How can we use unlimited routers or beyond 255 hopes practically for a big network while using OSPF routing protocol if we know that maximum limit for TTL value is 255?

How can we use unlimited routers or beyond 255 hopes practically for a big network while using OSPF routing protocol if we know that maximum limit for TTL value is 255 ?
And If answer is No then OSPF is also having the limit of routers in the network as EIGRP has 255 maximum.

OSPF has no limits and you can configure OSPF Areas !!

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So does that mean that if we configure OSFPF areas then not all and only few routers of the particular area will be visited while routing and that is why TTL will not exhaust faster?

Thank You Sir for reply and yes I understand that we have areas in ospf to reduce the broadcast and also they help us to manage the big network and the routers performance, etc but sir lets take an example to understand it more
If we have 300 routers connected in my company and running ospf routing protocol and then I suppose to ping from router 1 to router 300 (my destination router ) so how can we ping if we know that maximum TTL value is 255 , won’t the packets be dropped at router 255 because of TTL limit ? won’t the router discards the packet and sends an ICMP Time Exceeded message back to the source ?

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Understanding TTL Limit in OSPF Network with 300 Routers

In the case of an OSPF network with 300 routers where the maximum TTL limit is 255, it might seem challenging for a ping from Router 1 to reach Router 300 due to the TTL limit.

Let’s calculate the scenario:

  • When Router 1 sends out a ping packet to Router 300, the initial TTL is set to 255.
  • As the packet traverses each router, the TTL decreases by 1.
  • For the ping packet to reach Router 300, it needs to pass through all 300 routers.

Let’s calculate the remaining TTL when the packet reaches Router 300: $$ 255 - 300 = -45 $$

Since the calculated TTL value is negative (-45) when reaching Router 300, the packet will be dropped at Router 255 due to TTL expiration. In this case, Router 255 will discard the packet and send an ICMP Time Exceeded message back to the source (Router 1).

Therefore, in a network with 300 routers and a maximum TTL limit of 255, a ping from Router 1 to Router 300 would not successfully reach its destination due to the TTL limit being exceeded before reaching the final destination.

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Thank you sir for making me understand about TTL process. But still I am not getting that EIGRP has limit of using 255 routers and ospf has unlimited use of router as we were taught in the ospf class while the time of learning difference between eigrp and ospf
And also I have been asked the same question in my recent Interview.

The distinction between EIGRP and OSPF in terms of router limits isn’t strictly about the protocols themselves being inherently limited to a specific number of routers. Instead, these constraints are more about practical network design, scalability, and performance considerations.

EIGRP: EIGRP does not have a hard limit of 255 routers; however, it uses a default administrative distance of 255 for routes learned from external sources. The confusion might arise from the fact that EIGRP packets use an 8-bit field for the hop count, limiting the hop count to 255. But this is not a limit on the number of routers. The actual limitation in deploying EIGRP in large-scale networks comes from its topology table maintenance and the requirement for all routers within an EIGRP autonomous system to be interconnected in a full mesh or near-full mesh topology for optimal routing. This can become complex and difficult to manage as the network grows, but the protocol itself doesn’t impose a strict router limit.

OSPF: OSPF is designed with scalability in mind and uses a hierarchical network design with areas to overcome limitations in large-scale deployments. OSPF does not have an inherent limit on the number of routers. Instead, the design of OSPF with areas allows for efficiently scaling to accommodate a large number of routers without overwhelming the network with routing information. The use of areas in OSPF helps to limit the scope of link-state advertisements (LSAs) to within an area, reducing the amount of routing information that each router needs to process and maintain.

In summary, while there’s no hard limit on the number of routers that EIGRP or OSPF can support, practical scalability is influenced by network design principles, the management of routing information, and performance considerations. EIGRP’s perceived limit is more about hop count and network design complexity, whereas OSPF’s design inherently supports larger and more scalable networks through its use of hierarchical areas.

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