OSPF Stuck in Exstart Adjacency State

  • According to this article two routers will get stuck in the exstart adjacency state when their configured MTU sizes do not match. Does a VLAN tag affect this as well even if the routers involved are unconcerned with which VLAN the packet is coming from or heading to?

    I'll try to clarify the question if it is unclear at all.

  • According to this article two routers will get stuck in the exstart adjacency state when their configured MTU sizes do not match. Does a VLAN tag affect this as well even if the routers involved are unconcerned with which VLAN the packet is coming from or heading to?

    Most Cisco routers and switches allow 1500 byte IP payloads by default, even when tagged with dot1q. You can verify this with show ip interface SomeIntfName...

    Router1#sh ip int vlan105
    Vlan105 is up, line protocol is up
      Internet address is 10.15.2.19/30
      Broadcast address is 255.255.255.255
      Address determined by setup command
      MTU is 1500 bytes  <-------------
    

    The reason dot1q on a Cisco interface works without bumping the physical interface MTU is because most Cisco routers support what Cisco calls "baby giants"... a baby giant has an Ethernet MTU above 1518 (which includes the eth header size), but not very much over 1518... usually the default values is 1522 bytes... see this MTU configuration doc for more information. Many Cisco platforms support configurable ethernet baby giant MTUs to 1532 bytes (or even higher), which is also why MPLS tag stacks (2 or 3 tags deep) can work through legacy ethernet links.

    Even though Cisco ethernet interfaces support baby giants, they leave the default IP MTU at 1500 bytes. As long as the default IP MTU of 1500 is maintained, you'll have no problems with the default settings of any other router with a 1500-byte IP MTU... quoting RFC 2328 Section 10.8:

    10.8.  Sending Database Description Packets
    
        This section describes how Database Description Packets are sent
        to a neighbor. The Database Description packet's Interface MTU
        field is set to the size of the largest IP datagram that can be
        sent out the sending interface, without fragmentation.
    

    What if I am using Cisco router and another router from a different vendor? The Cisco router's interface is configured for a 1500 byte MTU, but it is actually transmitting an IP packet that is 1522 bytes the other router will still accept it as long as the interface is configured with a 1500 byte MTU? That would be because of what @Puglet mentioned below and OSPF assigns the Interface MTU part of the DB Description packet with the MTU size explicitly configured on the interface and not what the actual size of the packet is?

    Correct, Cisco IOS uses the IP MTU of the local interface... the IP MTU of your Cisco and the other vendor's box must match. Sadly, I can't speak for the behavior of other vendor equipment. Is there a way you can conduct a ping test (using the DF option) before you deploy?

    I'm not actually experiencing this issue. I saw a similar question on link in a discussion about good interview questions. I figured it would be discussion for this Stack Exchange.

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Content dated before 7/24/2021 11:53 AM