The documentation set for this product strives to use bias-free language. For the purposes of this documentation set, bias-free is defined as language that does not imply discrimination based on age, disability, gender, racial identity, ethnic identity, sexual orientation, socioeconomic status, and intersectionality. Exceptions may be present in the documentation due to language that is hardcoded in the user interfaces of the product software, language used based on RFP documentation, or language that is used by a referenced third-party product. Learn more about how Cisco is using Inclusive Language.
This chapter describes how to configure the Hot Standby Router Protocol (HSRP) on the Cisco NX-OS switch.
HSRP is a first-hop redundancy protocol (FHRP) that allows a transparent failover of the first-hop IP router. HSRP provides first-hop routing redundancy for IP hosts on Ethernet networks configured with a default router IP address. You use HSRP in a group of routers for selecting an active router and a standby router. In a group of routers, the active router is the router that routes packets; the standby router is the router that takes over when the active router fails or when preset conditions are met.
Many host implementations do not support any dynamic router discovery mechanisms but can be configured with a default router. Running a dynamic router discovery mechanism on every host is not feasible for a number of reasons, including administrative overhead, processing overhead, and security issues. HSRP provides failover services to these hosts.
This section includes the following topics:
When you use HSRP, you configure the HSRP virtual IP address as the host’s default router (instead of the IP address of the actual router). The virtual IP address is an IPv4 or IPv6 address that is shared among a group of routers that run HSRP.
When you configure HSRP on a network segment, you provide a virtual MAC address and a virtual IP address for the HSRP group. You configure the same virtual address on each HSRP-enabled interface in the group. You also configure a unique IP address and MAC address on each interface that acts as the real address. HSRP selects one of these interfaces to be the active router. The active router receives and routes packets destined for the virtual MAC address of the group.
HSRP detects when the designated active router fails. At that point, a selected standby router assumes control of the virtual MAC and IP addresses of the HSRP group. HSRP also selects a new standby router at that time.
HSRP uses a priority mechanism to determine which HSRP-configured interface becomes the default active router. To configure an interface as the active router, you assign it with a priority that is higher than the priority of all the other HSRP-configured interfaces in the group. The default priority is 100, so if you configure just one interface with a higher priority, that interface becomes the default active router.
Interfaces that run HSRP send and receive multicast User Datagram Protocol (UDP)-based hello messages to detect a failure and to designate active and standby routers. When the active router fails to send a hello message within a configurable period of time, the standby router with the highest priority becomes the active router. The transition of packet forwarding functions between the active and standby router is completely transparent to all hosts on the network.
You can configure multiple HSRP groups on an interface.
Figure 17-1 shows a network configured for HSRP. By sharing a virtual MAC address and a virtual IP address, two or more interfaces can act as a single virtual router.
Figure 17-1 HSRP Topology with Two Enabled Routers
The virtual router does not physically exist but represents the common default router for interfaces that are configured to provide backup to each other. You do not need to configure the hosts on the LAN with the IP address of the active router. Instead, you configure them with the IP address (virtual IP address) of the virtual router as their default router. If the active router fails to send a hello message within the configurable period of time, the standby router takes over, responds to the virtual addresses, and becomes the active router, assuming the active router duties. From the host perspective, the virtual router remains the same.
Note Packets received on a routed port destined for the HSRP virtual IP address will terminate on the local router, regardless of whether that router is the active HSRP router or the standby HSRP router. This includes ping and Telnet traffic. Packets received on a Layer 2 (VLAN) interface destined for the HSRP virtual IP address will terminate on the active router.
HSRP routers communicate with each other by exchanging HSRP hello packets. These packets are sent to the destination IP multicast address 224.0.0.2 (reserved multicast address used to communicate to all routers) on UDP port 1985. The active router sources hello packets from its configured IP address and the HSRP virtual MAC address while the standby router sources hellos from its configured IP address and the interface MAC address, which may or may not be the burned-in address (BIA). The BIA is the last six bytes of the MAC address that is assigned by the manufacturer of the network interface card (NIC).
Because hosts are configured with their default router as the HSRP virtual IP address, hosts must communicate with the MAC address associated with the HSRP virtual IP address. This MAC address is a virtual MAC address, 0000.0C07.ACxy, where xy is the HSRP group number in hexadecimal based on the respective interface. For example, HSRP group 1 uses the HSRP virtual MAC address of 0000.0C07.AC01. Hosts on the adjoining LAN segment use the normal Address Resolution Protocol (ARP) process to resolve the associated MAC addresses.
HSRP version 2 uses the new IP multicast address 224.0.0.102 to send hello packets instead of the multicast address of 224.0.0.2, which is used by version 1. HSRP version 2 permits an expanded group number range of 0 to 4095 and uses a new MAC address range of 0000.0C9F.F000 to 0000.0C9F.FFFF
IPv6 hosts learn of available IPv6 routers through IPv6 neighbor discovery (ND) router advertisement (RA) messages. These messages are multicast periodically, or be solicited by hosts, but the time delay for detecting when a default route is down be 30 seconds or more. HSRP for IPv6 provides a much faster switchover to an alternate default router than the IPv6 ND protocol provides, less than a second if the milliseconds timers are used. HSRP for IPv6 provides a virtual first hop for IPv6 hosts.
When you configure an IPv6 interface for HSRP, the periodic RAs for the interface link-local address stop after IPv6 ND sends a final RA with a router lifetime of zero. No restrictions occur for the interface IPv6 link-local address. Other protocols continue to receive and send packets to this address.
IPv6 ND sends periodic RAs for the HSRP virtual IPv6 link-local address when the HSRP group is active. These RAs stop after a final RA is sent with a router lifetime of 0 when the HSRP group leaves the active state. HSRP uses the virtual MAC address for active HSRP group messages only (hello, coup, and redesign).
An HSRP IPv6 group has a virtual MAC address that is derived from the HSRP group number and a virtual IPv6 link-local address that is derived, by default, from the HSRP virtual MAC address. The default virtual MAC address for an HSRP IPv6 group always used to form the virtual IPv6 link-local address, regardless of the actual virtual MAC address used by the group.
Table 17-1 shows the MAC and IP addresses used for IPv6 neighbor discovery packets and HSRP packets.
|
|
|
|
|
---|---|---|---|---|
HSRP does not add IPv6 link-local addresses to the Unicast Routing Information Base (URIB). There are also no secondary virtual IP addresses for link-local addresses.
For global unicast addresses, HSRP adds the virtual IPv6 address to the URIB and IPv6 but does not register the virtual IPv6 addresses to ICMPv6. ICMPv6 redirects are not supported for HSRP IPv6 groups.
Cisco NX-OS supports HSRP version 1 by default. You can configure an interface to use HSRP version 2.
HSRP version 2 has the following enhancements to HSRP version 1:
When you change the HSRP version, Cisco NX-OS reinitializes the group because it now has a new virtual MAC address.
HSRP version 2 has a different packet format than HSRP version 1. The packet format uses a type-length-value (TLV) format. HSRP version 2 packets received by an HSRP version 1 router are ignored.
HSRP message digest 5 (MD5) algorithm authentication protects against HSRP-spoofing software and uses the industry-standard MD5 algorithm for improved reliability and security. HSRP includes the IPv4 or IPv6 address in the authentication TLVs.
Routers that are configured with HSRP exchange the following three types of multicast messages:
HSRP allows you to configure multiple groups on an interface. You can configure two overlapping IPv4 HSRP groups to load share traffic from the connected hosts while providing the default router redundancy expected from HSRP. Figure 17-2 shows an example of a load-sharing HSRP IPv4 configuration.
Figure 17-2 shows two routers (A and B) and two HSRP groups. Router A is the active router for group A but is the standby router for group B. Similarly, router B is the active router for group B and the standby router for group A. If both routers remain active, HSRP load balances the traffic from the hosts across both routers. If either router fails, the remaining router continues to process traffic for both hosts
Note HSRP for IPv6 load balances by default. If there are two HSRP IPv6 groups on the subnet, hosts learn of both from their router advertisements and choose to use one so that the load is shared between the advertised routers.
HSRP supports bidirectional forwarding detection (BFD). BFD is a detection protocol that provides fast forwarding-path failure detection times. BFD provides subsecond failure detection between two adjacent devices and can be less CPU-intensive than protocol hello messages because some of the BFD load can be distributed onto the data plane on supported modules. See the Cisco Nexus 5500 Series NX-OS Interfaces Configuration Guide, Release 6.0 for more information.
HSRP interoperates with virtual port channels (vPCs). vPCs allow links that are physically connected to two different Cisco Nexus 5500 switches to appear as a single port channel by a third switch. See the Cisco Nexus 5500 Series NX-OS Layer 2 Switching Configuration Guide, Release 6.0, for more information on vPCs.
vPC forwards traffic through both the active HSRP router and the standby HSRP router. You can configure a threshold on the priority of the standby HSRP router to determine when traffic should fail over to the vPC trunk. See the “Configuring the HSRP Priority” section.
Note You should configure HSRP on the primary vPC peer switch as active and HSRP on the vPC secondary switch as standby.
Some third-party devices can ignore the HSRP virtual MAC address and instead use the source MAC address of an HSRP router. in a vPC environment, the packets using this source MAC address may be sent across the vPC peer link, causing a potential dropped packet. Configure the vPC peer gateway to enable the HSRP routers to directly handle packets sent to the local vPC peer MAC address and the remote vPC peer MAC address, as well as the HSRP virtual MAC address. See the Cisco Nexus 5500 Series NX-OS Layer 2 Switching Configuration Guide, Release 6.0, for more information on the vPC peer gateway.
Note For mixed-chassis configurations where the vPC peer link is configured on an F-series module, configure the vPC peer gateway exclude option to exclude the Layer 3 backup route that traverses the vPC peer link. See the Cisco Nexus 5500 Series NX-OS Layer 2 Switching Configuration Guide, Release 6.0, for more information on the vPC peer gateway exclude option.
HSRP supports Virtual Routing and Forwarding instances (VRFs).
If you change the VRF membership of an interface, Cisco NX-OS removes all Layer 3 configuration, including HSRP.
The following table shows the licensing requirements for this feature:
HSRP has the following configuration guidelines and limitations:
Table 17-2 lists the default settings for HSRP parameters.
|
|
---|---|
This section includes the following topics:
Note If you are familiar with the Cisco IOS CLI, be aware that the Cisco NX-OS commands for this feature might differ from the Cisco IOS commands that you would use.
To enable the HSRP feature, use the following command in global configuration mode:
|
|
---|---|
To disable the HSRP feature and remove all associated configuration, use the following command in global configuration mode:
|
|
---|---|
You can configure the HSRP version. If you change the version for existing groups, Cisco NX-OS reinitializes HSRP for those groups because the virtual MAC address changes. The HSRP version applies to all groups on the interface.
Note IPv6 HSRP groups must be configured as HSRP version 2.
To configure the HSRP version, use the following command in interface configuration mode:
|
|
---|---|
You can configure an HSRP group on an IPv4 interface and configure the virtual IP address and virtual MAC address for the HSRP group.
Ensure that you have enabled the HSRP feature (see the “Enabling the HSRP Feature” section).
Cisco NX-OS enables an HSRP group once you configure the virtual IP address on any member interface in the group. You should configure HSRP attributes such as authentication, timers, and priority before you enable the HSRP group.
6. ip [ ip-address [ secondary ]]
Note You should use the no shutdown command to enable the interface after you finish the configuration.
This example shows how to configure an HSRP group on Ethernet 1/2:
You can configure an HSRP group on an IPv6 interface and configure the virtual MAC address for the HSRP group.
When you configure an HSRP group for IPv6, HSRP generates a link-local address from the link-local prefix. HSRP also generates a modified EUI-64 format interface identifier in which the EUI-64 interface identifier is created from the relevant HSRP virtual MAC address.
Ensure that you have enabled the HSRP feature (see the “Enabling the HSRP Feature” section).
Ensure that you have enabled HSRP version 2 on the interface that you want to configure an IPv6 HSRP group on.
Ensure that you have configured HSRP attributes such as authentication, timers, and priority before you enable the HSRP group.
6. ip ipv6-address [secondary]
Note You should use the no shutdown command to enable the interface after you finish the configuration.
The following example shows how to configure an IPv6 HSRP group on Ethernet 3/2:
switch(config)# interface ethernet 3/2
switch(config-if)# ip 12001:0DB8:0001:0001:/64
switch(config-if)# hsrp 2 ipv6
switch(config-if)# no shutdown
You can override the default virtual MAC address that HSRP derives from the configured group number.
Note You must configure the same virtual MAC address on both vPC peers of a vPC link.
To manually configure the virtual MAC address for an HSRP group, use the following command in hsrp configuration mode:
|
|
---|---|
Configures the virtual MAC address for an HSRP group. The string uses the standard MAC address format (xxxx.xxxx.xxxx). |
To configure HSRP to use the burned-in MAC address of the interface for the virtual MAC address, use the following command in interface configuration mode:
You can configure HSRP to authenticate the protocol using cleartext or MD5 digest authentication. MD5 authentication uses a key chain (see the Cisco Nexus 5500 Series NX-OS Security Configuration Guide, Release 6.0).
Ensure that you have enabled the HSRP feature (see the “Enabling the HSRP Feature” section).
You must configure the same authentication and keys on all members of the HSRP group.
Ensure that you have created the key chain if you are using MD5 authentication.
2. interface interface- type slot/port
4. hsrp group- number [ ipv4 | ipv6 ]
5. authentication text string
or
authentication md5 { key-chain key-chain | key-string { 0 | 7 } text [ timeout seconds ]}
This example shows how to configure MD5 authentication for HSRP on Ethernet 1/2 after creating the key chain:
switch(config-if)# no switchport
You can configure the HSRP priority on an interface. HSRP uses the priority to determine which HSRP group member acts as the active router. If you configure HSRP on a vPC-enabled interface, you can optionally configure the upper and lower threshold values to control when to fail over to the vPC trunk If the standby router priority falls below the lower threshold, HSRP sends all standby router traffic across the vPC trunk to forward through the active HSRP router. HSRP maintains this scenario until the standby HSRP router priority increases above the upper threshold.
For IPv6 HSRP groups, if all group members have the same priority, HSRP selects the active router based on the IPv6 link-local address.
To configure the HSRP priority, use the following command in interface configuration mode:
You can optionally customize the behavior of HSRP. Be aware that as soon as you enable an HSRP group by configuring a virtual IP address, that group is now operational. If you first enable an HSRP group before customizing HSRP, the router could take control over the group and become the active router before you finish customizing the feature. If you plan to customize HSRP, you should do so before you enable the HSRP group.
To customize HSRP, use the following commands in interface configuration mode:
To display the HSRP configuration information, perform one of the following tasks:
This example shows how to enable HSRP on an interface with MD5 authentication and interface tracking:
For additional information related to implementing HSRP, see the following sections:
|
|
---|---|
Cisco Nexus 5000 Series Command Reference, Cisco NX-OS Releases 4.x, 5.x |