FIB Entries

Code View: Scroll / Show All

Router#show ip route 192.168.30.0 255.255.255.0

Routing entry for 192.168.30.0/24

Known via "connected", distance 0, metric 0 (connected,

via interface)

Routing Descriptor Blocks:

* directly connected, via Serial0/1

Route metric is 0, traffic share count is 1

Router#show ip cef 192.168.30.0 255.255.255.0

192.168.30.0/24, version 50, attached, connected, cached

adjacency to

Serial0/1

0 packets, 0 bytes

via Serial0/1, 0 dependencies

valid cached adjacency

CEF uses a receive FIB entry when the IP address is configured on one of the router's interfaces. The router must accept and process packets destined to receive entry, 172.17.1.12, rather than switch the packets through the router out another interface. In Example 2-5, note that the FIB entry is flagged as receive because it is the IP address assigned to the interface.

Example 2-5. Receive FIB Entry

Router#show running-configuration interface Serial0/3 interface Serial0/3 ip address 172.17.1.12 255.255.255.0

Router#show ip cef 172.17.1.12 255.255.255.255 172.17.1.12/32, version 17, epoch 0, receive

The following list describes two special receive entries:

• 172.17.1.0/32— A receive FIB entry is built for the all-0s host address of a network. This entry allows the router to receive broadcasts sent to the all-0s host address on the network segment.

• 172.17.1.255/32— A receive FIB entry is built for the all-1s host address of a network. This entry allows the router to receive broadcasts sent to the all-1s host address on the network segment.

Recursive FIB Entry

The concept of performing a lookup for the next-hop IP address toward a destination is called a recursive lookup. Recursive routes are routes that require the forwarding and adjacency information of another route. A recursive FIB entry is used when the router must look up the next-hop IP address of a prefix referenced to find the corresponding Layer 2 header rewrite string and output interface for the destination prefix. Border Gateway

Protocol (BGP) is the most common source of recursive entries, because the IP addresses used as the next hop for advertised prefixes are almost never directly connected to the router. In Example 2-6, note the FIB entry shows as recursive.

Example 2-6. Recursive FIB Entry

Router#show ip route 172.31.12.0 255.255.255.0 Routing entry for 172.31.12.0/24

Known via "static", distance 1, metric 0 Routing Descriptor Blocks: * 192.170.1.10

Route metric is 0, traffic share count is 1

Router#show ip cef 172.31.12.0 255.255.255.0 172.31.12.0/24, version 55, cached adjacency 192.170.1.10 0 packets, 0 bytes tag information set local tag: 4 via 192.170.1.10, 0 dependencies, recursive next hop 192.170.1.10, FastEthernet0/1 via 192.170.1.10/32 valid cached adjacency

Figure 2-9 illustrates recursive routing.

Figure 2-9. Recursive Routing [View full size image]

Figure 2-9. Recursive Routing [View full size image]

In Figure 2-9, Router A's only knowledge of the 172.31.12.0/24 network is through the static route configured. The router installs this static route with a next-hop IP address of 192.170.1.10. When switching a packet toward a destination on the 172.31.12.0/24 network, Router A must perform another lookup on 192.170.1.10 to determine the respective outbound interface, Layer 2 header rewrite string, and other next-hop information installed for that destination.

Internet backbone routers carry large routing tables that are mostly comprised of BGP routes. The majority of BGP routes are recursive routes. For example, most BGP peering is performed between loopback interfaces. The next-hop IP address of received prefixes is generally the peer's loopback address, which is not connected to the local router. Therefore, the router must perform multiple route lookups on a prefix's next-hop IP address for a single destination, so performance becomes a factor. To reduce performance impact, CEF performs recursive resolution.

CEF performs recursive resolution by following the recursive chain of lookups down to the leaf level. The FIB entry is flagged as recursive and is populated with the forwarding details from the path of the next hop, also referred to as the parent. Recursive routes are resolved when a recursive path is created. If a valid next hop is not found upon initial creation, the recursive path is queued in a list for periodic reresolution by the CEF background resolution process. CEF maintains a dependency list to keep track of changes. All the children paths depending on a parent's path are installed in the parent's dependents list. If a parent's path information changes, any dependents will also be reresolved.

CEF does not limit the amount of levels required for recursion. However, multilevel recursion used for load balancing was not supported until the CEF/MFI rewrite in Cisco IOS Software Release 12.2S, which is covered in Chapter 3. On a router running pre-MFI IOS, multilevel recursion with load balancing results in CEF using all the paths at the first level and then choosing only one of the paths at the subsequent levels. The results do not guarantee that load balancing will occur because of lack of code support.

Default Route Handler FIB Entry

The FIB installs the default route handler entry as a default entry placeholder. Packets destined to nonspecific destinations installed in the FIB use this entry and are discarded. Any default routes that are statically configured or learned from dynamic routing protocols are installed in place of this entry. However, if the default route is removed from the routing table, the FIB default route handler entry is reinstalled as a placeholder. Example 2-7 provides an example of the default route handler FIB entry.

Example 2-7. Default Route Handler FIB Entry

0.0.0.0/0, version 2, epoch 0, attached, default route handler 0 packets, 0 bytes via 0.0.0.0, 0 dependencies valid no route adjacency

ADJFIB FIB Entry

The ADJFIB entry is a special FIB entry that reflects an entry in the adjacency table. The best way to describe this entry is by providing an example. For a broadcast network, the FIB table will represent the connected network entry as 1.1.1.0/24. This entry will point to a glean adjacency, indicating that packets matching host prefixes within the connected network entry, such as 1.1.1.1, will need to glean more information to forward the packet. On an Ethernet interface, the router uses ARP to learn the necessary details used to forward packets destined to these host entries. Upon receiving an ARP response, the router installs the Layer 2 rewrite string and output interface details in the ARP table and the adjacency table. The router then installs an ADJFIB entry in the FIB table to reflect this adjacency. Example 2-8 shows a representation of an ADJFIB FIB entry.

Example 2-8. ADJFIB FIB Entry interface Ethernet0/0 ip address 1.1.1.2 255.255.255.0

Router#show ip cef 1.1.1.0 255.255.255.0 1.1.1.0/24, version 4, epoch 0, attached, connected 0 packets, 0 bytes via Ethernet0/0, 0 dependencies valid glean adjacency

Router#show ip cef 1.1.1.1

1.1.1.1/32, version 10, epoch 0, connected, cached adjacency 1.1.1.1 0 packets, 0 bytes via 1.1.1.1, Ethernet0/0, 0 dependencies next hop 1.1.1.1, Ethernet0/0 valid cached adjacency

Learned from IGPs

Entries learned from IGPs are the most common source of prefixes installed in the FIB. Example 2-9 illustrates a route learned through Open Shortest Path First (OSPF), which is programmed in the FIB table as a valid cached adjacency.

Example 2-9. FIB Entry Learned Through an IGP

Code View: Scroll / Show All

Router#show ip route 100.1.1.0 Routing entry for 100.1.1.0/24

Known via "ospf 1", distance 110, metric 10, type extern 2, forward metric 10

Last update from 38.0.0.2 on Ethernet1/0, 00:03:11 ago Routing Descriptor Blocks:

* 38.0.0.2, from 46.0.0.2, 00:03:11 ago, via Ethernet1/0 Route metric is 10, traffic share count is 1

Router#show ip cef 100.1.1.0

100.1.1.0/24, version 17, epoch 0, cached adjacency 38.0.0.2 0 packets, 0 bytes via 38.0.0.2, Ethernet1/0, 0 dependencies next hop 38.0.0.2, Ethernet1/0 valid cached adjacency

A set of generic FIB entries are also created on every router; these entries are covered in the next section. Generic FIB Entries

Any router can have, at the minimum, the FIB entries listed in Example 2-10, which are not tied to any particular interface. Note that the Interface column for these entries is empty.