MTU Limitations ..


The MTU of a sub-network is the largest size packet or frame, specified in octets that can be sent over it. Both OSPF and IS-IS require communicating routers to have matching MTU sizes in order to form adjacencies. This is needed so that routers will not advertise packets larger than a neighbor can receive and process. However, each protocol uses a different mechanism to check against MTU mismatch.  For this discussion the term MTU is used for a links Maximum Receive Unit (MRU) too.  
        
IS-IS   
        
– IS-IS works over the link layer, which does not provide for fragmentation and reassembly.   
          
– Hello’s are sent padded to MTU size till an adjacency comes up. If there is an MTU mismatch, the side having the lesser MTU would drop the bigger than MTU hello. This would not allow adjacencies to be formed between interfaces having different MTU’s.   
          
– The hello MTU match is an insufficient condition for IS-IS as LSP’s are flooded as is and not packed into any other packets. For the LSP’s to be successfully synchronized across the subdomain, all LSP’s need to be of a size lesser than the smallest link MTU in the subdomain, else the flooding of the LSP on the link will fail resulting in inconsistent routing tables.    
          
– Mis-configuration of the maximum packet size that a router sends out can cause problems across the subdomain as there is no way to check the value between routers that are not adjacent.            

OSPF  

– OSPF works over IP, so the fragmentation and reassembly of any OSPF packet is taken care by the IP layer. However for some link technologies where MTU is configurable but not negotiated, we can have packet black-holes whenever packets larger than the receiving sides MTU are sent.   
          
– The MTU is exchanged in the database description packets. If the value of MTU received in the first DB description packet is greater than that can be accepted on an interface, the packet is rejected and the adjacency is not formed. Retransmissions of DB description packets occur because the packets are never acknowledged. The adjacency therefore gets stuck in EXstart state.   

– As LS Update’s are assembled in each router, the MTU of another link does not affect the size of the LS Update packet.   
          
– As the MTU match is done at the database exchange state after the DR election has been completed, in case the DR itself cannot form adjacencies with the rest of the routers, it can cause the network to become a stub. 

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About Manav Bhatia

Manav Bhatia is a SDN/NFV dataplane architect at Ionos Networks and has co-authored several IETF standards on routing protocols, BFD, IPv6, security, etc. He is also a member of IETF Routing Area Directorate where he helps the Area Directors review the IETF standards for their impact on the Routing Area. View all posts by Manav Bhatia

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