یکی از terms هائی که در طراحی دیتا سنتر باید توجه عمیقی بهش بشه مسئله Redundancy و Load Balancing هست. منابع دیتا سنتر باید در هر شرایطی در دسترس باشند اصل شکل گیری دیتا سنتر هم بر همین اساس بوده، مکانی جهت امنیت و دسترسی سریع و بی وقفه به اطلاعات.
بنا به اسکیل هر دیتا سنتر بار متفاوتی بر روی دیوایس های مسیریابی شبکه وجود خواهد داشت و بر خلاف ایران که سرعت بالا چیزی نیست که عموم اقشار باهاش مانوس باشند در کشور های توسعه یافته مردم میتونند فیبر با سرعت چند گیگ درب منزلشون تحویل بگیرند ADSL2+ با سرعت up to 35m که چیزه ساده ایه! البته این امکانات کار رو برای سرویس دهنده ها سخت تر میکنه چرا که اونوقت با ترابیت سر و کار دارند نه مثل ایران با مگابیت.
سیسکو به عنوان یکی از پیشتازان صنعت شبکه های کامپیوتری یه سری پروتکل خودساز proprietary داره که از جمله اون میشه پروتکل تقسیم بار بر روی درگاه ها یا همون GLBP رو نامبرد! این پروتکل Load Sharing و Redundancy رو همزمان با هم در دل خودش داره و این خودش مزیت بزرگیه واقعا! خوب اساس کار اینه که یه سری دیوایس GLBP Support مثل روتر داریم که در لب مرز دیتا سنتر ما قرار دارن، ما تا حد ممکن میخوایم مسئله رو ساده نشون بدیم که اصل داستان دستگیرتون بشه و بعد خودتون بسته به خلاقیتتون میتونید اونو بست بدید وگرنه هیچ دیتا سنتری شبکه ای به این سادگی نداره.
این روتر ها وظیفه مسیریابی ترافیک مرزی دیتا سنتر رو دارن و از طرفی گیت وی یه سری کلاینت دیگه هستن که میخوان به این منابع دسترسی پیدا کنن اینجوری که ما طرح کردیم مسئله رو جهت ساده سازیه والا اصلا اینجوری نیس!
طبق این شماتیک ما 5 تا روتر داریم که یک پورتشون خورده به یه سویچ که ارتباط با بقیه سازمانه و بقیه سازمان از این طریق به منابع دیتا سنتر دسترسی پیدا میکنن و یک پورت دیگه هم به منابع دیتا سنتر متصله:
Default Gateway تمامی این کلاینت ها 192.168.0.1 هست که در واقع IP گروه GLBP ماست.
حالا نمونه کانفیگ روترها:
Router1
interface FastEthernet0/0
ip address 192.168.0.11
glbp 5 ip 192.168.0.1
glbp 5 priority 200
glbp 5 preempt
Router2
interface FastEthernet0/0
ip address 192.168.0.12
glbp 5 ip 192.168.0.1
glbp 5 priority 190
glbp 5 preempt
Router3
interface FastEthernet0/0
ip address 192.168.0.13
glbp 5 ip 192.168.0.1
glbp 5 priority 180
glbp 5 preempt
Router4
interface FastEthernet0/0
ip address 192.168.0.14
glbp 5 ip 192.168.0.1
glbp 5 priority 170
glbp 5 preempt
Router5
interface FastEthernet0/0
ip address 192.168.0.15
glbp 5 ip 192.168.0.1
glbp 5 priority 160
glbp 5 preempt
ما در هر روتر اون پورتی که به کلایت ها متصل هست رو اینطوری کانفیگ میکنیم:
اول یه IP در همون رنج میدیم مثلا
ip address 192.168.0.11
و بعد IP گروه GLBP که همون دیفالت گیت وی کلاینت هاست رو نتظیم میکنیم که 192.168.0.1 شماره گروه GLBP هم 5 و در خط بعدی میزان ارزش این مسیریاب رو برای تقسیم بار مشخص میکنیم:
glbp 5 priority 200
پرایوریتی بالاتر به معنی Utilize بیشتر این گیت وی.
همونطور که قبلا هم ذکر شد GLBP مزیت Redandancy رو هم در دل خودش داره یعنی اگر یکی از این روتر ها دچار نقص شد از مجموعه کنار گذاشته و بار بین بقیه تقسیم میشه
مطالعات بیشتر در اینجا و اینجا
The Gateway Load Balancing Protocol feature provides automatic router backup for IP hosts configured with a single default gateway on an IEEE 802.3 LAN. Multiple first hop routers on the LAN combine to offer a single virtual first hop IP router while sharing the IP packet forwarding load. Other routers on the LAN may act as redundant GLBP routers that will become active if any of the existing forwarding routers fail.
Gateway Load Balancing Protocol performs a similar, but not identical, function for the user as the HSRP and the VRRP. HSRP and VRRP protocols allow multiple routers to participate in a virtual router group configured with a virtual IP address. One member is elected to be the active router to forward packets sent to the virtual IP address for the group. The other routers in the group are redundant until the active router fails. These standby routers have unused bandwidth that the protocol is not using. Although multiple virtual router groups can be configured for the same set of routers, the hosts must be configured for different default gateways, which results in an extra administrative burden. Gateway Load Balancing Protocol provides load balancing over multiple routers (gateways) using a single virtual IP address and multiple virtual MAC addresses. Each host is configured with the same virtual IP address, and all routers in the virtual router group participate in forwarding packets. GLBP members communicate between each other through hello messages sent every 3 seconds to the multicast address 224.0.0.102, User Datagram Protocol (UDP) port 3222 (source and destination).
Gateway Load Balancing Protocol Active Virtual Gateway
Members of a Gateway Load Balancing Protocol group elect one gateway to be the active virtual gateway (AVG) for that group. Other group members provide backup for the AVG in the event that the AVG becomes unavailable. The AVG assigns a virtual MAC address to each member of the GLBP group. Each gateway assumes responsibility for forwarding packets sent to the virtual MAC address assigned to it by the AVG. These gateways are known as active virtual forwarders (AVFs) for their virtual MAC address.
The AVG is responsible for answering Address Resolution Protocol (ARP) requests for the virtual IP address. Load sharing is achieved by the AVG replying to the ARP requests with different virtual MAC addresses.
In Figure 1, Router A is the AVG for a GLBP group, and is responsible for the virtual IP address 10.21.8.10. Router A is also an AVF for the virtual MAC address 0007.b400.0101. Router B is a member of the same GLBP group and is designated as the AVF for the virtual MAC address 0007.b400.0102. Client 1 has a default gateway IP address of 10.21.8.10 and a gateway MAC address of 0007.b400.0101. Client 2 shares the same default gateway IP address but receives the gateway MAC address 0007.b400.0102 because Router B is sharing the traffic load with Router A.
GLBP Virtual MAC Address Assignment
A Gateway Load Balancing Protocol group allows up to four virtual MAC addresses per group. The AVG is responsible for assigning the virtual MAC addresses to each member of the group. Other group members request a virtual MAC address after they discover the AVG through hello messages. Gateways are assigned the next MAC address in sequence. A virtual forwarder that is assigned a virtual MAC address by the AVG is known as a primary virtual forwarder. Other members of the Gateway Load Balancing Protocol group learn the virtual MAC addresses from hello messages. A virtual forwarder that has learned the virtual MAC address is referred to as a secondary virtual forwarder.
GLBP Virtual Gateway Redundancy
GLBP operates virtual gateway redundancy in the same way as HSRP. One gateway is elected as the AVG, another gateway is elected as the standby virtual gateway, and the remaining gateways are placed in a listen state.
If an AVG fails, the standby virtual gateway will assume responsibility for the virtual IP address. A new standby virtual gateway is then elected from the gateways in the listen state.
GLBP Virtual Forwarder Redundancy
Virtual forwarder redundancy is similar to virtual gateway redundancy with an AVF. If the AVF fails, one of the secondary virtual forwarders in the listen state assumes responsibility for the virtual MAC address.
The new AVF is also a primary virtual forwarder for a different forwarder number. GLBP migrates hosts away from the old forwarder number using two timers that start as soon as the gateway changes to the active virtual forwarder state. GLBP uses the hello messages to communicate the current state of the timers.
The redirect time is the interval during which the AVG continues to redirect hosts to the old virtual forwarder MAC address. When the redirect time expires, the AVG stops redirecting hosts to the virtual forwarder, although the virtual forwarder will continue to forward packets that were sent to the old virtual forwarder MAC address.
The secondary holdtime is the interval during which the virtual forwarder is valid. When the secondary holdtime expires, the virtual forwarder is removed from all gateways in the GLBP group. The expired virtual forwarder number becomes eligible for reassignment by the AVG.
GLBP Gateway Priority
Gateway Load Balancing Protocol gateway priority determines the role that each GLBP gateway plays and what happens if the AVG fails.
Priority also determines if a GLBP router functions as a backup virtual gateway and the order of ascendancy to becoming an AVG if the current AVG fails. You can configure the priority of each backup virtual gateway with a value of 1 through 255 using the glbp priority command.
In Figure 1, if Router A, the AVG in a LAN topology, fails, an election process takes place to determine which backup virtual gateway should take over. In this example, Router B is the only other member in the group so it will automatically become the new AVG. If another router existed in the same GLBP group with a higher priority, then the router with the highest priority would be elected. If both routers have the same priority, the backup virtual gateway with the higher IP address would be elected to become the active virtual gateway.
By default, the GLBP gateway preemptive scheme is disabled. A backup virtual gateway can become the AVG only if the current AVG fails, regardless of the priorities assigned to the virtual gateways. You can enable the GLBP preemptive scheme using the glbp preempt command. Preemption allows a backup virtual gateway to become the AVG, if the backup virtual gateway is assigned a higher priority than the current AVG.
GLBP Gateway Weighting and Tracking
Gateway Load Balancing Protocol uses a weighting scheme to determine the forwarding capacity of each router in the Gateway Load Balancing Protocol group. The weighting assigned to a router in the Gateway Load Balancing Protocol group determines whether it will forward packets and, if so, the proportion of hosts in the LAN for which it will forward packets. Thresholds can be set to disable forwarding when the weighting falls below a certain value, and when it rises above another threshold, forwarding is automatically reenabled.
The Gateway Load Balancing Protocol group weighting can be automatically adjusted by tracking the state of an interface within the router. If a tracked interface goes down, the GLBP group weighting is reduced by a specified value. Different interfaces can be tracked to decrement the GLBP weighting by varying amounts.
GLBP Benefits
Load Sharing
You can configure Gateway Load Balancing Protocol in such a way that traffic from LAN clients can be shared by multiple routers, thereby sharing the traffic load more equitably among available routers.
Multiple Virtual Routers
Gateway Load Balancing Protocol supports up to 1024 virtual routers (GLBP groups) on each physical interface of a router, and up to 4 virtual forwarders per group.
Preemption
The redundancy scheme of Gateway Load Balancing Protocol enables you to preempt an active virtual gateway with a higher priority backup virtual gateway that has become available. Forwarder preemption works in a similar way, except that forwarder preemption uses weighting instead of priority and is enabled by default.
Authentication
You can use a simple text password authentication scheme between GLBP group members to detect configuration errors. A router within a Gateway Load Balancing Protocol group with a different authentication string than other routers will be ignored by other group members.
