HSRP Virtual MAC and BIA MAC address

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Sep 13th, 2007
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Which are the case where we are using virtual MAC address? and when do we need to implement BIA MAC address?

what are the conditions to use them in different place? or what are the benifits?

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lgijssel Thu, 09/13/2007 - 11:25
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The virtual mac-adress is used by the host that is HSRP-active. When it fails, the standby takes over using the same virtual mac adress.

In that way, end nodes do not need to relearn the mac adress for their gateway and so they can keep on sending data.

In all other applications, the mac-adress should be unique and we use the BIA for that.



sundar.palaniappan Thu, 09/13/2007 - 13:28
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Leo has explained the operational difference between the two options. As far as the benefits are concerned the first method, using virtual MAC address, should work in most cases and is the default setting.

However there can be very few situations with certain platforms, like network disruption or active rotuer transitioning, that would warrant the use of 'standby use-bia' command to address HSRP instability. This command is also used to address the IOS bug CSCdr02376. Have a look at this document that explains quite well the reasoning behind that.




m.volodko Thu, 09/13/2007 - 23:58
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Q. What is the standby use-bia command and how does it work?

A. By default, HSRP uses the preassigned HSRP virtual MAC address on Ethernet and FDDI, or the functional address on Token Ring. To configure HSRP to use the interface's burnt-in address as its virtual MAC address, instead of the default, use the standby use-bia command.

For example, on Token Ring, if Source Route Bridging is in use, a Routing Information Field (RIF) is stored with the virtual MAC address in the host's RIF cache. The RIF indicates the path and final ring used to reach the MAC address. As routers transition to the active state, they send gratuitous Address Resolution Protocols (ARPs) in order to update the host's ARP table. However, this does not affect the RIF cache of the hosts that are on the bridged ring. This situation can lead to packets being bridged to the ring for the previous active router. To avoid this situation, use the standby use-bia command. The router now uses its burnt-in MAC address as the virtual MAC address.

Note: Using the standby use-bia command has these disadvantages:

* When a router becomes active the virtual IP address is moved to a different MAC address. The newly active router sends a gratuitous ARP response, but not all host implementations handle the gratuitous ARP correctly.

* Proxy ARP breaks when use-bia is configured. A standby router cannot cover for the lost proxy ARP database of the failed router.

from: http://www.cisco.com/warp/public/619/3.html


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