CCIE Chapter 6: IP routing by zDr0vSg


									CCIE Chapter 6: IP routing

Cisco Press CCNP Self-Study BCMSN Official Exam Certification Guide 4th Edition

How a packet is routed

1. A router receives the frame and checks the received frame check sequence (FCS); if errors
occurred, the frame is discarded. The router makes no attempt to recover the lost packet.
2. If no errors occurred, the router checks the Ethernet Type field for the packet type, and extracts
the packet. The Data Link header and trailer can now be discarded.
3. Assuming an IP packet, the router checks its IP routing table for the most specific prefix match
of the packet’s destination IP address.
4. The matched routing table entry includes the outgoing interface and next-hop router; this
information points the router to the adjacency information needed to build a new Data Link
5. Before creating a new frame, the router updates the IP header TTL field, requiring a
recomputation of the IP header checksum.
6. The router encapsulates the IP packet in a new Data Link header (including the destination
address) and trailer (including a new FCS) to create a new frame.

Fast switching, also know as route once switch many, this works by following the steps above for the first
packet of a particular flow. The information like destination Mac address etc that needs to be changed is
added to a cache. The next time the router sees a packet matching the same flow it doesn’t need to
recalculate all the information it can get most of it form the cache. This reduces the amount of processor
load need to route the packet.

CEF: cisco express forward, CE using a thing called FIB ( forward information base) the FIB contains
information about all the known routes in the routing table. The FIB is made up of a tree like structure
called mtrie, in the FIB table the most specific routes are entered first. This is done because CEF will
match on the first entry it finds in the FIB table so if there where two routes and unless the /25 entry was first in the FIB table the less specific route would be chosen. The
FIB also contains the next hop address for the route.

. For every entry in the FIB table there is a point to an entry In the CEF adjacency table.

Packets that cant be routed via CEf are marked as cefpunt and sent to the layer 3 engine, some reasons a
packet might be sent to the layer 3 engine:

An entry cannot be located in the FIB

The FIB table is full

The IP Time To Live (TTL) has expired

The maximum transmission unit (MTU) is exceeded, and the packet must be fragmented
An Internet Control Message Protocol (ICMP) redirect is involved
The encapsulation type is not supported
Packets are tunneled, requiring a compression or encryption operation
An access list with the log option is triggered
A Network Address Translation (NAT) operation must be performed (except on the Catalyst 6500
Supervisor 720, which can handle NAT in hardware)

CEF hardware optimisations

■ Accelerated   CEF (aCEF)—CEF is distributed across multiple Layer 3 forwarding engines,
typically located on Catalyst 6500 line cards. These engines do not have the capability to store
and use the entire FIB, so only a portion of the FIB is downloaded to them at any time. This
functions as an FIB “cache,” containing entries that are likely to be used again. If FIB entries
are not found in the cache, requests are sent to the Layer 3 engine for more FIB information.
The net result is that CEF is accelerated on the line cards, but not necessarily at a sustained
wire-speed rate.
■ Distributed CEF (dCEF)—CEF can be distributed completely among multiple Layer 3
forwarding engines for even greater performance. Because the FIB is self-contained for
complete Layer 3 forwarding, it can be replicated across any number of independent Layer 3
forwarding engines. The Catalyst 6500 has line cards that support dCEF, each with its own
FIB table and forwarding engine. A central Layer 3 engine (the MSFC3, for example)
maintains the routing table and generates the FIB, which is then dynamically downloaded in
full to each of the line cards.

Adjacency Table:

the adjacency table acts just like a mac address table but each entry is mapped to a particular entry in the
FIB. If there is no mac entry in the adjacency table for a FIB entry the entry is maked as “CEFglean” and
the first packet matching that entry is sent to the layer 3 engine so it can perform the ARP request.

Once the next hop has been found the layer 2 headers and the TTL field needs to be updated.
The switch has an additional functional block that performs a packet rewrite in real time. The
packet rewrite engine makes the following changes to the packet just before forwarding:
■ Layer 2 destination address—Changed to the next-hop device’s MAC address
■ Layer 2 source address—Changed to the outbound Layer 3 switch interface’s MAC address
■ Layer 3 IP Time To Live (TTL)—Decremented by one because one router hop has just occurred
■ Layer 3 IP checksum—Recalculated to include changes to the IP header
■ Layer 2 frame checksum—Recalculated to include changes to the Layer 2 and Layer 3 headers

Classless and Classful routing

Classless routing—When a default route exists, and no specific match is made when comparing
the destination of the packet and the routing table, the default route is used.

Classful routing—When a default route exists, and the class A, B, or C network for the
destination IP address does not exist at all in the routing table, the default route is used. If any
part of that classful network exists in the routing table, but the packet does not match any
of the existing subnets of that classful network, the router does not use the default route and
thus discards the packet.

Multilayer Switching

 Mls uses layer 3 information to make a switching decision ( eg CEF), MLS differs slightly
because it uses vlan (SVI) interface, routed interfaces and port channel’s.

When using VLAN interfaces, the switch must take one noticeable but simple additional step
when routing a packet. Like typical routers, MLS makes a routing decision to forward a packet.
As with routers, the routes in an MLS routing table entry list an outgoing interface (a VLAN
interface in this case), as well as a next-hop layer 3 address. The adjacency information (for example,
the IP ARP table or the CEF adjacency table) lists the VLAN number and the next-hop device’s
MAC address to which the packet should be forwarded—again, typical of normal router operation.

At this point, a true router would know everything it needs to know to forward the packet. An MLS
switch, however, then also needs to use Layer 2 logic to decide out which physical interface to
physically forward the packet. The switch will simply find the next-hop device’s MAC address in
the CAM and forward the frame to that address based on the CAM.

Routed port, a routed port is a port that has had the NO switchport command issued on an MLS
switch. It acts like a router port on a router does and requires the ip address information to be
entered on the port.

Etherchannel routed port, can be config’d as above ( no switchport command on channel group
and physical port) but the load balancing method should be changed from mac ( either soruce or
dest) to ip ( either source or dest)

Policy based routing,
Configured on an interface and used to make routing decisions that differ from the routing table,
can use lots of different metric or matches to make a decision. The decision can be be changing
the next hop address or changing something with a header of the packet.

Policy routes are noting more then a sophisticated static route.
Policy routing can also be used for route tagging, tag the route with an identifier that can be used to
distinguish it from other routes.

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