preempt issue (CCNP Switching)
Well, i engage the CCNP Switching topics. One of them in which i am stuck is redundancy protocols. HSRP, VRRP and GLBP. Have read them plenty of times but still not accurately understand.
As you know, GLBP is said to differ from the other two. In HSRP and VRRP, there is only one router performing packet forwarding whereas others are waiting on standby eager to become an active router; in GLBP, routers rather than the router which is currently down, continue to forward packets across the network. However, one feature makes my mind confused. The preempt feature.
Regarding preempt,
1) Imagine the protocol used is GLBP. Both interfaces of both routers are shut down,(AVFs are still operating) both routers have preempt enabled but previously active router(AVG) has lower priority than SVG.In this case, will the other router(SVG) take over the active role ?
2) in order for preempt feature to work on a router, do that router always have to have higher priority?
3) is preempt feature more prior to the "priority"?
As you know, GLBP is said to differ from the other two. In HSRP and VRRP, there is only one router performing packet forwarding whereas others are waiting on standby eager to become an active router; in GLBP, routers rather than the router which is currently down, continue to forward packets across the network. However, one feature makes my mind confused. The preempt feature.
Regarding preempt,
1) Imagine the protocol used is GLBP. Both interfaces of both routers are shut down,(AVFs are still operating) both routers have preempt enabled but previously active router(AVG) has lower priority than SVG.In this case, will the other router(SVG) take over the active role ?
2) in order for preempt feature to work on a router, do that router always have to have higher priority?
3) is preempt feature more prior to the "priority"?
Monster PC specs(Packard Bell VR46) : Intel Celeron Dual-Core 1.2 GHz CPU , 4096 MB DDR3 RAM, Intel Media Graphics (R) 4 Family with IntelGMA 4500 M HD graphics. 
5 year-old laptop PC specs(Toshiba Satellite A210) : AMD Athlon 64 x2 1.9 GHz CPU, ATI Radeon X1200 128 MB Video Memory graphics card, 3072 MB 667 Mhz DDR2 RAM. (1 stick 2 gigabytes and 1 stick 1 gigabytes)
5 year-old laptop PC specs(Toshiba Satellite A210) : AMD Athlon 64 x2 1.9 GHz CPU, ATI Radeon X1200 128 MB Video Memory graphics card, 3072 MB 667 Mhz DDR2 RAM. (1 stick 2 gigabytes and 1 stick 1 gigabytes)
Comments
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xXErebuS
Member Posts: 230
If both routers have preempt enabled then the router with the lower priority wasn't the AVG. Priority works at initialization; if you have router A that has priority 10 and router B priority 11 then if they both go through election process at the same time router B will be the AVG.... If router B is started up after router A then router A will be AVG unless preempt is enabled (disabled by default).
Preempt and Priority are not the same thing.... Priority sets which router should be the AVG; preempt assures that it will be come the AVG if it reboots / loses connection
Consider more scenarios lets keep A = PRI 10 and B = PRI 11
NO Preempt
Both reboot at same time then B is AVG
B Reboots A becomes AVG and stays AVG even if B comes back online
throw router C into the mix with PRI 8; A&B reboot C becomes and stays AVG even if A&B come back online
Preempt USED
Both Reboot Same time B becomes AVG
B reboots; A Becomes AVG while B is down / not responding / once back up and seen in GLBP group it will resume Role as AVG
throw router C into Mix with PRI 8; B goes down; A becomes AVG; A goes down C becomes AVG; then A&B come back up B resumes role as AVG.... -
thedrama
Member Posts: 291 ■□□□□□□□□□
Thank you so much.
So, if the AVG which gets down had lower priority, it won't take over the active role again?
One more question,
If the active router goes down, only other routers including SVG will be able to load-balance and traffic sharing?Monster PC specs(Packard Bell VR46) : Intel Celeron Dual-Core 1.2 GHz CPU , 4096 MB DDR3 RAM, Intel Media Graphics (R) 4 Family with IntelGMA 4500 M HD graphics.
5 year-old laptop PC specs(Toshiba Satellite A210) : AMD Athlon 64 x2 1.9 GHz CPU, ATI Radeon X1200 128 MB Video Memory graphics card, 3072 MB 667 Mhz DDR2 RAM. (1 stick 2 gigabytes and 1 stick 1 gigabytes) -
xXErebuS
Member Posts: 230
Not sure what your asking; if the AVG goes down another router will take over as AVG; the load balancing is done through the different AVF's....
So AVG A goes down (group of BCDEF); B has second highest priority so it will become AVG and replay to ARP requests with the virtual macs of BCDEF achieving load balancing....
This is the simple version; there are plenty of paramaters i.e. weight to control how the load balancing is done.