Why OSPF doesn't have AS number and EIGRP have the AS number, after all both are IGP

[ABINTHOMAS6431]

Hi, 

I came to know that ospf does not have AS number concept even it is an IGP... then why EIGRP have AS number since it is also an IGP

[Mark Malone · Accepted solution]

Hi
differently designed protocols ,OSPF uses a process ID and domains it has no concept of AS boundary,RIP is similar it does not use AS,BGP and EIGRP use AS numbers , BGP is not really an IGP though its a transport protocol, OSPF open standard , EIGRP started out as proprietary Cisco feature but i think that has been opened up but its still mostly used on Cisco devices

[Giuseppe Larosa · Accepted solution]

Hello Abinthomas6431,

there are different design choices made by respective protocol designers.

OSPFv2 the original IGP for IPv4 works on the area id concept and area flags (area type). The OSPF process-id is not sent out in OSPFv2 messages.

An OSPF neighborship is formed if routers share a common IP subnet on their primary IP address they agree on subnet mask, they agree on OSPF hello/dead timers and they agree on area id and area type.

They must also have the same IP MTU to be able to exchange the link state database.

To be noted in OSPFv3 originated for routing IPv6 an instance id is sent in messages.

OSPFv3 uses instance-id to support routing of different protocols like IPv6 unicast and IPv4 unicast.

Instance-id 0-31 are for IPv6 unicast routing and instance-id 64-95 are for IPv4 unicast.

EIGRP uses the AS number as a way to discriminate between different routing domains.

EIGRP routers need to agree on AS number and K values to build a neighborship.

The K values says how the metric components are used.

The EIGRP timers can be different on the two devices.

Hope to help

Giuseppe