Re: [eng/rtg] changing loopbacks
It's worth noting that C's don't need actual IP address space assigned to the router-id for OSPF. It's just an arbitrary value; it's probably better karma to set it to whatever you want (maybe something that doesn't look like an IP address). RFC 2328: Router ID A 32-bit number assigned to each router running the OSPF protocol. This number uniquely identifies the router within an Autonomous System. CCO: <http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_command_reference_chapter09186a008010a39c.html#wp1049279> Usage Guidelines You can configure an arbitrary value in the IP address format for each router. However, each router ID must be unique. If this command is used on an OSPF router process which is already active (has neighbors), the new router-ID is used at the next reload or at a manual OSPF process restart. To manually restart the OSPF process, use the clear ip ospf command.
--- Austin <amckinle@andrew.cmu.edu> wrote:
It's worth noting that C's don't need actual IP address space assigned to the router-id for OSPF. It's just an arbitrary value; it's probably better karma to set it to whatever you want (maybe something that doesn't look like an IP address).
RFC 2328:
Router ID A 32-bit number assigned to each router running the OSPF protocol. This number uniquely identifies the router within an Autonomous System.
eek! There are a couple of downsides to having the router-ID divorced from a physical address: 1) you get an additional number which you have to have to track to ensure uniqueness. 2) you lose the benefit of being able to double check reachability (ping/ssh to router ID) 3) RFC 1403 says that the BGP router identifier must be the same as the OSPF router ID, and do you really want your BGP to reflect an unreachable ID? I've had a customer who used unreachable router IDs, and it made their NOC work quite a bit harder than they otherwise would have had to... -David __________________________________ Yahoo! Mail - PC Magazine Editors' Choice 2005 http://mail.yahoo.com
<thegameiam@yahoo.com> wrote:
eek! There are a couple of downsides to having the router-ID divorced from a physical address:
1) you get an additional number which you have to have to track to ensure uniqueness.
2) you lose the benefit of being able to double check reachability (ping/ssh to router ID)
No doubt, but the OP was trying to fend off OSPF adjacency teardowns when renumbering loopbacks.
3) RFC 1403 says that the BGP router identifier must be the same as the OSPF router ID, and do you really want your BGP to reflect an unreachable ID?
Wait a second... <RFC 1403> 3. BGP Identifier and OSPF router ID The BGP identifier MUST be the same as the OSPF router id at all times that the router is up. This characteristic is required for two reasons. i Synchronisation between OSPF and BGP Consider the scenario in which 3 ASBRs, RT1, RT2, and RT3, belong to the same autonomous system. +-----+ | RT3 | +-----+ | Autonomous System running OSPF / \ +-----+ +-----+ | RT1 | | RT2 | +-----+ +-----+ Both RT1 and RT2 have routes to an external network X and import it into the OSPF routing domain. RT3 is advertising the route to network X to other external BGP speakers. RT3 must use the OSPF router ID to determine whether it is using RT1 or RT2 to forward packets to network X and hence build the correct AS_PATH to advertise to other external speakers. More precisely, RT3 must determine which ASBR it is using to reach network X by matching the OSPF router ID for its route to network X with the BGP Identifier of one of the ASBRs, and use the corresponding route for further advertisement to external BGP peers. </RFC 1403> Can someone explain that? Why would RT3 care about the BPG identifiers of the other ASBR's? Why would the ASBR's even have BGP identifiers? What BGP attribute has anything to do with this? Austin --On Sunday, October 02, 2005 12:55 PM -0700 David Barak <thegameiam@yahoo.com> wrote:
eek! There are a couple of downsides to having the router-ID divorced from a physical address:
1) you get an additional number which you have to have to track to ensure uniqueness.
2) you lose the benefit of being able to double check reachability (ping/ssh to router ID)
3) RFC 1403 says that the BGP router identifier must be the same as the OSPF router ID, and do you really want your BGP to reflect an unreachable ID?
I've had a customer who used unreachable router IDs, and it made their NOC work quite a bit harder than they otherwise would have had to...
-David
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David Barak