TelePresence codecs route all traffic through their default gateway router. If (for whatever reason) the default gateway router fails, TelePresence calls hang or self-terminate, depending on the amount of time it takes for the default gateway router to recover (or be replaced).
The Cisco Hot Standby Router Protocol (HSRP) is the first of three First-Hop Redundancy Protocols (FHRP) discussed in this chapter (the other two being VRRP and GLBP). An FHRP provides increased availability by allowing for transparent failover of the first-hop (or default gateway) router.
HSRP is used in a group of routers for selecting an active router and a standby router. In a group of router interfaces, the active router is the router of choice for routing packets; the standby router is the router that takes over when the active router fails or when preset conditions are met.
Endpoint devices, or IP hosts, have an IP address of a single router configured as the default gateway. When HSRP is used, the HSRP virtual IP address is configured as the host’s default gateway instead of the actual IP address of the router.
When HSRP is configured on a network segment, it provides a virtual MAC address and an IP address that is shared among a group of routers running HSRP. The address of this HSRP group is referred to as the virtual IP address. One of these devices is selected by the HSRP to be the active router. The active router receives and routes packets destined for the MAC address of the group.
HSRP detects when the designated active router fails; at which point, a selected standby router assumes control of the MAC and IP addresses of the Hot Standby group. A new standby router is also selected at that time.
HSRP uses a priority mechanism to determine which HSRP configured router is to be the default active router. To configure a router as the active router, you assign it a priority that is higher than the priority of all the other HSRP-configured routers. The default priority is 100, so if just one router is configured to have a higher priority, that router will be the default active router.
Devices that run HSRP send and receive multicast User Datagram Protocol (UDP)-based hello messages to detect router 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 routers is completely transparent to all hosts on the network.
Multiple Hot Standby groups can be configured on an interface, thereby making fuller use of redundant routers and load sharing.
Figure 1 shows a network configured for HSRP. By sharing a virtual MAC address and IP address, two or more routers can act as a single virtual router. The virtual router does not physically exist but represents the common default gateway for routers that are configured to provide backup to each other. All IP hosts are configured with the IP address of the virtual router as their default gateway. If the active router fails to send a hello message within the configurable period of time, the standby router takes over and responds to the virtual addresses and becomes the active router, assuming the active router duties.
HSRP also supports object tracking so that the HSRP priority of a router can dynamically change when an object that is tracked goes down. Examples of objects that can be tracked are the line protocol state of an interface or the reachability of an IP route. If the specified object goes down, the HSRP priority is reduced.
Furthermore, HSRP supports SSO awareness so that HRSP can alter its behavior when a router with redundant Route Processors (RP) is configured as SSO redundancy mode. When an RP is active and the other RP is standby, SSO enables the standby RP to take over if the active RP fails.
With this functionality, HSRP SSO information is synchronized to the standby RP, allowing traffic that is sent using the HSRP virtual IP address to be continuously forwarded during a switchover without a loss of data or a path change. Additionally, if both RPs fail on the active HSRP router, the standby HSRP router takes over as the active HSRP router.
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SSO awareness for HSRP is enabled by default when the RP’s redundancy mode of operation is set to SSO (as shown in the “NSF with SSO” section of this chapter).
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Example 1 demonstrates the HSRP configuration that you can use on the LAN interface of the active router from Figure 1. Each HSRP group on a given subnet requires a unique number; in this example the HSRP group number is set to 10. The virtual router’s IP address (which is what each IP host on the network uses as a default-gateway address) is set to 172.16.128.3. The HRSP priority of this router has been set to 105, and preemption has been enabled on it; preemption allows for the router to immediately take over as the virtual router (provided it has the highest priority on the segment). Finally, object tracking has been configured so that if the line protocol state of interface Serial0/1 goes down (the WAN link for the active router, which is designated as object-number 110), the HSRP priority for this interface dynamically decrements (by a value of 10, by default).
Example 1: HSRP Example
Router(config)# track 110 interface Serial0/1 line-protocol Router(config)# interface GigabitEthernet0/0 Router(config-if)# ip address 172.16.128.1 255.255.255.0 Router(config-if)# standby 10 ip 172.16.128.3 Router(config-if)# standby 10 priority 105 preempt Router(config-if)# standby 10 track 110
As HRSP was the first FHRP and because it was invented by Cisco, it is Cisco proprietary. However, to support multivendor interoperability, aspects of HSRP were standardized in the Virtual Router Redundancy Protocol
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