Doubts about CEF and FIB table

[ciscolover]

HI all ¡¡ I'm studing about cef but i have some doubts...

1)It talks in some times about "the most specific route"... What is this??? The best route?

2)And when it talks about the longer subnet mask-->¿Longer subnet?

3)When it talks about the prefixes in the FIB table... Waht is this,,, A network address that contains another subnets??

Thanks all, and sorry , i speak bad english..

[Peter Paluch]

Hello,

1)It talks in some times about "the most specific route"... What is this??? The best route?

The  most specific route is an entry in the routing table (or in the FIB)  that matches the destination IP address in the maximum available number  of bits. In other words, it is the longest prefix match between the  destination IP address and the routing table entry.

Let's  have two routing table entries: 192.0.2.0/24 and 192.0.2.88/30. The  destination IP address 192.0.2.90 would match both these routing table  entries but the most specific route from these two is the 192.0.2.88/30  because it matches the destination address in the first 30 bits, which  is more specific than the 192.0.2.0/24 which matches the destination IP  in the first 24 bits.

2)And when it talks about the longer subnet mask-->¿Longer subnet?

Longer subnet mask is a subnet mask having more bits set to 1, e.g., the mask 255.255.255.248 is longer that 255.255.255.0.

3)When it talks about the prefixes in the FIB table... Waht is this,,, A network address that contains another subnets??

Are you asking about prefixes or about the FIB table?

Best regards,

Peter

[ciscolover]

Hi ¡¡¡

When i make show ip cef command, i view network and mask... This is in the "prefix"column...

This is a prefix¿?Why its a prefix? Really its a ip address for a host, for a network ¿or not?

Thanks ¡¡¡

[Peter Paluch]

Hello,

A prefix is the network portion of an IP address. Each IP address, as you certainly know, consists of two parts - the network part or the network prefix that is common for all stations in the same network, and the host part that identifies the idividual host in the network.

The address of a network, say, 192.168.1.0/24, is simply the network prefix with the host part entirely set to zero. Note that in this example, the prefix part is /24 bits long, i.e. the first three bytes: 192.168.1. The host part, i.e. the last byte, is set to zero. This holds for absolutely all network addresses - they contain the network prefix and have their host part set to zero. Therefore, sometimes, these two terms: network prefix and network address - are used interchangeably, although it is not completely precise.

The FIB is an optimized version of the routing table that can be searched very fast. But just as the routing table contains network addresses - in essence network prefixes - so does the FIB. In fact, the FIB is computed using the routing table contents.

Best regards,

Peter

[ciscolover]

Ok Ok, peter sorry its a stupid question.

Ok, prefix its the network portion of an ip address...Did not know -->prefix=network part ¡¡¡

Perfect Peter, now i understand better how works the FIB...

Thnaks for the help, i hope no need make more questions about CEF ¡¡¡¡

Curiously it talks about prefix when it uses a host ip (/32) without network part...

Regards

[Peter Paluch]

Hello,

Thnaks for the help, i hope no need make more questions about CEF

CEF can get quite intricate to understand I would be surprised if these were the only issues preventing you from understanding the CEF completely

Curiously it talks about prefix when it uses a host ip (/32) without network part...

Again, that is a convention only. Technically, an address with a /32 mask consists of network prefix only - actually without any host part (remember that the mask specifies the length of the network prefix in an IP address, and a mask of /32 allocates the entire IP address just to the network prefix). However, this kind of IP address/mask is simply given a special treatment and exactly as you have yourself correctly commented, it is processed as a host IP route.

Best regards,

Peter

[ciscolover]

Ok ¡¡

Thanks again peter... Its not easy to understand CEF '''¡¡¡