Confusion Over Tables Again

dppagc

In another thread I learnt that FIB + ADJ = CEF table.

I got 4 other questions:

1) Is the adjacency table derived from "good" routes of the mac address table?
2) Is the ip routing table (show ip route) same as the CEF table?
3) How about the topology table come into play?
4) Any other tables?

I am very confused.

joetest

dppagc wrote: »

In another thread I learnt that FIB + ADJ = CEF table. I got 4 other questions: 1) Is the adjacency table derived from "good" routes of the mac address table? 2) Is the ip routing table (show ip route) same as the CEF table? 3) How about the topology table come into play? 4) Any other tables? I am very confused.

1. No such thing as a route in the mac addr table. Mac address table is used to bind Mac addresses to ports/interfaces once learned in the flooding/switching process: xxxx.xxxx.xxxx mac is connected to port 1. But yes the Adj table contains the L2 information of CEF pulled from the Mac address table. 2. The FIB(aka CEF aka IP Forwarding table) table is build on L3 info based on the IP routing table(Routing Information Base/RIB) - they're 2 different tables with different structures with the same L3 information. 3. CEF has no topology table. Routing Protocols like EIGRP/OSPF has topology tables(or Link state Database for OSPF) for all their known/learned IP prefixes/networks. All the best routes from the topology table will then go to the IP routing table. 4. Don't know any other tables regarding CEF. Awesome formatting....

dppagc

With regards to point 2, I thought that the FIB and CEF and ip forwarding table are all different.

EdTheLad

1) When a cisco port is unshut it sends a "gratuitous arp reply" to tell its neighbors its port mac address. This means two cisco routers back to back will exchange layer 2 info. Both the arp and adjacency tables will be updated with the newly learned remote mac.

2) No, the routing table is built by choosing the best route to a specific destination, this is populated from routing protocols, "admin distance" is used between different routing protocols, metric is used within a routing protocol.
CEF is a switching mechanism that uses the FIB and Adj table, Adj table previously explained, the FIB is created using the routing table.
The routing table (RIB) consists of destination prefixes mapped to egress ports or next-hop ip, if the next-hop is an ip prefix, the router needs to perform route recursion until it finally knows the egress interface. The FIB is created from the result of route recursion lookup on every prefix in the rib.
The fib consists of destination prefix mapped to egress interface, the adj table holds all dst mac addresses for neighbors on egress interfaces.
CEF leverages the info from both tables, it can prebuild a L2 header i.e. src mac/dst mac/type for every destination prefix in RIB by utilizing both the FIB and ADJ table, thus no arp lookups or route recursion required while forwarding a packet.

3) Topology tables are used by routing protocols, they typically hold all possible routes to a destination, the best route is selected as best candidate to go in the RIB. This best candidate route will compete with other routing protocols, the routing protocol with the lowest admin distance wins and gets placed in the RIB.