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Community Member

fast switching

hi

i have q question about fast switching

i read in cisco press book. 1/20 of cache is invalidated every minute . what would happen if a packet is received within 60 seconds but the destination host has already moved somewhere? in that case cache holds a outdated information with regards to outgoing interface and mac address, will the router drop the packet or process switch it? thanks alot and have a nice day

1 ACCEPTED SOLUTION

Accepted Solutions
Community Member

Re: fast switching

It depends on the platform.

If we're talking about an actual router, and the device is at least a hop downstream, then the router will just forward it out the interface it has to the next hop it has and the receiving router will have to deal with it. Hopefully that device has reconverged, if not the packet will be discarded there, or bounced around a little.

If the hosts are connected to a router platform or an L2 network hanging from it, there's little danger as a result of that. To move to a different outgoing IP interface the device would have to move to a different subnet, have its arp time out, or gratuitously arp for itself. Any of those things will immediately invalidate the route cache entry.

If we're talking about a switching platform, its slightly different but still not bad. The route cache will always point out a L3 interface, and the MAC address table will be aged differently. Whenever a MAC moves around in the L2 network, it gets learned by the L2 process and the route cache still points to the same VLAN or BVI. You can get into an issue with certain redundancy schemes like HSRP and GLBP if your MAC timers and ARP timers aren't set right, then you could also black hole traffic in that case to a first hop address that has moved.

Remember that any serious adjacency event will invalidate a route cache entry, like an interface going down, an arp entry aging out, etc.

Lastly, CEF resolves almost all of this. CEF doesn't really age quite the same way, and depends on ARP or other L2 adjacencies at all times to trigger a change, and the entire available route table is pre-populated in memory so that there's a path to every network that router can get to at all times. Very efficient on both memory and CPU.

Hope this helps.

NS

1 REPLY
Community Member

Re: fast switching

It depends on the platform.

If we're talking about an actual router, and the device is at least a hop downstream, then the router will just forward it out the interface it has to the next hop it has and the receiving router will have to deal with it. Hopefully that device has reconverged, if not the packet will be discarded there, or bounced around a little.

If the hosts are connected to a router platform or an L2 network hanging from it, there's little danger as a result of that. To move to a different outgoing IP interface the device would have to move to a different subnet, have its arp time out, or gratuitously arp for itself. Any of those things will immediately invalidate the route cache entry.

If we're talking about a switching platform, its slightly different but still not bad. The route cache will always point out a L3 interface, and the MAC address table will be aged differently. Whenever a MAC moves around in the L2 network, it gets learned by the L2 process and the route cache still points to the same VLAN or BVI. You can get into an issue with certain redundancy schemes like HSRP and GLBP if your MAC timers and ARP timers aren't set right, then you could also black hole traffic in that case to a first hop address that has moved.

Remember that any serious adjacency event will invalidate a route cache entry, like an interface going down, an arp entry aging out, etc.

Lastly, CEF resolves almost all of this. CEF doesn't really age quite the same way, and depends on ARP or other L2 adjacencies at all times to trigger a change, and the entire available route table is pre-populated in memory so that there's a path to every network that router can get to at all times. Very efficient on both memory and CPU.

Hope this helps.

NS

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