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Router-ID question

Hi All,

In cisco, Does thr router-ID has to be reachable for OSPF,BGP or eigrp protocols.

logically, it has to be reachable, but I found the answer at one of ccie eritten exams mentioning it doesnt have to be reachable.

please feedback,

Thanks,

6 REPLIES
Silver

Re: Router-ID question

Hi,

The router-id command is used to configure a fixed router ID for a local routing process. The router ID is entered in the IP address format.

A routing protocol like EIGRP, OSPF or BGP automatically selects an IP address to use as the router ID when a process is started. The highest local IP address is selected and loopback interfaces are preferred. If you use an IP address from a local interface, we recommend that you use the address of a loopback interface rather than a physical interface. (The loopback interface is more effective than a fixed interface as an identifier because there is no physical link to go down.)

Remember, most important: manually any valid IP address can be used, even an address that is not locally configured on the router. That is, reachable or not (and a best practice is to use the router-id only as a "identifier" not reachable)

HTH

Andrea

Re: Router-ID question

Hi there,

The answer is NO, the router ID is something by which the router originating the packet can be uniquely distinguished from all other routers (as if its name), it doesn't have to be reachable at all.

HTH, please rate if it does,

Mohammed Mahmoud.

Re: Router-ID question

Hi,

what about ospf, how would ABR inject type-3 LSA to other routers if the router-ID is not reachable by some routers in different areas.

Thanks,

Re: Router-ID question

Hi there,

Router-ID is just an ID for the router (just like a naming tag), it doesn't have to be reachable at all (i mean routing wise, its just an ID).

HTH,

Mohammed Mahmoud.

Silver

Re: Router-ID question

Please remember how is created an OSPF adjacency, in particular:

Init: this state specifies that the router has received a hello packet from its neighbor, but the receiving router's ID was not included in the hello packet. When a router receives a hello packet from a neighbor, it should list the sender's router ID in its hello packet as an acknowledgment that it received a valid hello packet.

2-Way: this state designates that bi-directional communication has been established between two routers. Bi-directional means that each router has seen the other's hello packet. This state is attained when the router receiving the hello packet sees its own Router ID within the received hello packet's neighbor field. At this state, a router decides whether to become adjacent with this neighbor.

Router-ID is only a "tag" used to identify routers in the neighborship process.

HTH

Andrea

Re: Router-ID question

Hi Andrea,

Thanks for reminding, yes you are right.

Once review it back, I refreshed my memory.

Thanks again,

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