3 ms·

Can someone provide a lay explanation?

by Zikes 12y ago

namecast 12y ago
Sure. Lots of old/older routers can't hold more than 512K IPv4 routes in their "really fast routing table" memory (TCAM). This means that they'll have to either drop some routes - meaning some places on the internet will not be accessible from those routers - or that they'll have to use slower memory to store those routes - which means routing in general will be slow across the board. Many, many ISPs, hosting and content providers use routers with this limitation. So their customers are going to be impacted, in some indeterminable way. (Network geeks - I've glossed over a point or two, I know. Caveat lector.)
- Zikes 12y ago
  Is this limitation specific to IPv4? Would those same routers be able to handle a larger quantity of IPv6 addresses (if any), or does this highlight the need to upgrade those routers?
  - namecast 12y ago
    Ah, there's the rub! Assuming a Cisco 6500/7200 model router - the one most commonly affected by this issue - there's a certain amount of TCAM available by default for IPv4 routing, IPv6 routes, and multicast. By default, IPv4 only gets 512K routes. You can re-allocate more space to IPv4, but there's a potential cost... given the current rate of IPv6 adoption though, you could say that cost would be negligible for most people. More details here: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/116132-problem-catalyst6500-00.html http://www.cisco.com/c/en/us/support/docs/switches/catalyst-... (The question I have is, why are people with these routers taking a full BGP feed of 512K routes? I'd assume that 90% of the time a default route will do, or even just default to your transit + their direct peer routes...)
  - yebyen 12y ago
    As a person of similar layness but some parts bit-wise... I think the answer is no, plain and simple because IPv6 addresses are literally four times as large. If we were using IPv6 for years leading up to this, would it still be an issue? Obviously not the same question, but IPv6 having much larger potential address space and more opportunities for abbreviation in expressions of "what is a netblock range" means for one thing that you don't always need as many network blocks, you can have larger network blocks where a smaller one would be used in IPv4 because of limits and efficiency with the overall available and provisioned blocks... Disclaimer: I have never been in charge of a large router, let alone a large network.
    - tjgq 12y ago
      IPv6 addresses are larger, but routers only store prefixes, not full addresses. And yes, one of the design goals of IPv6 is to reduce address fragmentation, allowing the global routing table to contain fewer and shorter prefixes. That's only achievable in the long term, though - the currently assigned IPv4 prefixes will have to coexist with the IPv6 ones for a long time.
      - yebyen 12y ago
        I was trying to say something like this; I don't think it's true that you don't ever store full addresses in a router (You don't always store the address, sometimes just a prefix), but when you are describing a destination for a block, you would need a full address to finger that gateway, wouldn't you? Unless the gateway is always at the front of the address block, which I'm sure "they never taught us" in small network design school, at least I believe I've seen routes where the gateway was inside the netblock range or at the end rather than beginning, but again... big networks maybe. What I was trying to say was that when you have the freedom to write bigger ranges by an exponent factor of four like you have if you're the governing bodies of the internet with IPv6 vs IPv4, the expressive part of your network number can be made shorter, at least until your new network begins to approach the size of your old network. You don't avoid this problem at all, even if you are not storing 128 bits of address there are probably never scenarios where two internets of the same size last longer on the same hardware because IPv6. You're also right that you don't really ever have a scenario in the real world where we are on IPv6 and there is no need for IPv4, at least not for a very long time.

Can someone provide a lay explanation?

by Zikes 12y ago

Can someone provide a lay explanation?

Sure. Lots of old/older routers can't hold more than 512K IPv4 routes in their "really fast routing table" memory (TCAM). This means that they'll have to either drop some routes - meaning some places on the internet will not be accessible from those routers - or that they'll have to use slower memory to store those routes - which means routing in general will be slow across the board. Many, many ISPs, hosting and content providers use routers with this limitation. So their customers are going to be impacted, in some indeterminable way. (Network geeks - I've glossed over a point or two, I know. Caveat lector.)

Zikes 12y ago
Is this limitation specific to IPv4? Would those same routers be able to handle a larger quantity of IPv6 addresses (if any), or does this highlight the need to upgrade those routers?
- namecast 12y ago
  Ah, there's the rub! Assuming a Cisco 6500/7200 model router - the one most commonly affected by this issue - there's a certain amount of TCAM available by default for IPv4 routing, IPv6 routes, and multicast. By default, IPv4 only gets 512K routes. You can re-allocate more space to IPv4, but there's a potential cost... given the current rate of IPv6 adoption though, you could say that cost would be negligible for most people. More details here: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/116132-problem-catalyst6500-00.html http://www.cisco.com/c/en/us/support/docs/switches/catalyst-... (The question I have is, why are people with these routers taking a full BGP feed of 512K routes? I'd assume that 90% of the time a default route will do, or even just default to your transit + their direct peer routes...)
- yebyen 12y ago
  As a person of similar layness but some parts bit-wise... I think the answer is no, plain and simple because IPv6 addresses are literally four times as large. If we were using IPv6 for years leading up to this, would it still be an issue? Obviously not the same question, but IPv6 having much larger potential address space and more opportunities for abbreviation in expressions of "what is a netblock range" means for one thing that you don't always need as many network blocks, you can have larger network blocks where a smaller one would be used in IPv4 because of limits and efficiency with the overall available and provisioned blocks... Disclaimer: I have never been in charge of a large router, let alone a large network.
  - tjgq 12y ago
    IPv6 addresses are larger, but routers only store prefixes, not full addresses. And yes, one of the design goals of IPv6 is to reduce address fragmentation, allowing the global routing table to contain fewer and shorter prefixes. That's only achievable in the long term, though - the currently assigned IPv4 prefixes will have to coexist with the IPv6 ones for a long time.
    - yebyen 12y ago
      I was trying to say something like this; I don't think it's true that you don't ever store full addresses in a router (You don't always store the address, sometimes just a prefix), but when you are describing a destination for a block, you would need a full address to finger that gateway, wouldn't you? Unless the gateway is always at the front of the address block, which I'm sure "they never taught us" in small network design school, at least I believe I've seen routes where the gateway was inside the netblock range or at the end rather than beginning, but again... big networks maybe. What I was trying to say was that when you have the freedom to write bigger ranges by an exponent factor of four like you have if you're the governing bodies of the internet with IPv6 vs IPv4, the expressive part of your network number can be made shorter, at least until your new network begins to approach the size of your old network. You don't avoid this problem at all, even if you are not storing 128 bits of address there are probably never scenarios where two internets of the same size last longer on the same hardware because IPv6. You're also right that you don't really ever have a scenario in the real world where we are on IPv6 and there is no need for IPv4, at least not for a very long time.