Linux typically makes use of memory protection and virtual memory and can also handle non-uniform memory access,[100] however the project has absorbed μClinux which also makes it possible to run Linux on microcontrollers without virtual memory.[101]
Linux implements virtual memory with 4 and 5-levels page tables. As said, only user memory space is always pageable. It maintains information about each page frame of RAM in apposite data structures (of type struct page) that are populated immediately after boots and that are kept until shutdown, regardless of them being or not associated with virtual pages. Furthermore, it classifies all page frames in zones, according to their architecture dependent constraints and intended use. For example, pages reserved for DMA operations are in ZONE_DMA, pages that are not permanently mapped to virtual addresses are in ZONE_HIGHMEM (in x86_32 architecture this zone is for physical addresses above 896 MB, while x86_64 does not need it because x86_64 can permanently map physical pages that reside in higher addresses), and all that remains (with the exception of other less used classifications) is in ZONE_NORMAL.
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Small chunks of memory can be dynamically allocated via the family of kmalloc() API and freed with the appropriate variant of kfree(). vmalloc() and kvfree() are used for large virtually contiguous chunks. alloc_pages() allocates the desired number of entire pages.
Two or more routers can act as a single, virtual router if they share an IP address and a MAC (Layer 2 [L2]) address. The address is necessary for host workstation default gateway redundancy. Most host workstations do not contain routing tables and use only a single next hop IP and MAC address. This address is known as a default gateway. With HSRP, members of the virtual router group continually exchange status messages. One router can assume the routing responsibility of another if a router goes out of commission for either planned or unplanned reasons. Hosts are configured with a single default gateway and continue to forward IP packets to a consistent IP and MAC address. The changeover of devices that do the routing is transparent to the end workstations.
A set of routers that run HSRP works in concert to present the illusion of a single default gateway router to the hosts on the LAN. This set of routers is known as an HSRP group or standby group. A single router that is elected from the group is responsible to forward the packets that hosts send to the virtual router. This router is known as the active router. Another router is elected as the standby router. If the active router fails, the standby assumes the packet forwarding duties. Although an arbitrary number of routers can run HSRP, only the active router forwards the packets that are sent to the virtual router IP address.
Each standby group emulates a single virtual router (default gateway). For each group, a single well-known MAC and IP address is allocated to that group. Multiple standby groups can coexist and overlap on a LAN, and individual routers can participate in multiple groups. In this case, the router maintains a separate state and timers for each group.
Routers that run HSRP communicate HSRP information between each other through HSRP hello packets. These packets are sent to the destination IP multicast address 224.0.0.2 on User Datagram Protocol (UDP) port 1985. IP multicast address 224.0.0.2 is a reserved multicast address that is used to communicate to all routers. The active router sources hello packets from its configured IP address and the HSRP virtual MAC address. The standby router sources hellos from its configured IP address and the burned-in MAC address (BIA). This use of source addressing is necessary so that HSRP routers can correctly identify each other.
Because host workstations are configured with their default gateway as the HSRP standby IP address, hosts must communicate with the MAC address that is associated with the HSRP standby IP address. This MAC address is a virtual MAC address that is composed of 0000.0c07.ac**. The ** 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 in order to resolve the associated MAC addresses.
HSRP peer routers that protect a subnet are able to provide access to all other subnets in the network. This is the basis of HSRP. Therefore, which router becomes the active HSRP router is irrelevant. In Cisco IOS software releases earlier than Cisco IOS Software Release 12.1(3)T, ICMP redirects are automatically disabled on an interface when HSRP is used on that interface. Without this configuration, the hosts can be redirected away from the HSRP virtual IP address and toward an interface IP and MAC address of a single router. Redundancy is lost.
Cisco IOS Software introduces a method to allow ICMP redirects with HSRP. This method filters outbound ICMP redirect messages through HSRP. The next hop IP address is changed to an HSRP virtual address. The gateway IP address in the outbound ICMP redirect message is compared to a list of HSRP active routers that are present on that network. If the router that corresponds to the gateway IP address is an active router for an HSRP group, the gateway IP address is replaced with that group virtual IP address. This solution allows hosts to learn optimal routes to remote networks and, at the same time, maintain the resilience that HSRP provides.
You must understand the concept behind packet flow when you obtain sniffer traces to troubleshoot HSRP problems. Router A uses the priority of 200 and becomes the active router on both interfaces. In the example in this section, packets from the router that are destined for a host workstation have the source MAC address of the router physical MAC address (BIA). Packets from the host machines that are destined for the HSRP IP address have the destination MAC address of the HSRP virtual MAC address. Note that the MAC addresses are not the same for each flow between the router and the host.
In Catalyst Switches, the switch reports a host MAC address that moves if the host MAC address moves twice within 15 seconds. A possible cause is an STP loop. The switch discards packets from this host for about 15 seconds in an effort to minimize the impact of an STP loop. If the MAC address move between two ports that is reported is the HSRP virtual MAC address, the problem is most likely an issue in which both HSRP routers go into the active state.
If the MAC address that is reported is not the HSRP virtual MAC address, the issue can indicate the loop, duplication, or reflection of packets in the network. These types of conditions can contribute to HSRP problems. The most common causes for the move of MAC addresses are spanning tree problems or physical layer problems.
The workaround is to use the standby use-bia interface configuration command, which uses the Burned-In Address (BIA) of the interface as its virtual MAC address, instead of the preassigned MAC address.
Unless HSRP is configured on a Token Ring interface, only use the standby use-bia command in special circumstances. This command tells the router to use its BIA instead of the virtual HSRP MAC address for the HSRP group. On a Token Ring network, if source-route bridging (SRB) is in use, the standby use-bia command allows the new active router to update the host Routing Information Field (RIF) cache with a gratuitous ARP. But, not all of the host implementations handle the gratuitous ARP correctly. Another caveat for the standby use-bia command involves proxy ARP. A standby router cannot cover for the lost proxy ARP database of the failed active router.
Verify that the MAC address table entries exist on the switch for the HSRP routers for the HSRP virtual MAC address and the physical BIAs. The show standby command on the router provides the virtual MAC address. The show interface command provides the physical BIA. Here are sample outputs:
BPL is the delivery of broadband over the existing low- and medium-voltage electric power distribution network. BPL speeds are comparable to DSL and cable modem speeds. BPL can be provided to homes using existing electrical connections and outlets. BPL is an emerging technology that is available in very limited areas. It has significant potential because power lines are installed virtually everywhere, alleviating the need to build new broadband facilities for every customer.
Virtualization support: The SATA ports can be used with virtualization hypervisors, including VMware ESXi, Linux KVM, Xen, and Microsoft Hyper-V, however support is limited to AHCI (non-RAID) mode. RSTe mode is not supported with virtualization hypervisors.
Lenovo also offers software plug-in modules, Lenovo XClarity Integrators, to manage physical infrastructure from leading external virtualization management software tools including those from Microsoft and VMware.
Virtualization support: The onboard SATA ports of the server can be used with virtualization hypervisors, including VMware ESXi, Linux KVM, Xen, and Microsoft Hyper-V, however support is limited to AHCI (non-RAID) mode. RSTe mode is not supported with virtualization hypervisors.
If you are still trying to figure out what a virtual influencer is, you are forgiven, considering that the influencer marketing landscape continues to grow at such an immense pace. In short, a virtual influencer is a digital character that was created using computer graphics software. This character is then given a personality and will at all times act on social media platforms as if he/she is the influencer.
A study in the US found that 58 percent of respondents were following a virtual influencer at the time, establishing the influencers as a strong force to be reckoned with. According to Christopher Travers, the founder of VirtualHumans.org, virtual influencers can do anything that human influencers can do, but with more control and engagement. As a matter of fact, virtual influencers can offer nearly three times the engagement rates of real influencers. The team at HypeAuditor and VirtualHumans.org worked together to compile a list of the top virtual influencers. Without further ado, let us introduce you to the virtual characters that you are bound to hear a lot more about in the future. 2ff7e9595c
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