.
To display global RIP statistics, enter:
# show rip globalsTable 6-4 describes the fields in the show rip globals command output.
To display the RIP interface statistics for all RIP interface entries, enter:
# show rip statisticsTable 6-5 describes the fields in the show rip statistics command output.
Configuring the Internet Protocol
Use the ip command to specify Internet Protocol (IP) configuration commands for the CSS. This command is available in global configuration mode.
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Configuring an IP Route
A static route consists of a destination network address and mask, as well as the next hop to reach the destination. You can also specify a default static route (using 0.0.0.0 as the destination network address and a valid next hop address) to direct frames for which no other destination is listed in the routing table. Default static routes are useful for forwarding otherwise unrouteable packets by the CSS.
When you configure a static route, the CSS creates an internal service that periodically polls the configured next hop address with an ICMP echo (or ping) keepalive. The internal service is called an implicit service. If the router fails, the CSS removes any entries from the routing table that point to the failed router and stops sending network traffic to the failed router. When the router recovers, the CSS:
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The implicit service does not determine if the default or static route appears in the routing table. This decision is based on the CSS having a viable ARP entry for the next hop router IP address so the CSS can forward traffic to that destination. The CSS uses the ICMP keepalive as a means to ensure the next hop router MAC address is available and current. However, in certain situations, the next hop router may block ICMP message transmitted by the CSS, which results in a failed ICMP keepalive (the ICMP keepalive is in the Down state). As long as the CSS has the ARP entry of the next hop router the static route is still placed in the routing table.
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Use the ip route command to configure an IP route. You can configure a static route, a default static IP route, a blackhole route (where the CSS drops any packets addressed to the route), or a firewall IP route. Each ip route command requires one of the following:
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The syntax for this global configuration command is:
ip route ip_address subnet_mask[blackhole|ip_address2{distance|
originated-packets}|firewall index {distance}]The syntax and options for the command are as follows:
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For example, to configure a static IP route to destination network address 192.168.0.0 /16 and a next hop address of 192.167.1.1, enter:
(config)# ip route 192.168.0.0 /16 192.167.1.1For example, to configure a default IP route using a destination address of 0.0.0.0 /0 and a next hop address of 192.167.1.1, enter:
(config)# ip route 0.0.0.0 /0 192.167.1.1For example, to configure a blackhole route, enter:
(config)# ip route 192.168.1.0 /24 blackholeFor example, to configure a firewall IP route with an index number of 3 and an administrative distance of 2, enter:
(config)# ip route 192.168.1.0 /24 firewall 3 2To remove a static route, enter:
(config)# no ip route 0.0.0.0 /0 10.0.1.1To disable the dropping of packets to a blackhole route, enter:
(config)# no ip route 192.168.1.0 /24 blackholeTo remove a firewall route, enter:
(config)# no ip route 192.168.1.0 /24 firewall 3Disabling an Implicit Service for the Static Route Next Hop
Use the ip no-implicit-service command when you do not want the CSS to start an implicit service for the next hop of a static route. By default, the CSS establishes an implicit (or internal) service for the gateway address when a static route is defined. The ip no-implicit-service command specifies that no implicit service is established to the next hop of the static route, which disables the internal service ICMP keepalive. In this case, if the ARP address for the next hop is not known to the CSS, the address will not appear in the routing table.
The purpose of the implicit service to the next hop of a static route is to monitor the availability of the next hop to forward data traffic. When the ip no-implicit-service command is in effect, traffic is forwarded to the next hop even when the next hop is unavailable. Because of the possibility of data being lost if the next hop becomes unavailable, use of the ip no-implicit-service command is strongly discouraged.
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When you implement the ip no-implicit-service global configuration command, this action does not affect previously configured static routes. The ip no-implicit-service command affects only those static routes added after you enable the command. We recommend you reboot the CSS after you modify the configuration to ensure all static routes are the same, which is useful for network monitoring and troubleshooting. If you wish to stop the implicit service for a previously configured static route, then you must delete and reconfigure the static route.
For example:
(config)# ip no-implicit-serviceTo reset the default setting, enter:
(config)# no ip no-implicit-serviceConfiguring an IP Source Route
Use the ip source-route command to enable the CSS to process frames with information that overrides the default routing. For example:
(config)# ip source-route
Caution
To disable the processing of frames with the IP source-route option (the default behavior), enter:
(config)# no ip source-routeConfiguring the IP Record Route
Use the ip record-route command to enable the CSS to process frames with the IP address of each router along a path. For example:
(config)# ip record-route
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To disable the processing of frames with the record-route option (the default behavior), enter:
(config)# no ip record-routeConfiguring Box-to-Box Redundancy
Use the ip redundancy command to enable box-to-box redundancy. Box-to-box redundancy provides chassis-level redundancy between two identically configured CSSs. Refer to the Cisco Content Services Switch Advanced Configuration Guide for information about configuring box-to-box redundancy.
The CSS does not support simultaneous box-to-box redundancy and VIP or interface redundancy configurations.
For example:
(config)# ip redundancyTo disable box-to-box redundancy, enter:
(config)# no ip redundancyConfiguring IP Equal-Cost Multipath
Use the ip ecmp command to set the equal-cost multipath (ECMP) selection algorithm and the preferred reverse egress path. The CSS supports a maximum of 15 ECMP paths.
The syntax for this global configuration command is:
ip ecmp [address|no-prefer-ingress|roundrobin]
The options for this global configuration mode command are as follows:
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(config)# ip ecmp no-prefer-ingressTo reset the ingress path of a flow for its preferred reverse egress path, enter:
(config)# no ip ecmp no-prefer-ingress•
(config)# ip ecmp roundrobin
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Forwarding IP Subnet Broadcast Addressed Frames
Use the ip subnet-broadcast command to enable the CSS to forward subnet broadcast addressed frames.
For example:
(config)# ip subnet-broadcastTo disable forwarding of subnet broadcast addressed frames (the default behavior), enter:
(config)# no ip subnet-broadcast
Caution
If a "smurf" attack is successful, all the destination subnet hosts reply to the echo and flood the path back to the source. By disabling subnet broadcast forwarding, the original echo never reaches the hosts.
Configuring IP Unconditional Bridging
By default, the routing table lookup of a destination path by the CSS on received packets overrides bridging decisions to be made for those packets. If the routing table specifies that the CSS use a different physical Ethernet port than what is specified for port bridging, the CSS ignores the bridging decision. If you have a network that you want to bridge through the CSS to an upstream router, you may want to force the CSS to make a bridging decision on the received packets instead of making a routing table decision.
Use the ip uncond-bridging global configuration command to always make a bridging decision on the received packets. With this command, the bridging decision always takes precedence over a routing table decision.
For example:
(config)# ip uncond-bridgingTo restore the default behavior of the CSS, enter:
(config)# no ip uncond-bridgingConfiguring IP Opportunistic Layer 3 Forwarding
The CSS opportunistic Layer 3 forwarding feature allows the CSS to reduce the number of network device hops for certain packets or flows. The CSS forwards packets at Layer 3 if the destination MAC address in the Ethernet header is the CSS MAC address. Use the ip opportunistic command to enable opportunistic Layer 3 forwarding and allow the CSS to make Layer 3 forwarding decisions even if the Layer 2 packet destination MAC address does not belong to the CSS.
For example, Figure 6-1 shows a CSS connected to VLAN1 and VLAN2. Each VLAN has an end station and an uplink to Router1. End stations A and B both point to Router1 as their default router. When End Station A transmits a packet to End Station B, it uses its default route to Router1. The packet contains Router1's destination MAC address. A traditional Layer 2 device forwards the packet to Router1, and Router1 forwards the packet to End Station B on VLAN2.
Using opportunistic Layer 3 forwarding, the CSS inspects the IP packet header to determine the destination IP address. Instead of forwarding the packet to Router1, the CSS forwards the packet directly to End Station B. Because the CSS handles the packet only once, the router and uplink are not used and network resources are conserved.
Figure 6-1 Example of Opportunistic Layer 3 Forwarding
The options for this global configuration mode command are as follows:
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For example, to configure IP opportunistic Layer 3 forwarding to all, enter:
(config)# ip opportunistic allTo reconfigure IP opportunistic Layer 3 forwarding to the default of local enter:
(config)# no ip opportunisticWhen you configure ip opportunistic all, you can use the ip route originated-packets command (see the "Configuring an IP Route" section) to configure routes that the CSS uses to reach devices, but does not use as opportunistic routes for forwarding traffic. Routes created using the ip route originated-packets command apply only to packets that originate on the CSS. Packets and flows forwarded by the CSS do not use these routes.
For example:
(config)# ip route 0.0.0.0 /0 192.168.1.7 originated-packetsShowing IP Configuration Information
Use the show ip command to display IP information for the CSS. This section includes the following topics:
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Showing IP Global Configuration Parameters
Use the show ip config command to display IP global configuration parameters. These parameters show the state (enabled or disabled) of the source route option, forward IP broadcasts, record-route option, and IP route change logging. The show ip config command also shows the value for the orphaned route timer.
Table 6-6 describes the fields in the show ip config output.
Showing IP Interface Information
Use the show ip interfaces command to display configured IP interfaces on the CSS. The display includes the circuit state, IP address, broadcast address, Internet Control Message Protocol (ICMP) settings, and Router Discovery Program (RDP) settings.
Table 6-7 describes the fields in the show ip interfaces command output.
Showing IP Routing Information
Use the show ip routes command to display IP routing information. The syntax and options for this command are as follows:
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The variables are as follows:
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To show all IP routes in the CSS, enter:
# show ip routesTable 6-8 describes the fields in the show ip routes command output.
Showing IP Statistics
Use the show ip statistics command to display aggregate TCP statistics for the unit. Table 6-9 describes the fields in the show ip statistics output.
Showing a Summary of IP Global Statistics
Use the show ip summary command to display a summary of IP global statistics. The statistics include data on reachable and total routes, reachable and total hosts, memory in use for each, and total IP routing memory in use.
Table 6-10 describes the fields in the show ip summary command output.
Configuring the Cisco Discovery Protocol
The Cisco Discovery Protocol (CDP) is a medium-independent protocol that runs over Layer 2 (the data link layer) on the CSS and other Cisco manufactured equipment, such as routers, switches, bridges, and access servers. Use the cdp global configuration command to allow the CSS to advertise itself to all other neighboring Cisco CDP-compatible devices on a network. The CSS transmits CDP advertisements to other CDP-compatible devices on the network; the CSS does not listen for CDP messages from the other CDP-compatible devices.
Any Cisco device with CDP support can learn about the CSS by listening to the periodic messages transmitted by the CSS and determining when the CSS is active. Network operators and analysts can use this information for configuration monitoring, topology discovery, and fault diagnosis.
CDP messages contain specific information about the CSS, such as:
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CDP advertisements also include hold time information, which defines the length of time the receiving device is to hold CDP information before discarding it.
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Enabling CDP
Use the cdp run global configuration command to enable CDP transmissions from the CSS to other neighboring Cisco CDP-compatible devices on the network. By default, CDP is disabled for the CSS.
For example:
(config)# cdp runTo disable CDP transmissions on the CSS, enter:
(config)# no cdp runSetting the CDP Hold Time
Use the cdp holdTime global configuration command to specify the amount of time a receiving device retains the CDP information sent by the CSS (time-to-live information) before discarding this information. If a neighboring device does not receive a CDP message before the hold time expires, the neighboring device drops the CSS as a neighbor. Valid entries are 10 to 255 seconds. The default is 180 seconds.
To specify a CDP hold time of 255 seconds for the receiving device, enter:
(config)# cdp holdTime 255To reset the CDP hold time back to the default value of 180 seconds, enter:
(config)# no cdp holdTimeSetting the CDP Transmission Rate
Use the cdp timer global configuration command to specify the frequency at which the CSS transmits CDP packets to all receiving CDP-compatible devices. Valid entries are 5 to 254 seconds. The default is 60 seconds.
To change the CDP transmission rate for the CSS to 120 seconds, enter:
(config)# cdp timer 120To reset the CDP timer to the default rate of 60 seconds, enter:
(config)# no cdp timerShowing CDP Information
Use the show cdp command to display and verify CDP information for the CSS, such as frequency of transmissions and the hold time for transmitted CSS CDP information.
For example:
(config)# show cdpGlobal CDP information:Sending CDP packets every 60 secondsSending a holdtime value of 16 secondsTimeLastCdpSent: 0 days 00:00:30The following example illustrates the CDP output on a Cisco Catalyst 8540 router using the Cisco IOS show cdp neighbors command.
24-8540-1>show cdp neighborsCapability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge S - Switch, H - Host, I - IGMP, r - RepeaterDevice ID Local Intrfce Holdtme Capability Platform Port ID00-10-58-01-4d-e3 Eth 0 178 R T S CSS 11050 Eth-MgmtSCA043801A5 Eth 0 144 T S WS-C6009 3/125-8540-1 Fas 0/0/7 142 R T C8540CSR Fas 0/0/425-8540-1 Eth 0 142 R T C8540CSR Eth 0SCA043801HU(bxb11 Eth 0 151 T S WS-C6009 2/4800-07-85-43-14-1d Eth 0 170 R T S CSS11150 Eth-MgmtConfiguring Spanning-Tree Bridging for the CSS
The CSS supports configuration of Spanning-Tree Protocol (STP) bridging. Spanning-tree bridging detects, and then prevents, loops in the network. Use the bridge command to configure global spanning-tree bridging options for the CSS, such as bridge aging time, forward delay time, hello time interval, and maximum age. Make sure you configure the spanning-tree bridging parameters the same on all switches running STP in the network.
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For details about configuring spanning-tree bridging paremeter for an Ethernet interface or for a trunked Ethernet interface and VLAN pair, refer to Chapter 5, Configuring Interfaces and Circuits.
Configuring Spanning-Tree Bridge Aging-Time
Use the bridge aging-time command to set the bridge filtering database aging time for the CSS. The aging time is the timeout period, in seconds, for aging out dynamically learned forwarding information. Enter an integer from 10 to 1000000. The default is 300.
To set the bridge aging time to 600, enter:
(config)# bridge aging-time 600To restore the default aging time of 300, enter:
(config)# no bridge aging-timeConfiguring Spanning-Tree Bridge Forward-Time
Use the bridge forward-time command to set the bridge forward delay time. The forward time is the delay time, in seconds, that all bridges use for forward delay when this bridge is acting as the root. Enter an integer from 4 to 30. The default is 4.
To set the bridge forward time to 9, enter:
(config)# bridge forward-time 9To restore the default delay time of 4, enter:
(config)# no bridge forward-timeConfiguring Spanning-Tree Bridge Hello-Time
Use the bridge hello-time command to set the bridge hello time interval. The hello time is the time, in seconds, that all bridges wait before sending a hello packet (when the bridge acts as the root). Enter an integer from 1 to 10. The default is 1.
To set the bridge hello time to 9, enter:
(config)# bridge hello-time 9To restore the default hello time interval of 1, enter:
(config)# no bridge hello-timeConfiguring Spanning-Tree Bridge Max-Age
Use the bridge max-age command to set the bridge spanning-tree maximum age. The maximum age is the time, in seconds, that protocol information received on a port is stored by the CSS (when a bridge acts as the root). Enter an integer from 6 to 40. The default is 6.
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To set the bridge maximum age to 21, enter:
(config)# bridge max-age 21To restore the default maximum age of 6, enter:
(config)# no bridge max-ageConfiguring Spanning-Tree Bridge Priority
To set the priority that spanning tree uses to choose the root bridge in the network, use the global bridge priority command. In spanning tree, the 2-octet field is prepended to the 6-octet MAC address to form an 8-octet bridge identifier. The device with the lowest bridge identifier is considered the highest priority bridge and becomes the root bridge. The range for bridge priority is 0 to 65535. The default is 32768.
For example:
(config)# bridge priority 1700To restore the bridge priority to the default of 32768, enter:
(config)# no bridge priorityDisabling Bridge Spanning-Tree
Spanning-tree bridging is enabled by default. When you disable spanning-tree bridging, the CSS drops those bridge protocol data units (BPDUs) that it recognizes as BPDUs, but forwards the Cisco Systems 802.1Q BPDUs (tagged with the proprietary 01-00-0c-cc-cc-cc-cd destination MAC address) on an 802.1Q VLAN trunk. The CSS can still operate in an 802.1Q spanning-tree environment as long as you do not require that the CSS put any of its ports into a blocking state.
To disable spanning-tree bridging, enter:
(config)# bridge spanning-tree disable
Caution
To reenable spanning-tree bridging, enter:
(config)# bridge spanning-tree enableShowing Bridge Configurations
Use the show bridge forwarding command to display bridge forwarding information. Table 6-11 describes the fields in the show bridge forwarding command output.
Use the show bridge status command to display bridge status information. Table 6-12 describes the fields in the show bridge status output.
Configuring the DHCP Relay Agent
The Dynamic Host Configuration Protocol (DHCP) servers provide configuration parameters to DHCP clients. When DHCP clients and associated servers do not reside on the same IP network or subnet, a DHCP relay agent can transfer DHCP messages between them. To configure a DHCP relay agent on a CSS, define DHCP server destinations on a circuit and enable the DHCP relay agent on the circuit.
You must first assign an IP address on the circuit to be able to configure the DHCP relay agent for the circuit. Use the ip address command in the specific circuit mode to assign the IP address and a subnet mask. For example:
(config-circuit[VLAN2])# ip address 178.3.6.53/8This section includes the following topics:
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Adding a DHCP Destination on a Circuit
A CSS circuit acts as the DHCP relay agent. For each circuit on the CSS, you can configure a maximum of five DHCP destinations. The initial DHCP broadcast request is sent to all of the configured destinations.
Do not configure a relay destination on a circuit when the relay destination is directly connected to or reachable from one of the ports on the same circuit. In this case, the DHCP packets reach the relay destination through normal broadcast and a relay agent is not required.
Use the dhcp relay-to command to specify the DHCP relay destination address. This command is available in circuit configuration mode. Enter an IP address in dotted-decimal notation.
For example, to add a destination address of 192.168.22.25 to a DHCP server, enter:
(config-circuit[VLAN2])# dhcp relay-to 192.168.22.25To remove the relay destination address, enter:
(config-circuit[VLAN2])# no dhcp relay-to 192.168.22.25Enabling and Disabling DHCP on the Circuit
After you enable the DHCP relay agent on the CSS circuit, the CSS transfers DHCP messages between DHCP clients and servers. Use the dhcp-relay-agent command to enable the agent on the circuit. This command is available in circuit configuration mode.
For example:
(config-circuit[VLAN2])# dhcp-relay-agentTo disable the DHCP relay agent on the circuit, enter:
(config-circuit[VLAN2])# no dhcp-relay-agentDefining the Hops Field Value for Forwarding DHCP Messages
The CSS forwards or discards a DHCP message based on the hops field value in the BOOTP header. When messages have values in the hops fields that exceed the maximum value set on the CSS, the CSS discards the message. Use the dhcp-agent max-hops global configuration command to set the maximum allowable number in the hops field. By default, the maximum allowable number is 4. You can set a number from 1 to 15.
For example, to set the maximum allowable value of 10, enter:
(config)# dhcp-agent max-hops 10To reset the maximum allowable number in the hops field to the default of 4, enter:
(config)# no dhcp-agent max-hopsDisplaying the DHCP Relay Configuration
Use the show dhcp-relay-agent global command to display the DHCP configuration information on a CSS. This command is available in all modes. For example:
# show dhcp-relay-agent globalTable 6-13 describes the fields in the show dhcp-relay-agent global command output.
Where to Go Next
Chapter 7, Configuring Open Shortest Path First (OSPF) describes how to configure and view information for the OSPF protocol.
