MTU mismatches are the primary reason an OSPF adjacency becomes stuck in the EXSTART state. After hellos are exchanged and the routers become neighbors, each OSPF speaker advertises the IP MTU of it’s local interface in a Data Base Description (DBD) LSA. If there is a mismatch you’ll probably just adjust the configuration to be identical on both ends of the link and be done. However, when you try to peer two OSPF routers with different network operating systems, things start to fall apart fairly quickly.
OSPF Sequence Numbers
When an OSPF router originates an LSA for the first time, it will choose the sequence number 0×80000001. The 0x prefix means it’s a hexadecimal number, where each hex character represents a four bit binary word. This post discusses why the OSPF sequence number begins with 0×8, and some quirks when counting with signed numbers.
Unicast Hello packets
I was debugging an issue recently and ran across something unusual. I had thought that all OSPF hellos on BCAST and P2P intefaces were sent as multicasts. However as you can see from the wireshark capture below, packet 10 is clearly sent as a unicast.
# Time Source Dest Proto Info 04 15.503997 192.168.12.2 184.108.40.206 OSPF Hello Packet 09 23.507483 192.168.12.1 220.127.116.11 OSPF Hello Packet 10 23.518430 192.168.12.2 192.168.12.1 OSPF Hello Packet 14 25.400245 192.168.12.2 18.104.22.168 OSPF Hello Packet
A test – What LSAs will broadcast interfaces generate?
You have just added a new workgroup switch, switch_1 to your network. You have the following conditions.
- Router_A and Router_B were already fully adjacent over an OSPF point-to-point link.
- Network 192.168.12.0/24 is immediately advertised by Router_A to Router_B.
- There are no other OSPF routers connected to Switch_1.
- Router_A interface Gi1/0 is up/up, has OSPF network-type BCAST and has assumed the role of DR for that segment.
My question to you is, “What LSA(s) will Router_A send to Router_B to describe this new OSPF link?”. Your choices are:
- Add a ‘Stub’ type-3 link to Router LSA
- Add a ‘Transit’ type-2 link to Router LSA
- A new network-LSA
- BOTH a new Network-LSA, AND add a ’Transit’ type-2 link to Router LSA
When a single-homed router is isolated by link failure, the LSAs it originated can live for up to 60 minutes in the OSPF LSDB of the surviving routers. This may not be what you were expecting and cause a lot of confusion when troubleshooting OSPF. In this post we’ll look at why LSAs from an isolated router linger and how OSPF still knows how to ‘do the right thing’.
Navigating the OSPF LSDB to find relevant LSAs can be tricky. Even when you find the right LSA, there is no guarantee that OSPF will include that LSA in it’s SPF calculation. Thankfully Cisco routers will tell you if the router advertising that LSA is reachable or not.