![]() This will make the transmissions more efficient as more information can be packed into one larger packet. This occurs if the MTU of the outgoing interface is greater than 1500 bytes. This is another inefficiency.Īfter CSCse01519, OSPF in IOS can pack OSPF packets to be greater than 1500 bytes. OSPF builds OSPF packets up to 1500 bytes and they then get fragmented before the router transmits them. The added overhead of the encapsulation bytes of the tunnel leads to an MTU which is lower than 1500 bytes. One example where this typically occurs, is an IPSec tunnel between 2 routers running OSPF. This typically occurs when an OSPF router has many links and hence the router LSA because bigger than the link MTU.Įqually so, if the MTU of the outgoing interface is smaller than 1500 bytes, then the OSPF process would still build or pack OSPF packets up 1500 bytes and the IP stack of the router would fragment this into smaller IP packets in order to fit the MTU of the outgoing link. The IP stack of the router then fragments it to fit the MTU of the outgoing interface. There is one exception to this: if the LSA is so big that one LSA holds more than 1500 bytes, then OSPF builds that packet, no matter what the size (OSPF cannot fragment one LSA). This is somewhat inefficient because OSPF could send bigger packets on the link and achieve a greater throughput. So, if the interface MTU is bigger than 1500 bytes, OSPF would still pack only up to 1500 bytes into an OSPF packet. This is a regardless of the interface MTU. Before CSCse01519, OSPF in IOS would build OSPF packets up to a maximum of 1500 bytes.
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