Users Guide

OSPF and OSPFv3 1191
Flood blocking cannot be enabled on virtual interfaces. While the feature
could be allowed on virtual interfaces, it is less likely to be used on a virtual
interface, since virtual interfaces are created specifically to allow flooding
between two backbone routers. So the option of flood blocking on virtual
interfaces is not supported.
See "Configuring Flood Blocking" on page 1266 for a configuration example.
MTU
OSPF database description packets announce the IP MTU of the interface
where they are transmitted. Two routers form an OSPF adjacency only if their
IP MTUs are the same. If OSPF receives a database description packet whose
IP MTU is larger than the local IP MTU, it drops the packet. Adjacencies in
this situation remain in Exchange Start state. A log message identifies the IP
MTU mismatch:
<11> JAN 01 00:00:51 192.168.75.1-1 OSPF[175099648]:
spnbo.c(672) 12 %% Dropping a DD packet received on interface
0/1. DD MTU is 2000. Local MTU is 1500.
The administrator can configure OSPF to ignore MTU mismatches using the
ip ospf mtu-ignore command. Configuring one end of an OSPF-enabled link
to ignore an MTU mismatch can cause issues when the DD or LSA packets
are dropped on the end of the link with the smaller MTU.
OSPF does not provide a way to fragment protocol packets. Larger OSPF
networks, e.g. those with more than ~100 OSPF links require a larger IP
MTU in order to transmit/receive the LSA update. Use the system jumbo
mtu command to configure an MTU sufficiently large to contain the largest
expected LSA packet.